[PATCH v4 0/5] virtiofs: propagate sync() to file server

[PATCH v4 0/5] virtiofs: propagate sync() to file server

[Greg Kurz]

This was a single patch until v3. Some preliminary cleanups were
introduced for submounts in this v4.

This can be tested with a custom virtiofsd implementing FUSE_SYNCFS, here:

https://gitlab.com/gkurz/qemu/-/tree/fuse-sync

v4: - submount fixes
    - set nodeid of the superblock in the request (Miklos)

v3: - just keep a 64-bit padding field in the arg struct (Vivek)

v2: - clarify compatibility with older servers in changelog (Vivek)
    - ignore the wait == 0 case (Miklos)
    - 64-bit aligned argument structure (Vivek, Miklos)

Greg Kurz (5):
  fuse: Fix leak in fuse_dentry_automount() error path
  fuse: Call vfs_get_tree() for submounts
  fuse: Make fuse_fill_super_submount() static
  virtiofs: Skip submounts in sget_fc()
  virtiofs: propagate sync() to file server

 fs/fuse/dir.c             | 45 +++++---------------
 fs/fuse/fuse_i.h          | 12 +++---
 fs/fuse/inode.c           | 87 ++++++++++++++++++++++++++++++++++++++-
 fs/fuse/virtio_fs.c       |  9 ++++
 include/uapi/linux/fuse.h | 10 ++++-
 5 files changed, 120 insertions(+), 43 deletions(-)

-- 
2.26.3


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[PATCH v4 1/5] fuse: Fix leak in fuse_dentry_automount() error path

[Greg Kurz]

Some rollback was forgotten during the addition of crossmounts.

Fixes: bf109c64040f ("fuse: implement crossmounts")
Cc: mreitz@redhat.com
Signed-off-by: Greg Kurz <groug@kaod.org>
---
 fs/fuse/dir.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 1b6c001a7dd1..fb2af70596c3 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -339,8 +339,11 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 
 	/* Initialize superblock, making @mp_fi its root */
 	err = fuse_fill_super_submount(sb, mp_fi);
-	if (err)
+	if (err) {
+		fuse_conn_put(fc);
+		kfree(fm);
 		goto out_put_sb;
+	}
 
 	sb->s_flags |= SB_ACTIVE;
 	fsc->root = dget(sb->s_root);
-- 
2.26.3

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Re: [PATCH v4 1/5] fuse: Fix leak in fuse_dentry_automount() error path

[Al Viro]

On Thu, May 20, 2021 at 05:46:50PM +0200, Greg Kurz wrote:
> Some rollback was forgotten during the addition of crossmounts.

Have you actually tested that?  Because I strongly suspect that
by that point the ownership of fc and fm is with sb and those
should be taken care of by deactivate_locked_super().
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Re: [PATCH v4 1/5] fuse: Fix leak in fuse_dentry_automount() error path

[Greg Kurz]

On Thu, 20 May 2021 19:45:13 +0000
Al Viro <viro@zeniv.linux.org.uk> wrote:

> On Thu, May 20, 2021 at 05:46:50PM +0200, Greg Kurz wrote:
> > Some rollback was forgotten during the addition of crossmounts.
> 
> Have you actually tested that?  Because I strongly suspect that
> by that point the ownership of fc and fm is with sb and those
> should be taken care of by deactivate_locked_super().

My bad, I didn't test but now I did and the issue is actually
worse than just a memory leak. This error path crashes upstream
without this patch:

[   26.206673] BUG: kernel NULL pointer dereference, address: 0000000000000000
[   26.209560] #PF: supervisor read access in kernel mode
[   26.211699] #PF: error_code(0x0000) - not-present page
[   26.214574] PGD 0 P4D 0 
[   26.216016] Oops: 0000 [#1] SMP PTI
[   26.217451] CPU: 0 PID: 3380 Comm: ls Kdump: loaded Not tainted 5.13.0-virtio-fs-sync+ #30
[   26.220839] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[   26.226362] RIP: 0010:__list_del_entry_valid+0x25/0x90
[   26.228449] Code: c3 0f 1f 40 00 48 8b 17 4c 8b 47 08 48 b8 00 01 00 00 00 00 ad de 48 39 c2 74 26 48 b8 22 01 00 00 00 00 ad de 49 39 c0 74 2b <49> 8b 30 48 39 fe 75 3a 48 8b 52 08 48 39 f2 75 48 b8 01 00 00 00
[   26.234256] RSP: 0018:ffffaa37006cbb18 EFLAGS: 00010217
[   26.235473] RAX: dead000000000122 RBX: ffff8f6844098200 RCX: 0000000000000000
[   26.236922] RDX: 0000000000000000 RSI: ffffffff99264e92 RDI: ffff8f6844098210
[   26.238401] RBP: ffff8f68420b3c00 R08: 0000000000000000 R09: 000000000000002a
[   26.239852] R10: 0000000000000000 R11: ffff8f6840402480 R12: ffff8f6844098210
[   26.241160] R13: ffff8f68420b3da8 R14: ffff8f6844098200 R15: 0000000000000000
[   26.242398] FS:  00007f547b93f200(0000) GS:ffff8f687bc00000(0000) knlGS:0000000000000000
[   26.243698] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   26.244693] CR2: 0000000000000000 CR3: 0000000104e50000 CR4: 00000000000006f0
[   26.245936] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   26.246961] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   26.247938] Call Trace:
[   26.248300]  fuse_mount_remove+0x2c/0x70 [fuse]
[   26.248892]  virtio_kill_sb+0x22/0x160 [virtiofs]
[   26.249487]  deactivate_locked_super+0x36/0xa0
[   26.250077]  fuse_dentry_automount+0x178/0x1a0 [fuse]


The crash happens because we're assuming fm was already added to
fc->mounts...

bool fuse_mount_remove(struct fuse_mount *fm)
{
	struct fuse_conn *fc = fm->fc;
	bool last = false;

	down_write(&fc->killsb);
	list_del_init(&fm->fc_entry); <=== HERE
	if (list_empty(&fc->mounts))
		last = true;
	up_write(&fc->killsb);

	return last;
}

but fm is added to fc->mounts much later after the superblock is fully
configured. Looking again at what is done for the root mount in
virtio_fs_get_tree(), I now realize sb->s_fs_info is used as a flag
to decide whether fuse_mount_remove() should be called:

static int virtio_fs_get_tree(struct fs_context *fsc)
{
...
	if (!sb->s_root) {
		err = virtio_fs_fill_super(sb, fsc);
		if (err) {
			fuse_conn_put(fc);
			kfree(fm);
CLEARED HERE =>		sb->s_fs_info = NULL;
			deactivate_locked_super(sb);
			return err;
		}

		sb->s_flags |= SB_ACTIVE;
	}
...
}

static void virtio_kill_sb(struct super_block *sb)
{
	struct fuse_mount *fm = get_fuse_mount_super(sb);

I.E. sb->s_fs_info

	bool last;

	/* If mount failed, we can still be called without any fc */
	if (fm) {

TESTED HERE ^^

		last = fuse_mount_remove(fm);
		if (last)
			virtio_fs_conn_destroy(fm);
	}
	kill_anon_super(sb);
}

The natural fix is to do the same in the automount case : take
back the ownership on fm by clearing sb->s_fs_info, which thus
implies to do the freeing.

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Re: [PATCH v4 1/5] fuse: Fix leak in fuse_dentry_automount() error path

[Greg Kurz]

On Fri, 21 May 2021 09:54:19 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> On Thu, 20 May 2021 at 21:45, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Thu, May 20, 2021 at 05:46:50PM +0200, Greg Kurz wrote:
> > > Some rollback was forgotten during the addition of crossmounts.
> >
> > Have you actually tested that?  Because I strongly suspect that
> > by that point the ownership of fc and fm is with sb and those
> > should be taken care of by deactivate_locked_super().
> 
> Not quite.  Patch looks correct because destruction of fm is done in
> fuse_put_super(), which only gets called if the sb initialization gets
> as far as setting up sb->s_root, which only happens after the
> successful fuse_fill_super_submount() call in this case.
> 
> Doing the destruction from the various ->kill_sb() instances instead
> of from ->put_super() would also fix this, but I'm not quite sure that
> that would be any cleaner.
> 

As saying in the answer I've just posted, a failure in
fuse_fill_super_submount() causes an actual crash because
fuse_mount_remove() logically assumes fm to already be in
fc->mounts, which isn't the case at this point.

In the root mount case, this is handled by taking back
the ownership on fm, i.e. do the rollback *and* clear
sb->s_fs_info. It seems that the same should be done
for submounts.

> Thanks,
> Miklos

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[PATCH v4 2/5] fuse: Call vfs_get_tree() for submounts

[Greg Kurz]

We don't set the SB_BORN flag on submounts superblocks. This is wrong
as these superblocks are then considered as partially constructed or
dying in the rest of the code and can break some assumptions.

One such case is when you have a virtiofs filesystem and you try to
mount it again : virtio_fs_get_tree() tries to obtain a superblock
with sget_fc(). The matching criteria in virtio_fs_test_super() is
the pointer of the underlying virtiofs device, which is shared by
the root mount and its submounts. This means that any submount can
be picked up instead of the root mount. This is itself a bug :
submounts should be ignored in this case. But, most importantly, it
then triggers an infinite loop in sget_fc() because it fails to grab
the superblock (very easy to reproduce).

