Re: [PATCH 0/5] Enable per-file/directory DAX operations

Ira Weiny
2019-10-25T20:49:32Z

On Fri, Oct 25, 2019 at 11:36:03AM +1100, Dave Chinner wrote:
> On Fri, Oct 25, 2019 at 02:29:04AM +0300, Boaz Harrosh wrote:
> > On 25/10/2019 00:35, Dave Chinner wrote:
>
> If something like a find or backup program brings the inode into
> cache, the app may not even get the behaviour it wants, and it can't
> change it until the inode is evicted from cache, which may be never.

Why would this be never?

> Nobody wants implicit/random/uncontrollable/unchangeable behaviour
> like this.

I'm thinking this could work with a bit of effort on the users part. While the
behavior does have a bit of uncertainty, I feel like there has to be a way to
get the inode to drop from the cache when a final iput() happens on the inode.

Admin programs should not leave files open forever, without the users knowing
about it. So I don't understand why the inode could not be evicted from the
cache if the FS knew that this change had been made and the inode needs to be
"re-loaded". See below...

>
> > (And never change the flag on the fly)
> > (Just brain storming here)
>
> We went over all this ground when we disabled the flag in the first
> place. We disabled the flag because we couldn't come up with a sane
> way to flip the ops vector short of tracking the number of aops
> calls in progress at any given time. i.e. reference counting the
> aops structure, but that's hard to do with a const ops structure,
> and so it got disabled rather than allowing users to crash
> kernels....

Agreed. We can't change the a_ops without some guarantee that no one is using
the file. Which means we need all fds to close and a final iput(). I thought
that would mean an eviction of the inode and a subsequent reload.

Yesterday I coded up the following (applies on top of this series) but I can't
seem to get it to work because I believe xfs is keeping a reference on the
inode. What am I missing? I think if I could get xfs to recognize that the
inode needs to be cleared from it's cache this would work, with some caveats.

Currently this works if I remount the fs or if I use <procfs>/drop_caches like
Boaz mentioned.

Isn't there a way to get xfs to do that on it's own?

Ira

From 7762afd95a3e21a782dffd2fd8e13ae4a23b5e4a Mon Sep 17 00:00:00 2001
From: Ira Weiny <ira.weiny@intel.com>
Date: Fri, 25 Oct 2019 13:32:07 -0700
Subject: [PATCH] squash: Allow phys change on any length file

delay the changing of the effective bit to when the inode is re-read
into the cache.

Currently a work in Progress because xfs seems to cache the inodes as
well and I'm not sure how to get xfs to release it's reference.
---
fs/xfs/xfs_ioctl.c | 18 +++++++-----------
include/linux/fs.h | 5 ++++-
2 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 89cf47ef273e..4d730d5781d9 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1233,10 +1233,13 @@ xfs_diflags_to_linux(
inode->i_flags |= S_NOATIME;
else
inode->i_flags &= ~S_NOATIME;
- if (xflags & FS_XFLAG_DAX)
- inode->i_flags |= S_DAX;
- else
- inode->i_flags &= ~S_DAX;
+ /* NOTE: we do not allow the physical DAX flag to immediately change
+ * the effective IS_DAX() flag tell the VFS layer to remove the inode
+ * from