The only viable solution is to set SB_BORN at some point. This
must be done with vfs_get_tree() because setting SB_BORN requires
special care, i.e. a memory barrier for super_cache_count() which
can check SB_BORN without taking any lock.

This requires to split out some code from fuse_dentry_automount() to
a new dedicated fuse_get_tree_submount().

The fs_private field of the filesystem context isn't used with
submounts : hijack it to pass the FUSE inode of the mount point
down to fuse_get_tree_submount().

Finally, adapt virtiofs to use this.

Signed-off-by: Greg Kurz <groug@kaod.org>
---
 fs/fuse/dir.c       | 48 +++++++++++----------------------------------
 fs/fuse/fuse_i.h    |  6 ++++++
 fs/fuse/inode.c     | 43 ++++++++++++++++++++++++++++++++++++++++
 fs/fuse/virtio_fs.c |  3 +++
 4 files changed, 63 insertions(+), 37 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index fb2af70596c3..4c8dafe4f69e 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -309,12 +309,9 @@ static int fuse_dentry_delete(const struct dentry *dentry)
 static struct vfsmount *fuse_dentry_automount(struct path *path)
 {
 	struct fs_context *fsc;
-	struct fuse_mount *parent_fm = get_fuse_mount_super(path->mnt->mnt_sb);
-	struct fuse_conn *fc = parent_fm->fc;
 	struct fuse_mount *fm;
 	struct vfsmount *mnt;
 	struct fuse_inode *mp_fi = get_fuse_inode(d_inode(path->dentry));
-	struct super_block *sb;
 	int err;
 
 	fsc = fs_context_for_submount(path->mnt->mnt_sb->s_type, path->dentry);
@@ -323,36 +320,19 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 		goto out;
 	}
 
-	err = -ENOMEM;
-	fm = kzalloc(sizeof(struct fuse_mount), GFP_KERNEL);
-	if (!fm)
+	/*
+	 * Hijack fsc->fs_private to pass the mount point inode to
+	 * fuse_get_tree_submount(). It *must* be NULLified afterwards
+	 * to avoid the inode pointer to be passed to kfree() when
+	 * the context gets freed.
+	 */
+	fsc->fs_private = mp_fi;
+	err = vfs_get_tree(fsc);
+	fsc->fs_private = NULL;
+	if (err)
 		goto out_put_fsc;
 
-	fsc->s_fs_info = fm;
-	sb = sget_fc(fsc, NULL, set_anon_super_fc);
-	if (IS_ERR(sb)) {
-		err = PTR_ERR(sb);
-		kfree(fm);
-		goto out_put_fsc;
-	}
-	fm->fc = fuse_conn_get(fc);
-
-	/* Initialize superblock, making @mp_fi its root */
-	err = fuse_fill_super_submount(sb, mp_fi);
-	if (err) {
-		fuse_conn_put(fc);
-		kfree(fm);
-		goto out_put_sb;
-	}
-
-	sb->s_flags |= SB_ACTIVE;
-	fsc->root = dget(sb->s_root);
-	/* We are done configuring the superblock, so unlock it */
-	up_write(&sb->s_umount);
-
-	down_write(&fc->killsb);
-	list_add_tail(&fm->fc_entry, &fc->mounts);
-	up_write(&fc->killsb);
+	fm = get_fuse_mount_super(fsc->root->d_sb);
 
 	/* Create the submount */
 	mnt = vfs_create_mount(fsc);
@@ -364,12 +344,6 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 	put_fs_context(fsc);
 	return mnt;
 
-out_put_sb:
-	/*
-	 * Only jump here when fsc->root is NULL and sb is still locked
-	 * (otherwise put_fs_context() will put the superblock)
-	 */
-	deactivate_locked_super(sb);
 out_put_fsc:
 	put_fs_context(fsc);
 out:
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 7e463e220053..d7fcf59a6a0e 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1090,6 +1090,12 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx);
 int fuse_fill_super_submount(struct super_block *sb,
 			     struct fuse_inode *parent_fi);
 
+/*
+ * Get the mountable root for the submount
+ * @fsc: superblock configuration context
+ */
+int fuse_get_tree_submount(struct fs_context *fsc);
+
 /*
  * Remove the mount from the connection
  *
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 393e36b74dc4..74e5205f203c 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1313,6 +1313,49 @@ int fuse_fill_super_submount(struct super_block *sb,
 	return 0;
 }
 
+/* Filesystem context private data holds the FUSE inode of the mount point */
+int fuse_get_tree_submount(struct fs_context *fsc)
+{
+	struct fuse_mount *fm;
+	struct fuse_inode *mp_fi = fsc->fs_private;
+	struct fuse_conn *fc = get_fuse_conn(&mp_fi->inode);
+	struct super_block *sb;
+	int err;
+
+	fm = kzalloc(sizeof(struct fuse_mount), GFP_KERNEL);
+	if (!fm)
+		return -ENOMEM;
+
+	fsc->s_fs_info = fm;
+	sb = sget_fc(fsc, NULL, set_anon_super_fc);
+	if (IS_ERR(sb)) {
+		kfree(fm);
+		return PTR_ERR(sb);
+	}
+	fm->fc = fuse_conn_get(fc);
+
+	/* Initialize superblock, making @mp_fi its root */
+	err = fuse_fill_super_submount(sb, mp_fi);
+	if (err) {
+		fuse_conn_put(fc);
+		deactivate_locked_super(sb);
+		kfree(fm);
+		return err;
+	}
+
+	sb->s_flags |= SB_ACTIVE;
+	fsc->root = dget(sb->s_root);
+	/* We are done configuring the superblock, so unlock it */
+	up_write(&sb->s_umount);
+
+	down_write(&fc->killsb);
+	list_add_tail(&fm->fc_entry, &fc->mounts);
+	up_write(&fc->killsb);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fuse_get_tree_submount);
+
 int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
 {
 	struct fuse_dev *fud = NULL;
diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index bcb8a02e2d8b..e12e5190352c 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -1420,6 +1420,9 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
 	unsigned int virtqueue_size;
 	int err = -EIO;
 
+	if (fsc->purpose == FS_CONTEXT_FOR_SUBMOUNT)
+		return fuse_get_tree_submount(fsc);
+
 	/* This gets a reference on virtio_fs object. This ptr gets installed
 	 * in fc->iq->priv. Once fuse_conn is going away, it calls ->put()
 	 * to drop the reference to this object.
-- 
2.26.3

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Re: [PATCH v4 2/5] fuse: Call vfs_get_tree() for submounts

[Greg Kurz]

On Fri, 21 May 2021 10:19:48 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> On Thu, May 20, 2021 at 05:46:51PM +0200, Greg Kurz wrote:
> > We don't set the SB_BORN flag on submounts superblocks. This is wrong
> > as these superblocks are then considered as partially constructed or
> > dying in the rest of the code and can break some assumptions.
> > 
> > One such case is when you have a virtiofs filesystem and you try to
> > mount it again : virtio_fs_get_tree() tries to obtain a superblock
> > with sget_fc(). The matching criteria in virtio_fs_test_super() is
> > the pointer of the underlying virtiofs device, which is shared by
> > the root mount and its submounts. This means that any submount can
> > be picked up instead of the root mount. This is itself a bug :
> > submounts should be ignored in this case. But, most importantly, it
> > then triggers an infinite loop in sget_fc() because it fails to grab
> > the superblock (very easy to reproduce).
> > 
> > The only viable solution is to set SB_BORN at some point. This
> > must be done with vfs_get_tree() because setting SB_BORN requires
> > special care, i.e. a memory barrier for super_cache_count() which
> > can check SB_BORN without taking any lock.
> 
> Looks correct, but...
> 
> as an easily backportable and verifiable bugfix I'd still go with the
> simple two liner:
> 
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -351,6 +351,9 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
>  	list_add_tail(&fm->fc_entry, &fc->mounts);
>  	up_write(&fc->killsb);
>  
> +	smp_wmb();
> +	sb->s_flags |= SB_BORN;
> +

plus the mandatory comment one must put to justify the
need for a memory barrier.

>  	/* Create the submount */
>  	mnt = vfs_create_mount(fsc);
>  	if (IS_ERR(mnt)) {
> 
> And have this patch be the cleanup.
> 

Fair enough.

> Also we need Fixes: and a Cc: stable@... tags on that one.
> 

Oops, I'll add these in the next round.

> Thanks,
> Miklos

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Re: [PATCH v4 2/5] fuse: Call vfs_get_tree() for submounts

[kernel test robot]

[-- Attachment #1: Type: text/plain, Size: 5294 bytes --]

Hi Greg,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on fuse/for-next]
[also build test WARNING on linux/master linus/master v5.13-rc2 next-20210521]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Greg-Kurz/virtiofs-propagate-sync-to-file-server/20210522-210652
base:   https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git for-next
config: nds32-randconfig-r011-20210522 (attached as .config)
compiler: nds32le-linux-gcc (GCC) 9.3.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/ee3cc45c5a2311efc82021bd5463271507bef828
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Greg-Kurz/virtiofs-propagate-sync-to-file-server/20210522-210652
        git checkout ee3cc45c5a2311efc82021bd5463271507bef828
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=nds32 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

   fs/fuse/dir.c: In function 'fuse_dentry_automount':
>> fs/fuse/dir.c:312:21: warning: variable 'fm' set but not used [-Wunused-but-set-variable]
     312 |  struct fuse_mount *fm;
         |                     ^~


vim +/fm +312 fs/fuse/dir.c

8fab010644363f Miklos Szeredi 2018-08-15  303  
bf109c64040f5b Max Reitz      2020-04-21  304  /*
bf109c64040f5b Max Reitz      2020-04-21  305   * Create a fuse_mount object with a new superblock (with path->dentry
bf109c64040f5b Max Reitz      2020-04-21  306   * as the root), and return that mount so it can be auto-mounted on
bf109c64040f5b Max Reitz      2020-04-21  307   * @path.
bf109c64040f5b Max Reitz      2020-04-21  308   */
bf109c64040f5b Max Reitz      2020-04-21  309  static struct vfsmount *fuse_dentry_automount(struct path *path)
bf109c64040f5b Max Reitz      2020-04-21  310  {
bf109c64040f5b Max Reitz      2020-04-21  311  	struct fs_context *fsc;
bf109c64040f5b Max Reitz      2020-04-21 @312  	struct fuse_mount *fm;
bf109c64040f5b Max Reitz      2020-04-21  313  	struct vfsmount *mnt;
bf109c64040f5b Max Reitz      2020-04-21  314  	struct fuse_inode *mp_fi = get_fuse_inode(d_inode(path->dentry));
bf109c64040f5b Max Reitz      2020-04-21  315  	int err;
bf109c64040f5b Max Reitz      2020-04-21  316  
bf109c64040f5b Max Reitz      2020-04-21  317  	fsc = fs_context_for_submount(path->mnt->mnt_sb->s_type, path->dentry);
bf109c64040f5b Max Reitz      2020-04-21  318  	if (IS_ERR(fsc)) {
bf109c64040f5b Max Reitz      2020-04-21  319  		err = PTR_ERR(fsc);
bf109c64040f5b Max Reitz      2020-04-21  320  		goto out;
bf109c64040f5b Max Reitz      2020-04-21  321  	}
bf109c64040f5b Max Reitz      2020-04-21  322  
ee3cc45c5a2311 Greg Kurz      2021-05-20  323  	/*
ee3cc45c5a2311 Greg Kurz      2021-05-20  324  	 * Hijack fsc->fs_private to pass the mount point inode to
ee3cc45c5a2311 Greg Kurz      2021-05-20  325  	 * fuse_get_tree_submount(). It *must* be NULLified afterwards
ee3cc45c5a2311 Greg Kurz      2021-05-20  326  	 * to avoid the inode pointer to be passed to kfree() when
ee3cc45c5a2311 Greg Kurz      2021-05-20  327  	 * the context gets freed.
ee3cc45c5a2311 Greg Kurz      2021-05-20  328  	 */
ee3cc45c5a2311 Greg Kurz      2021-05-20  329  	fsc->fs_private = mp_fi;
ee3cc45c5a2311 Greg Kurz      2021-05-20  330  	err = vfs_get_tree(fsc);
ee3cc45c5a2311 Greg Kurz      2021-05-20  331  	fsc->fs_private = NULL;
ee3cc45c5a2311 Greg Kurz      2021-05-20  332  	if (err)
bf109c64040f5b Max Reitz      2020-04-21  333  		goto out_put_fsc;
bf109c64040f5b Max Reitz      2020-04-21  334  
ee3cc45c5a2311 Greg Kurz      2021-05-20  335  	fm = get_fuse_mount_super(fsc->root->d_sb);
bf109c64040f5b Max Reitz      2020-04-21  336  
bf109c64040f5b Max Reitz      2020-04-21  337  	/* Create the submount */
bf109c64040f5b Max Reitz      2020-04-21  338  	mnt = vfs_create_mount(fsc);
bf109c64040f5b Max Reitz      2020-04-21  339  	if (IS_ERR(mnt)) {
bf109c64040f5b Max Reitz      2020-04-21  340  		err = PTR_ERR(mnt);
bf109c64040f5b Max Reitz      2020-04-21  341  		goto out_put_fsc;
bf109c64040f5b Max Reitz      2020-04-21  342  	}
bf109c64040f5b Max Reitz      2020-04-21  343  	mntget(mnt);
bf109c64040f5b Max Reitz      2020-04-21  344  	put_fs_context(fsc);
bf109c64040f5b Max Reitz      2020-04-21  345  	return mnt;
bf109c64040f5b Max Reitz      2020-04-21  346  
bf109c64040f5b Max Reitz      2020-04-21  347  out_put_fsc:
bf109c64040f5b Max Reitz      2020-04-21  348  	put_fs_context(fsc);
bf109c64040f5b Max Reitz      2020-04-21  349  out:
bf109c64040f5b Max Reitz      2020-04-21  350  	return ERR_PTR(err);
bf109c64040f5b Max Reitz      2020-04-21  351  }
bf109c64040f5b Max Reitz      2020-04-21  352  

---
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Re: [PATCH v4 2/5] fuse: Call vfs_get_tree() for submounts

[kernel test robot]

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Hi Greg,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on fuse/for-next]
[also build test WARNING on linux/master linus/master v5.13-rc2 next-20210521]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Greg-Kurz/virtiofs-propagate-sync-to-file-server/20210522-210652
base:   https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git for-next
config: sparc64-randconfig-p002-20210522 (attached as .config)
compiler: sparc64-linux-gcc (GCC) 9.3.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/ee3cc45c5a2311efc82021bd5463271507bef828
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Greg-Kurz/virtiofs-propagate-sync-to-file-server/20210522-210652
        git checkout ee3cc45c5a2311efc82021bd5463271507bef828
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=sparc64 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

   fs/fuse/dir.c: In function 'fuse_dentry_automount':
>> fs/fuse/dir.c:312:21: warning: variable 'fm' set but not used [-Wunused-but-set-variable]
     312 |  struct fuse_mount *fm;
         |                     ^~

Kconfig warnings: (for reference only)
   WARNING: unmet direct dependencies detected for FRAME_POINTER
   Depends on DEBUG_KERNEL && (M68K || UML || SUPERH) || ARCH_WANT_FRAME_POINTERS || MCOUNT
   Selected by
   - LOCKDEP && DEBUG_KERNEL && LOCK_DEBUGGING_SUPPORT && !MIPS && !PPC && !ARM && !S390 && !MICROBLAZE && !ARC && !X86


vim +/fm +312 fs/fuse/dir.c

8fab010644363f Miklos Szeredi 2018-08-15  303  
bf109c64040f5b Max Reitz      2020-04-21  304  /*
bf109c64040f5b Max Reitz      2020-04-21  305   * Create a fuse_mount object with a new superblock (with path->dentry
bf109c64040f5b Max Reitz      2020-04-21  306   * as the root), and return that mount so it can be auto-mounted on
bf109c64040f5b Max Reitz      2020-04-21  307   * @path.
bf109c64040f5b Max Reitz      2020-04-21  308   */
bf109c64040f5b Max Reitz      2020-04-21  309  static struct vfsmount *fuse_dentry_automount(struct path *path)
bf109c64040f5b Max Reitz      2020-04-21  310  {
bf109c64040f5b Max Reitz      2020-04-21  311  	struct fs_context *fsc;
bf109c64040f5b Max Reitz      2020-04-21 @312  	struct fuse_mount *fm;
bf109c64040f5b Max Reitz      2020-04-21  313  	struct vfsmount *mnt;
bf109c64040f5b Max Reitz      2020-04-21  314  	struct fuse_inode *mp_fi = get_fuse_inode(d_inode(path->dentry));
bf109c64040f5b Max Reitz      2020-04-21  315  	int err;
bf109c64040f5b Max Reitz      2020-04-21  316  
bf109c64040f5b Max Reitz      2020-04-21  317  	fsc = fs_context_for_submount(path->mnt->mnt_sb->s_type, path->dentry);
bf109c64040f5b Max Reitz      2020-04-21  318  	if (IS_ERR(fsc)) {
bf109c64040f5b Max Reitz      2020-04-21  319  		err = PTR_ERR(fsc);
bf109c64040f5b Max Reitz      2020-04-21  320  		goto out;
bf109c64040f5b Max Reitz      2020-04-21  321  	}
bf109c64040f5b Max Reitz      2020-04-21  322  
ee3cc45c5a2311 Greg Kurz      2021-05-20  323  	/*
ee3cc45c5a2311 Greg Kurz      2021-05-20  324  	 * Hijack fsc->fs_private to pass the mount point inode to
ee3cc45c5a2311 Greg Kurz      2021-05-20  325  	 * fuse_get_tree_submount(). It *must* be NULLified afterwards
ee3cc45c5a2311 Greg Kurz      2021-05-20  326  	 * to avoid the inode pointer to be passed to kfree() when
ee3cc45c5a2311 Greg Kurz      2021-05-20  327  	 * the context gets freed.
ee3cc45c5a2311 Greg Kurz      2021-05-20  328  	 */
ee3cc45c5a2311 Greg Kurz      2021-05-20  329  	fsc->fs_private = mp_fi;
ee3cc45c5a2311 Greg Kurz      2021-05-20  330  	err = vfs_get_tree(fsc);
ee3cc45c5a2311 Greg Kurz      2021-05-20  331  	fsc->fs_private = NULL;
ee3cc45c5a2311 Greg Kurz      2021-05-20  332  	if (err)
bf109c64040f5b Max Reitz      2020-04-21  333  		goto out_put_fsc;
bf109c64040f5b Max Reitz      2020-04-21  334  
ee3cc45c5a2311 Greg Kurz      2021-05-20  335  	fm = get_fuse_mount_super(fsc->root->d_sb);
bf109c64040f5b Max Reitz      2020-04-21  336  
bf109c64040f5b Max Reitz      2020-04-21  337  	/* Create the submount */
bf109c64040f5b Max Reitz      2020-04-21  338  	mnt = vfs_create_mount(fsc);
bf109c64040f5b Max Reitz      2020-04-21  339  	if (IS_ERR(mnt)) {
bf109c64040f5b Max Reitz      2020-04-21  340  		err = PTR_ERR(mnt);
bf109c64040f5b Max Reitz      2020-04-21  341  		goto out_put_fsc;
bf109c64040f5b Max Reitz      2020-04-21  342  	}
bf109c64040f5b Max Reitz      2020-04-21  343  	mntget(mnt);
bf109c64040f5b Max Reitz      2020-04-21  344  	put_fs_context(fsc);
bf109c64040f5b Max Reitz      2020-04-21  345  	return mnt;
bf109c64040f5b Max Reitz      2020-04-21  346  
bf109c64040f5b Max Reitz      2020-04-21  347  out_put_fsc:
bf109c64040f5b Max Reitz      2020-04-21  348  	put_fs_context(fsc);
bf109c64040f5b Max Reitz      2020-04-21  349  out:
bf109c64040f5b Max Reitz      2020-04-21  350  	return ERR_PTR(err);
bf109c64040f5b Max Reitz      2020-04-21  351  }
bf109c64040f5b Max Reitz      2020-04-21  352  

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[PATCH v4 3/5] fuse: Make fuse_fill_super_submount() static

[Greg Kurz]

This function used to be called from fuse_dentry_automount(). This code
was moved to fuse_get_tree_submount() in the same file since then. It
is unlikely there will ever be another user. No need to be extern in
this case.

Signed-off-by: Greg Kurz <groug@kaod.org>
---
 fs/fuse/fuse_i.h | 9 ---------
 fs/fuse/inode.c  | 4 ++--
 2 files changed, 2 insertions(+), 11 deletions(-)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index d7fcf59a6a0e..e2f5c8617e0d 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1081,15 +1081,6 @@ void fuse_send_init(struct fuse_mount *fm);
  */
 int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx);
 
-/*
- * Fill in superblock for submounts
- * @sb: partially-initialized superblock to fill in
- * @parent_fi: The fuse_inode of the parent filesystem where this submount is
- * 	       mounted
- */
-int fuse_fill_super_submount(struct super_block *sb,
-			     struct fuse_inode *parent_fi);
-
 /*
  * Get the mountable root for the submount
  * @fsc: superblock configuration context
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 74e5205f203c..123b53d1c3c6 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1275,8 +1275,8 @@ static void fuse_sb_defaults(struct super_block *sb)
 		sb->s_xattr = fuse_no_acl_xattr_handlers;
 }
 
-int fuse_fill_super_submount(struct super_block *sb,
-			     struct fuse_inode *parent_fi)
+static int fuse_fill_super_submount(struct super_block *sb,
+				    struct fuse_inode *parent_fi)
 {
 	struct fuse_mount *fm = get_fuse_mount_super(sb);
 	struct super_block *parent_sb = parent_fi->inode.i_sb;
-- 
2.26.3

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[PATCH v4 4/5] virtiofs: Skip submounts in sget_fc()

[Greg Kurz]

All submounts share the same virtio-fs device instance as the root
mount. If the same virtiofs filesystem is mounted again, sget_fc()
is likely to pick up any of these submounts and reuse it instead of
the root mount.

On the server side:

# mkdir ${some_dir}
# mkdir ${some_dir}/mnt1
# mount -t tmpfs none ${some_dir}/mnt1
# touch ${some_dir}/mnt1/THIS_IS_MNT1
# mkdir ${some_dir}/mnt2
# mount -t tmpfs none ${some_dir}/mnt2
# touch ${some_dir}/mnt2/THIS_IS_MNT2

On the client side:

# mkdir /mnt/virtiofs1
# mount -t virtiofs myfs /mnt/virtiofs1
# ls /mnt/virtiofs1
mnt1 mnt2
# grep virtiofs /proc/mounts
myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)

And now remount it again:

# mount -t virtiofs myfs /mnt/virtiofs2
# grep virtiofs /proc/mounts
myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
myfs /mnt/virtiofs2 virtiofs rw,seclabel,relatime 0 0
# ls /mnt/virtiofs2
THIS_IS_MNT2

Submount mnt2 was picked-up instead of the root mount.

Just skip submounts in virtio_fs_test_super().

Signed-off-by: Greg Kurz <groug@kaod.org>
---
 fs/fuse/virtio_fs.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index e12e5190352c..8962cd033016 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -1408,6 +1408,11 @@ static int virtio_fs_test_super(struct super_block *sb,
 	struct fuse_mount *fsc_fm = fsc->s_fs_info;
 	struct fuse_mount *sb_fm = get_fuse_mount_super(sb);
 
+
+	/* Skip submounts */
+	if (!list_is_first(&sb_fm->fc_entry, &sb_fm->fc->mounts))
+		return 0;
+
 	return fsc_fm->fc->iq.priv == sb_fm->fc->iq.priv;
 }
 
-- 
2.26.3

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Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc()

[Greg Kurz]

On Fri, 21 May 2021 10:26:27 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> On Thu, 20 May 2021 at 17:47, Greg Kurz <groug@kaod.org> wrote:
> >
> > All submounts share the same virtio-fs device instance as the root
> > mount. If the same virtiofs filesystem is mounted again, sget_fc()
> > is likely to pick up any of these submounts and reuse it instead of
> > the root mount.
> >
> > On the server side:
> >
> > # mkdir ${some_dir}
> > # mkdir ${some_dir}/mnt1
> > # mount -t tmpfs none ${some_dir}/mnt1
> > # touch ${some_dir}/mnt1/THIS_IS_MNT1
> > # mkdir ${some_dir}/mnt2
> > # mount -t tmpfs none ${some_dir}/mnt2
> > # touch ${some_dir}/mnt2/THIS_IS_MNT2
> >
> > On the client side:
> >
> > # mkdir /mnt/virtiofs1
> > # mount -t virtiofs myfs /mnt/virtiofs1
> > # ls /mnt/virtiofs1
> > mnt1 mnt2
> > # grep virtiofs /proc/mounts
> > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> >
> > And now remount it again:
> >
> > # mount -t virtiofs myfs /mnt/virtiofs2
> > # grep virtiofs /proc/mounts
> > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> > myfs /mnt/virtiofs2 virtiofs rw,seclabel,relatime 0 0
> > # ls /mnt/virtiofs2
> > THIS_IS_MNT2
> >
> > Submount mnt2 was picked-up instead of the root mount.
> 

> Why is this a problem?
> 

It seems very weird to mount the same filesystem again
and to end up in one of its submounts. We should have:

# ls /mnt/virtiofs2
mnt1 mnt2

> Thanks,
> Miklos

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Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc()

[Greg Kurz]

On Fri, 21 May 2021 10:50:34 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> On Fri, 21 May 2021 at 10:39, Greg Kurz <groug@kaod.org> wrote:
> >
> > On Fri, 21 May 2021 10:26:27 +0200
> > Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > > On Thu, 20 May 2021 at 17:47, Greg Kurz <groug@kaod.org> wrote:
> > > >
> > > > All submounts share the same virtio-fs device instance as the root
> > > > mount. If the same virtiofs filesystem is mounted again, sget_fc()
> > > > is likely to pick up any of these submounts and reuse it instead of
> > > > the root mount.
> > > >
> > > > On the server side:
> > > >
> > > > # mkdir ${some_dir}
> > > > # mkdir ${some_dir}/mnt1
> > > > # mount -t tmpfs none ${some_dir}/mnt1
> > > > # touch ${some_dir}/mnt1/THIS_IS_MNT1
> > > > # mkdir ${some_dir}/mnt2
> > > > # mount -t tmpfs none ${some_dir}/mnt2
> > > > # touch ${some_dir}/mnt2/THIS_IS_MNT2
> > > >
> > > > On the client side:
> > > >
> > > > # mkdir /mnt/virtiofs1
> > > > # mount -t virtiofs myfs /mnt/virtiofs1
> > > > # ls /mnt/virtiofs1
> > > > mnt1 mnt2
> > > > # grep virtiofs /proc/mounts
> > > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> > > >
> > > > And now remount it again:
> > > >
> > > > # mount -t virtiofs myfs /mnt/virtiofs2
> > > > # grep virtiofs /proc/mounts
> > > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> > > > myfs /mnt/virtiofs2 virtiofs rw,seclabel,relatime 0 0
> > > > # ls /mnt/virtiofs2
> > > > THIS_IS_MNT2
> > > >
> > > > Submount mnt2 was picked-up instead of the root mount.
> > >
> >
> > > Why is this a problem?
> > >
> >
> > It seems very weird to mount the same filesystem again
> > and to end up in one of its submounts. We should have:
> >
> > # ls /mnt/virtiofs2
> > mnt1 mnt2
> 
> Okay, sorry, I understand the problem.  The solution is wrong,
> however: the position of the submount on that list is no indication
> that it's the right one (it's possible that the root sb will go away
> and only a sub-sb will remain).
> 

Ah... I had myself convinced this could not happen, i.e. you can't
unmount a parent sb with a sub-sb still mounted.

How can this happen ?

> Even just setting a flag in the root, indicating that it's the root
> isn't fully going to solve the problem.
> 
> Here's issue in full:
> 
> case 1:  no connection for "myfs" exists
>     - need to create fuse_conn, sb
> 
> case 2: connection for "myfs" exists but only sb for submount

How would we know this sb isn't a root sb ?

>     - only create sb for root, reuse fuse_conn
> 
> case 3: connection for "myfs" as well as root sb exists
>    - reuse sb
> 
> I'll think about how to fix this properly, it's probably going to be
> rather more involved...
> 

Sure. BTW I'm wondering why we never reuse sbs for submounts ?

> Thanks,
> Miklos

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Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc()

[Greg Kurz]

On Fri, 21 May 2021 14:37:25 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> On Fri, 21 May 2021 at 12:06, Greg Kurz <groug@kaod.org> wrote:
> >
> > On Fri, 21 May 2021 10:50:34 +0200
> > Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > > On Fri, 21 May 2021 at 10:39, Greg Kurz <groug@kaod.org> wrote:
> > > >
> > > > On Fri, 21 May 2021 10:26:27 +0200
> > > > Miklos Szeredi <miklos@szeredi.hu> wrote:
> > > >
> > > > > On Thu, 20 May 2021 at 17:47, Greg Kurz <groug@kaod.org> wrote:
> > > > > >
> > > > > > All submounts share the same virtio-fs device instance as the root
> > > > > > mount. If the same virtiofs filesystem is mounted again, sget_fc()
> > > > > > is likely to pick up any of these submounts and reuse it instead of
> > > > > > the root mount.
> > > > > >
> > > > > > On the server side:
> > > > > >
> > > > > > # mkdir ${some_dir}
> > > > > > # mkdir ${some_dir}/mnt1
> > > > > > # mount -t tmpfs none ${some_dir}/mnt1
> > > > > > # touch ${some_dir}/mnt1/THIS_IS_MNT1
> > > > > > # mkdir ${some_dir}/mnt2
> > > > > > # mount -t tmpfs none ${some_dir}/mnt2
> > > > > > # touch ${some_dir}/mnt2/THIS_IS_MNT2
> > > > > >
> > > > > > On the client side:
> > > > > >
> > > > > > # mkdir /mnt/virtiofs1
> > > > > > # mount -t virtiofs myfs /mnt/virtiofs1
> > > > > > # ls /mnt/virtiofs1
> > > > > > mnt1 mnt2
> > > > > > # grep virtiofs /proc/mounts
> > > > > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > > > > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > > > > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> > > > > >
> > > > > > And now remount it again:
> > > > > >
> > > > > > # mount -t virtiofs myfs /mnt/virtiofs2
> > > > > > # grep virtiofs /proc/mounts
> > > > > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
> > > > > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
> > > > > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
> > > > > > myfs /mnt/virtiofs2 virtiofs rw,seclabel,relatime 0 0
> > > > > > # ls /mnt/virtiofs2
> > > > > > THIS_IS_MNT2
> > > > > >
> > > > > > Submount mnt2 was picked-up instead of the root mount.
> > > > >
> > > >
> > > > > Why is this a problem?
> > > > >
> > > >
> > > > It seems very weird to mount the same filesystem again
> > > > and to end up in one of its submounts. We should have:
> > > >
> > > > # ls /mnt/virtiofs2
> > > > mnt1 mnt2
> > >
> > > Okay, sorry, I understand the problem.  The solution is wrong,
> > > however: the position of the submount on that list is no indication
> > > that it's the right one (it's possible that the root sb will go away
> > > and only a sub-sb will remain).
> > >
> >
> > Ah... I had myself convinced this could not happen, i.e. you can't
> > unmount a parent sb with a sub-sb still mounted.
> 
> No, but it's possible for sub-sb to continue existing after it's no
> longer a submount of original mount.
> >
> > How can this happen ?
> 
> E.g. move the submount out of the way, then unmount the parent, or
> detach submount (umount -l) while keeping something open in there and
> umount the parent.
> 

Ok, I get it now. Thanks for the clarification.

> > > Even just setting a flag in the root, indicating that it's the root
> > > isn't fully going to solve the problem.
> > >
> > > Here's issue in full:
> > >
> > > case 1:  no connection for "myfs" exists
> > >     - need to create fuse_conn, sb
> > >
> > > case 2: connection for "myfs" exists but only sb for submount
> >
> > How would we know this sb isn't a root sb ?
> >
> > >     - only create sb for root, reuse fuse_conn
> > >
> > > case 3: connection for "myfs" as well as root sb exists
> > >    - reuse sb
> > >
> > > I'll think about how to fix this properly, it's probably going to be
> > > rather more involved...
> > >
> >
> > Sure. BTW I'm wondering why we never reuse sbs for submounts ?
> 
> Right, same general issue.
> 
> An sb can be identified by its root nodeid, so I guess the proper fix
> to make the root nodeid be the key for virtio_fs_test_super().
> 

Cool, I was thinking about doing this exactly. :)

> Thanks,
> Miklos

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[PATCH v4 5/5] virtiofs: propagate sync() to file server

[Greg Kurz]

Even if POSIX doesn't mandate it, linux users legitimately expect
sync() to flush all data and metadata to physical storage when it
is located on the same system. This isn't happening with virtiofs
though : sync() inside the guest returns right away even though
data still needs to be flushed from the host page cache.

This is easily demonstrated by doing the following in the guest:

$ dd if=/dev/zero of=/mnt/foo bs=1M count=5K ; strace -T -e sync sync
5120+0 records in
5120+0 records out
5368709120 bytes (5.4 GB, 5.0 GiB) copied, 5.22224 s, 1.0 GB/s
sync()                                  = 0 <0.024068>
+++ exited with 0 +++

and start the following in the host when the 'dd' command completes
in the guest:

$ strace -T -e fsync /usr/bin/sync virtiofs/foo
fsync(3)                                = 0 <10.371640>
+++ exited with 0 +++

There are no good reasons not to honor the expected behavior of
sync() actually : it gives an unrealistic impression that virtiofs
is super fast and that data has safely landed on HW, which isn't
the case obviously.

Implement a ->sync_fs() superblock operation that sends a new
FUSE_SYNCFS request type for this purpose. Provision a 64-bit
placeholder for possible future extensions. Since the file
server cannot handle the wait == 0 case, we skip it to avoid a
gratuitous roundtrip. Note that this is per-superblock : a
FUSE_SYNCFS is send for the root mount and for each submount.

Like with FUSE_FSYNC and FUSE_FSYNCDIR, lack of support for
FUSE_SYNCFS in the file server is treated as permanent success.
This ensures compatibility with older file servers : the client
will get the current behavior of sync() not being propagated to
the file server.

Note that such an operation allows the file server to DoS sync().
Since a typical FUSE file server is an untrusted piece of software
running in userspace, this is disabled by default.  Only enable it
with virtiofs for now since virtiofsd is supposedly trusted by the
guest kernel.

Reported-by: Robert Krawitz <rlk@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
---
 fs/fuse/fuse_i.h          |  3 +++
 fs/fuse/inode.c           | 40 +++++++++++++++++++++++++++++++++++++++
 fs/fuse/virtio_fs.c       |  1 +
 include/uapi/linux/fuse.h | 10 +++++++++-
 4 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index e2f5c8617e0d..01d9283261af 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -761,6 +761,9 @@ struct fuse_conn {
 	/* Auto-mount submounts announced by the server */
 	unsigned int auto_submounts:1;
 
+	/* Propagate syncfs() to server */
+	unsigned int sync_fs:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;
 
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 123b53d1c3c6..96b00253f766 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -506,6 +506,45 @@ static int fuse_statfs(struct dentry *dentry, struct kstatfs *buf)
 	return err;
 }
 
+static int fuse_sync_fs(struct super_block *sb, int wait)
+{
+	struct fuse_mount *fm = get_fuse_mount_super(sb);
+	struct fuse_conn *fc = fm->fc;
+	struct fuse_syncfs_in inarg;
+	FUSE_ARGS(args);
+	int err;
+
+	/*
+	 * Userspace cannot handle the wait == 0 case. Avoid a
+	 * gratuitous roundtrip.
+	 */
+	if (!wait)
+		return 0;
+
+	/* The filesystem is being unmounted. Nothing to do. */
+	if (!sb->s_root)
+		return 0;
+
+	if (!fc->sync_fs)
+		return 0;
+
+	memset(&inarg, 0, sizeof(inarg));
+	args.in_numargs = 1;
+	args.in_args[0].size = sizeof(inarg);
+	args.in_args[0].value = &inarg;
+	args.opcode = FUSE_SYNCFS;
+	args.nodeid = get_node_id(sb->s_root->d_inode);
+	args.out_numargs = 0;
+
+	err = fuse_simple_request(fm, &args);
+	if (err == -ENOSYS) {
+		fc->sync_fs = 0;
+		err = 0;
+	}
+
+	return err;
+}
+
 enum {
 	OPT_SOURCE,
 	OPT_SUBTYPE,
@@ -909,6 +948,7 @@ static const struct super_operations fuse_super_operations = {
 	.put_super	= fuse_put_super,
 	.umount_begin	= fuse_umount_begin,
 	.statfs		= fuse_statfs,
+	.sync_fs	= fuse_sync_fs,
 	.show_options	= fuse_show_options,
 };
 
diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 8962cd033016..f649a47efb68 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -1455,6 +1455,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
 	fc->release = fuse_free_conn;
 	fc->delete_stale = true;
 	fc->auto_submounts = true;
+	fc->sync_fs = true;
 
 	/* Tell FUSE to split requests that exceed the virtqueue's size */
 	fc->max_pages_limit = min_t(unsigned int, fc->max_pages_limit,
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 271ae90a9bb7..36ed092227fa 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -181,6 +181,9 @@
  *  - add FUSE_OPEN_KILL_SUIDGID
  *  - extend fuse_setxattr_in, add FUSE_SETXATTR_EXT
  *  - add FUSE_SETXATTR_ACL_KILL_SGID
+ *
+ *  7.34
+ *  - add FUSE_SYNCFS
  */
 
 #ifndef _LINUX_FUSE_H
@@ -216,7 +219,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 33
+#define FUSE_KERNEL_MINOR_VERSION 34
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -509,6 +512,7 @@ enum fuse_opcode {
 	FUSE_COPY_FILE_RANGE	= 47,
 	FUSE_SETUPMAPPING	= 48,
 	FUSE_REMOVEMAPPING	= 49,
+	FUSE_SYNCFS		= 50,
 
 	/* CUSE specific operations */
 	CUSE_INIT		= 4096,
@@ -971,4 +975,8 @@ struct fuse_removemapping_one {
 #define FUSE_REMOVEMAPPING_MAX_ENTRY   \
 		(PAGE_SIZE / sizeof(struct fuse_removemapping_one))
 
+struct fuse_syncfs_in {
+	uint64_t	padding;
+};
+
 #endif /* _LINUX_FUSE_H */
-- 
2.26.3

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Re: [PATCH v4 5/5] virtiofs: propagate sync() to file server

[Greg Kurz]

On Thu, 20 May 2021 17:46:54 +0200
Greg Kurz <groug@kaod.org> wrote:

> Even if POSIX doesn't mandate it, linux users legitimately expect
> sync() to flush all data and metadata to physical storage when it
> is located on the same system. This isn't happening with virtiofs
> though : sync() inside the guest returns right away even though
> data still needs to be flushed from the host page cache.
> 
> This is easily demonstrated by doing the following in the guest:
> 
> $ dd if=/dev/zero of=/mnt/foo bs=1M count=5K ; strace -T -e sync sync
> 5120+0 records in
> 5120+0 records out
> 5368709120 bytes (5.4 GB, 5.0 GiB) copied, 5.22224 s, 1.0 GB/s
> sync()                                  = 0 <0.024068>
> +++ exited with 0 +++
> 
> and start the following in the host when the 'dd' command completes
> in the guest:
> 
> $ strace -T -e fsync /usr/bin/sync virtiofs/foo
> fsync(3)                                = 0 <10.371640>
> +++ exited with 0 +++
> 
> There are no good reasons not to honor the expected behavior of
> sync() actually : it gives an unrealistic impression that virtiofs
> is super fast and that data has safely landed on HW, which isn't
> the case obviously.
> 
> Implement a ->sync_fs() superblock operation that sends a new
> FUSE_SYNCFS request type for this purpose. Provision a 64-bit
> placeholder for possible future extensions. Since the file
> server cannot handle the wait == 0 case, we skip it to avoid a
> gratuitous roundtrip. Note that this is per-superblock : a
> FUSE_SYNCFS is send for the root mount and for each submount.
> 

s/send/sent

Miklos,

Great thanks for the quick feedback on these patches ! :)

Apart from the fact that nothing is sent for submounts as long
as we don't set SB_BORN on them, this patch doesn't really
depends on the previous ones.

If it looks good to you, maybe you can just merge it and
I'll re-post the fixes separately ?

Cheers,

--
Greg

> Like with FUSE_FSYNC and FUSE_FSYNCDIR, lack of support for
> FUSE_SYNCFS in the file server is treated as permanent success.
> This ensures compatibility with older file servers : the client
> will get the current behavior of sync() not being propagated to
> the file server.
> 
> Note that such an operation allows the file server to DoS sync().
> Since a typical FUSE file server is an untrusted piece of software
> running in userspace, this is disabled by default.  Only enable it
> with virtiofs for now since virtiofsd is supposedly trusted by the
> guest kernel.
> 
> Reported-by: Robert Krawitz <rlk@redhat.com>
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  fs/fuse/fuse_i.h          |  3 +++
>  fs/fuse/inode.c           | 40 +++++++++++++++++++++++++++++++++++++++
>  fs/fuse/virtio_fs.c       |  1 +
>  include/uapi/linux/fuse.h | 10 +++++++++-
>  4 files changed, 53 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index e2f5c8617e0d..01d9283261af 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -761,6 +761,9 @@ struct fuse_conn {
>  	/* Auto-mount submounts announced by the server */
>  	unsigned int auto_submounts:1;
>  
> +	/* Propagate syncfs() to server */
> +	unsigned int sync_fs:1;
> +
>  	/** The number of requests waiting for completion */
>  	atomic_t num_waiting;
>  
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 123b53d1c3c6..96b00253f766 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -506,6 +506,45 @@ static int fuse_statfs(struct dentry *dentry, struct kstatfs *buf)
>  	return err;
>  }
>  
> +static int fuse_sync_fs(struct super_block *sb, int wait)
> +{
> +	struct fuse_mount *fm = get_fuse_mount_super(sb);
> +	struct fuse_conn *fc = fm->fc;
> +	struct fuse_syncfs_in inarg;
> +	FUSE_ARGS(args);
> +	int err;
> +
> +	/*
> +	 * Userspace cannot handle the wait == 0 case. Avoid a
> +	 * gratuitous roundtrip.
> +	 */
> +	if (!wait)
> +		return 0;
> +
> +	/* The filesystem is being unmounted. Nothing to do. */
> +	if (!sb->s_root)
> +		return 0;
> +
> +	if (!fc->sync_fs)
> +		return 0;
> +
> +	memset(&inarg, 0, sizeof(inarg));
> +	args.in_numargs = 1;
> +	args.in_args[0].size = sizeof(inarg);
> +	args.in_args[0].value = &inarg;
> +	args.opcode = FUSE_SYNCFS;
> +	args.nodeid = get_node_id(sb->s_root->d_inode);
> +	args.out_numargs = 0;
> +
> +	err = fuse_simple_request(fm, &args);
> +	if (err == -ENOSYS) {
> +		fc->sync_fs = 0;
> +		err = 0;
> +	}
> +
> +	return err;
> +}
> +
>  enum {
>  	OPT_SOURCE,
>  	OPT_SUBTYPE,
> @@ -909,6 +948,7 @@ static const struct super_operations fuse_super_operations = {
>  	.put_super	= fuse_put_super,
>  	.umount_begin	= fuse_umount_begin,
>  	.statfs		= fuse_statfs,
> +	.sync_fs	= fuse_sync_fs,
>  	.show_options	= fuse_show_options,
>  };
>  
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 8962cd033016..f649a47efb68 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -1455,6 +1455,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
>  	fc->release = fuse_free_conn;
>  	fc->delete_stale = true;
>  	fc->auto_submounts = true;
> +	fc->sync_fs = true;
>  
>  	/* Tell FUSE to split requests that exceed the virtqueue's size */
>  	fc->max_pages_limit = min_t(unsigned int, fc->max_pages_limit,
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index 271ae90a9bb7..36ed092227fa 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -181,6 +181,9 @@
>   *  - add FUSE_OPEN_KILL_SUIDGID
>   *  - extend fuse_setxattr_in, add FUSE_SETXATTR_EXT
>   *  - add FUSE_SETXATTR_ACL_KILL_SGID
> + *
> + *  7.34
> + *  - add FUSE_SYNCFS
>   */
>  
>  #ifndef _LINUX_FUSE_H
> @@ -216,7 +219,7 @@
>  #define FUSE_KERNEL_VERSION 7
>  
>  /** Minor version number of this interface */
> -#define FUSE_KERNEL_MINOR_VERSION 33
> +#define FUSE_KERNEL_MINOR_VERSION 34
>  
>  /** The node ID of the root inode */
>  #define FUSE_ROOT_ID 1
> @@ -509,6 +512,7 @@ enum fuse_opcode {
>  	FUSE_COPY_FILE_RANGE	= 47,
>  	FUSE_SETUPMAPPING	= 48,
>  	FUSE_REMOVEMAPPING	= 49,
> +	FUSE_SYNCFS		= 50,
>  
>  	/* CUSE specific operations */
>  	CUSE_INIT		= 4096,
> @@ -971,4 +975,8 @@ struct fuse_removemapping_one {
>  #define FUSE_REMOVEMAPPING_MAX_ENTRY   \
>  		(PAGE_SIZE / sizeof(struct fuse_removemapping_one))
>  
> +struct fuse_syncfs_in {
> +	uint64_t	padding;
> +};
> +
>  #endif /* _LINUX_FUSE_H */

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Re: [PATCH v4 5/5] virtiofs: propagate sync() to file server

[Vivek Goyal]

On Sun, Aug 15, 2021 at 05:14:06PM +0300, Amir Goldstein wrote:
> Hi Greg,
> 
> Sorry for the late reply, I have some questions about this change...
> 
> On Fri, May 21, 2021 at 9:12 AM Greg Kurz <groug@kaod.org> wrote:
> >
> > Even if POSIX doesn't mandate it, linux users legitimately expect
> > sync() to flush all data and metadata to physical storage when it
> > is located on the same system. This isn't happening with virtiofs
> > though : sync() inside the guest returns right away even though
> > data still needs to be flushed from the host page cache.
> >
> > This is easily demonstrated by doing the following in the guest:
> >
> > $ dd if=/dev/zero of=/mnt/foo bs=1M count=5K ; strace -T -e sync sync
> > 5120+0 records in
> > 5120+0 records out
> > 5368709120 bytes (5.4 GB, 5.0 GiB) copied, 5.22224 s, 1.0 GB/s
> > sync()                                  = 0 <0.024068>
> > +++ exited with 0 +++
> >
> > and start the following in the host when the 'dd' command completes
> > in the guest:
> >
> > $ strace -T -e fsync /usr/bin/sync virtiofs/foo
> > fsync(3)                                = 0 <10.371640>
> > +++ exited with 0 +++
> >
> > There are no good reasons not to honor the expected behavior of
> > sync() actually : it gives an unrealistic impression that virtiofs
> > is super fast and that data has safely landed on HW, which isn't
> > the case obviously.
> >
> > Implement a ->sync_fs() superblock operation that sends a new
> > FUSE_SYNCFS request type for this purpose. Provision a 64-bit
> > placeholder for possible future extensions. Since the file
> > server cannot handle the wait == 0 case, we skip it to avoid a
> > gratuitous roundtrip. Note that this is per-superblock : a
> > FUSE_SYNCFS is send for the root mount and for each submount.
> >
> > Like with FUSE_FSYNC and FUSE_FSYNCDIR, lack of support for
> > FUSE_SYNCFS in the file server is treated as permanent success.
> > This ensures compatibility with older file servers : the client
> > will get the current behavior of sync() not being propagated to
> > the file server.
> 
> I wonder - even if the server does not support SYNCFS or if the kernel
> does not trust the server with SYNCFS, fuse_sync_fs() can wait
> until all pending requests up to this call have been completed, either
> before or after submitting the SYNCFS request. No?

> 
> Does virtiofsd track all requests prior to SYNCFS request to make
> sure that they were executed on the host filesystem before calling
> syncfs() on the host filesystem?

Hi Amir,

I don't think virtiofsd has any such notion. I would think, that
client should make sure all pending writes have completed and
then send SYNCFS request.

Looking at the sync_filesystem(), I am assuming vfs will take care
of flushing out all dirty pages and then call ->sync_fs.

Having said that, I think fuse queues the writeback request internally
and signals completion of writeback to mm(end_page_writeback()). And
that's why fuse_fsync() has notion of waiting for all pending
writes to finish on an inode (fuse_sync_writes()).

So I think you have raised a good point. That is if there are pending
writes at the time of syncfs(), we don't seem to have a notion of
first waiting for all these writes to finish before we send
FUSE_SYNCFS request to server. 

In case of virtiofs, we could probably move away from the notion
of ending writeback immediately. IIUC, this was needed for regular
fuse where we wanted to make sure rouge/malfunctining fuse server
could not affect processes on system which are not dealing with
fuse. But in case of virtiofs, guest is trusting file server. I
had tried to get rid of this for virtiofs but ran into some
other issues which I could not resolve easily at the time and
then I got distracted in other things.

Anyway, irrespective of that, we probably need a way to flush
out all pending writes with fuse and then send FUSE_SYNCFS. (And
lost make sure writes coming after call to fuse_sync_fs(), continue
to be queued and we don't livelock.

BTW, in the context of virtiofs, this probably is problem only with
mmaped writes. otherwise cache=auto and cache=none are basically
writethrough caches. So write is sent to server immediately. So there
is nothing to be written back when syncfs() comes along. But mmaped()
writes are different and even with cache=auto there can be dirty pages.
(cache=none does not support mmap() at all).

> 
> I am not familiar enough with FUSE internals so there may already
> be a mechanism to track/wait for all pending requests?

fuse_sync_writes() does it for inode. I am not aware of anything
which can do it for the whole filesystem (all the inodes).

> 
> >
> > Note that such an operation allows the file server to DoS sync().
> > Since a typical FUSE file server is an untrusted piece of software
> > running in userspace, this is disabled by default.  Only enable it
> > with virtiofs for now since virtiofsd is supposedly trusted by the
> > guest kernel.
> 
> Isn't there already a similar risk of DoS to sync() from the ability of any
> untrusted (or malfunctioning) server to block writes?

I think fuse has some safeguards for this. Fuse signals completion
of writeback immediately so that vfs/mm/fs does not blocking trying
to writeback and if server is not finishing WRITES fast enough, the
there will be enough dirty pages in bdi that it will create back
pressure and block process dirtying pages.

Thanks
Vivek

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Re: [PATCH v4 5/5] virtiofs: propagate sync() to file server

[Vivek Goyal]

On Mon, Aug 16, 2021 at 09:57:08PM +0300, Amir Goldstein wrote:
> On Mon, Aug 16, 2021 at 6:29 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > On Sun, Aug 15, 2021 at 05:14:06PM +0300, Amir Goldstein wrote:
> > > Hi Greg,
> > >
> > > Sorry for the late reply, I have some questions about this change...
> > >
> > > On Fri, May 21, 2021 at 9:12 AM Greg Kurz <groug@kaod.org> wrote:
> > > >
> > > > Even if POSIX doesn't mandate it, linux users legitimately expect
> > > > sync() to flush all data and metadata to physical storage when it
> > > > is located on the same system. This isn't happening with virtiofs
> > > > though : sync() inside the guest returns right away even though
> > > > data still needs to be flushed from the host page cache.
> > > >
> > > > This is easily demonstrated by doing the following in the guest:
> > > >
> > > > $ dd if=/dev/zero of=/mnt/foo bs=1M count=5K ; strace -T -e sync sync
> > > > 5120+0 records in
> > > > 5120+0 records out
> > > > 5368709120 bytes (5.4 GB, 5.0 GiB) copied, 5.22224 s, 1.0 GB/s
> > > > sync()                                  = 0 <0.024068>
> > > > +++ exited with 0 +++
> > > >
> > > > and start the following in the host when the 'dd' command completes
> > > > in the guest:
> > > >
> > > > $ strace -T -e fsync /usr/bin/sync virtiofs/foo
> > > > fsync(3)                                = 0 <10.371640>
> > > > +++ exited with 0 +++
> > > >
> > > > There are no good reasons not to honor the expected behavior of
> > > > sync() actually : it gives an unrealistic impression that virtiofs
> > > > is super fast and that data has safely landed on HW, which isn't
> > > > the case obviously.
> > > >
> > > > Implement a ->sync_fs() superblock operation that sends a new
> > > > FUSE_SYNCFS request type for this purpose. Provision a 64-bit
> > > > placeholder for possible future extensions. Since the file
> > > > server cannot handle the wait == 0 case, we skip it to avoid a
> > > > gratuitous roundtrip. Note that this is per-superblock : a
> > > > FUSE_SYNCFS is send for the root mount and for each submount.
> > > >
> > > > Like with FUSE_FSYNC and FUSE_FSYNCDIR, lack of support for
> > > > FUSE_SYNCFS in the file server is treated as permanent success.
> > > > This ensures compatibility with older file servers : the client
> > > > will get the current behavior of sync() not being propagated to
> > > > the file server.
> > >
> > > I wonder - even if the server does not support SYNCFS or if the kernel
> > > does not trust the server with SYNCFS, fuse_sync_fs() can wait
> > > until all pending requests up to this call have been completed, either
> > > before or after submitting the SYNCFS request. No?
> >
> > >
> > > Does virtiofsd track all requests prior to SYNCFS request to make
> > > sure that they were executed on the host filesystem before calling
> > > syncfs() on the host filesystem?
> >
> > Hi Amir,
> >
> > I don't think virtiofsd has any such notion. I would think, that
> > client should make sure all pending writes have completed and
> > then send SYNCFS request.
> >
> > Looking at the sync_filesystem(), I am assuming vfs will take care
> > of flushing out all dirty pages and then call ->sync_fs.
> >
> > Having said that, I think fuse queues the writeback request internally
> > and signals completion of writeback to mm(end_page_writeback()). And
> > that's why fuse_fsync() has notion of waiting for all pending
> > writes to finish on an inode (fuse_sync_writes()).
> >
> > So I think you have raised a good point. That is if there are pending
> > writes at the time of syncfs(), we don't seem to have a notion of
> > first waiting for all these writes to finish before we send
> > FUSE_SYNCFS request to server.
> 
> Maybe, but I was not referring to inode writeback requests.
> I had assumed that those were handled correctly.
> I was referring to pending metadata requests.
> 
> ->sync_fs() in local fs also takes care of flushing metadata
> (e.g. journal). I assumed that virtiofsd implements FUSE_SYNCFS
> request by calling syncfs() on host fs,

Yes virtiofsd calls syncfs() on host fs.

> but it is does that than
> there is no guarantee that all metadata requests have reached the
> host fs from virtiofs unless client or server take care of waiting
> for all pending metadata requests before issuing FUSE_SYNCFS.

We don't have any journal in virtiofs. In fact we don't seem to
cache any metadta. Except probably the case when "-o writeback" 
where we can trust local time stamps.

If "-o writeback" is not enabled, i am not sure what metadata
we will be caching that we will need to worry about. Do you have
something specific in mind. (Atleast from virtiofs point of view,
I can't seem to think what metadata we are caching which we need
to worry about).

Thanks
Vivek

> 
> But maybe I am missing something.
> 
> It might be worth mentioning that I did not find any sync_fs()
> commands that request to flush metadata caches on the server in
> NFS or SMB protocols either.
> 
> Thanks,
> Amir.
> 

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Re: [PATCH v4 5/5] virtiofs: propagate sync() to file server

[Vivek Goyal]

On Sat, Aug 28, 2021 at 05:21:39PM +0200, Miklos Szeredi wrote:
> On Mon, Aug 16, 2021 at 11:29:02AM -0400, Vivek Goyal wrote:
> > On Sun, Aug 15, 2021 at 05:14:06PM +0300, Amir Goldstein wrote:
> 
> > > I wonder - even if the server does not support SYNCFS or if the kernel
> > > does not trust the server with SYNCFS, fuse_sync_fs() can wait
> > > until all pending requests up to this call have been completed, either
> > > before or after submitting the SYNCFS request. No?
> > 
> > > 
> > > Does virtiofsd track all requests prior to SYNCFS request to make
> > > sure that they were executed on the host filesystem before calling
> > > syncfs() on the host filesystem?
> > 
> > Hi Amir,
> > 
> > I don't think virtiofsd has any such notion. I would think, that
> > client should make sure all pending writes have completed and
> > then send SYNCFS request.
> > 
> > Looking at the sync_filesystem(), I am assuming vfs will take care
> > of flushing out all dirty pages and then call ->sync_fs.
> > 
> > Having said that, I think fuse queues the writeback request internally
> > and signals completion of writeback to mm(end_page_writeback()). And
> > that's why fuse_fsync() has notion of waiting for all pending
> > writes to finish on an inode (fuse_sync_writes()).
> > 
> > So I think you have raised a good point. That is if there are pending
> > writes at the time of syncfs(), we don't seem to have a notion of
> > first waiting for all these writes to finish before we send
> > FUSE_SYNCFS request to server.
> 
> So here a proposed patch for fixing this.  Works by counting write requests
> initiated up till the syncfs call.  Since more than one syncfs can be in
> progress counts are kept in "buckets" in order to wait for the correct write
> requests in each instance.
> 
> I tried to make this lightweight, but the cacheline bounce due to the counter is
> still there, unfortunately.  fc->num_waiting also causes cacheline bouce, so I'm
> not going to optimize this (percpu counter?) until that one is also optimizied.
> 
> Not yet tested, and I'm not sure how to test this.
> 
> Comments?
> 
> Thanks,
> Miklos
> 
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 97f860cfc195..8d1d6e895534 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -389,6 +389,7 @@ struct fuse_writepage_args {
>  	struct list_head queue_entry;
>  	struct fuse_writepage_args *next;
>  	struct inode *inode;
> +	struct fuse_sync_bucket *bucket;
>  };
>  
>  static struct fuse_writepage_args *fuse_find_writeback(struct fuse_inode *fi,
> @@ -1608,6 +1609,9 @@ static void fuse_writepage_free(struct fuse_writepage_args *wpa)
>  	struct fuse_args_pages *ap = &wpa->ia.ap;
>  	int i;
>  
> +	if (wpa->bucket && atomic_dec_and_test(&wpa->bucket->num_writepages))

Hi Miklos,

Wondering why this wpa->bucket check is there. Isn't every wpa is associated
bucket.  So when do we run into situation when wpa->bucket = NULL.


> +		wake_up(&wpa->bucket->waitq);
> +
>  	for (i = 0; i < ap->num_pages; i++)
>  		__free_page(ap->pages[i]);
>  
> @@ -1871,6 +1875,19 @@ static struct fuse_writepage_args *fuse_writepage_args_alloc(void)
>  
>  }
>  
> +static void fuse_writepage_add_to_bucket(struct fuse_conn *fc,
> +					 struct fuse_writepage_args *wpa)
> +{
> +	if (!fc->sync_fs)
> +		return;
> +
> +	rcu_read_lock();
> +	do {
> +		wpa->bucket = rcu_dereference(fc->curr_bucket);
> +	} while (unlikely(!atomic_inc_not_zero(&wpa->bucket->num_writepages)));

So this loop is there because fuse_sync_fs() might be replacing
fc->curr_bucket. And we are fetching this pointer under rcu. So it is
possible that fuse_fs_sync() dropped its reference and that led to
->num_writepages 0 and we don't want to use this bucket.

What if fuse_sync_fs() dropped its reference but still there is another
wpa in progress and hence ->num_writepages is not zero. We still don't
want to use this bucket for new wpa, right?

> +	rcu_read_unlock();
> +}
> +
>  static int fuse_writepage_locked(struct page *page)
>  {
>  	struct address_space *mapping = page->mapping;
> @@ -1898,6 +1915,7 @@ static int fuse_writepage_locked(struct page *page)
>  	if (!wpa->ia.ff)
>  		goto err_nofile;
>  
> +	fuse_writepage_add_to_bucket(fc, wpa);
>  	fuse_write_args_fill(&wpa->ia, wpa->ia.ff, page_offset(page), 0);
>  
>  	copy_highpage(tmp_page, page);
> @@ -2148,6 +2166,8 @@ static int fuse_writepages_fill(struct page *page,
>  			__free_page(tmp_page);
>  			goto out_unlock;
>  		}
> +		fuse_writepage_add_to_bucket(fc, wpa);
> +
>  		data->max_pages = 1;
>  
>  		ap = &wpa->ia.ap;
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 07829ce78695..ee638e227bb3 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -515,6 +515,14 @@ struct fuse_fs_context {
>  	void **fudptr;
>  };
>  
> +struct fuse_sync_bucket {
> +	atomic_t num_writepages;
> +	union {
> +		wait_queue_head_t waitq;
> +		struct rcu_head rcu;
> +	};
> +};
> +
>  /**
>   * A Fuse connection.
>   *
> @@ -807,6 +815,9 @@ struct fuse_conn {
>  
>  	/** List of filesystems using this connection */
>  	struct list_head mounts;
> +
> +	/* New writepages go into this bucket */
> +	struct fuse_sync_bucket *curr_bucket;
>  };
>  
>  /*
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index b9beb39a4a18..524b2d128985 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -506,10 +506,24 @@ static int fuse_statfs(struct dentry *dentry, struct kstatfs *buf)
>  	return err;
>  }
>  
> +static struct fuse_sync_bucket *fuse_sync_bucket_alloc(void)
> +{
> +	struct fuse_sync_bucket *bucket;
> +
> +	bucket = kzalloc(sizeof(*bucket), GFP_KERNEL | __GFP_NOFAIL);
> +	if (bucket) {
> +		init_waitqueue_head(&bucket->waitq);
> +		/* Initial active count */
> +		atomic_set(&bucket->num_writepages, 1);
> +	}
> +	return bucket;
> +}
> +
>  static int fuse_sync_fs(struct super_block *sb, int wait)
>  {
>  	struct fuse_mount *fm = get_fuse_mount_super(sb);
>  	struct fuse_conn *fc = fm->fc;
> +	struct fuse_sync_bucket *bucket, *new_bucket;
>  	struct fuse_syncfs_in inarg;
>  	FUSE_ARGS(args);
>  	int err;
> @@ -528,6 +542,31 @@ static int fuse_sync_fs(struct super_block *sb, int wait)
>  	if (!fc->sync_fs)
>  		return 0;
>  
> +	new_bucket = fuse_sync_bucket_alloc();
> +	spin_lock(&fc->lock);
> +	bucket = fc->curr_bucket;
> +	if (atomic_read(&bucket->num_writepages) != 0) {
> +		/* One more for count completion of old bucket */
> +		atomic_inc(&new_bucket->num_writepages);
> +		rcu_assign_pointer(fc->curr_bucket, new_bucket);
> +		/* Drop initially added active count */
> +		atomic_dec(&bucket->num_writepages);
> +		spin_unlock(&fc->lock);
> +
> +		wait_event(bucket->waitq, atomic_read(&bucket->num_writepages) == 0);
> +		/*
> +		 * Drop count on new bucket, possibly resulting in a completion
> +		 * if more than one syncfs is going on
> +		 */
> +		if (atomic_dec_and_test(&new_bucket->num_writepages))
> +			wake_up(&new_bucket->waitq);
> +		kfree_rcu(bucket, rcu);
> +	} else {
> +		spin_unlock(&fc->lock);
> +		/* Free unused */
> +		kfree(new_bucket);
When can we run into the situation when fc->curr_bucket is num_writepages
== 0. When install a bucket it has count 1. And only time it can go to
0 is when we have dropped the initial reference. And initial reference
can be dropped only after removing bucket from fc->curr_bucket.

IOW, we don't drop initial reference on a bucket if it is in
fc->curr_bucket. And that mean anything installed fc->curr_bucket should
not ever have a reference count of 0. What am I missing.

Thanks
Vivek

> +	}

> +
>  	memset(&inarg, 0, sizeof(inarg));
>  	args.in_numargs = 1;
>  	args.in_args[0].size = sizeof(inarg);
> @@ -770,6 +809,7 @@ void fuse_conn_put(struct fuse_conn *fc)
>  			fiq->ops->release(fiq);
>  		put_pid_ns(fc->pid_ns);
>  		put_user_ns(fc->user_ns);
> +		kfree_rcu(fc->curr_bucket, rcu);
>  		fc->release(fc);
>  	}
>  }
> @@ -1418,6 +1458,7 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx)
>  	if (sb->s_flags & SB_MANDLOCK)
>  		goto err;
>  
> +	fc->curr_bucket = fuse_sync_bucket_alloc();
>  	fuse_sb_defaults(sb);
>  
>  	if (ctx->is_bdev) {
> 

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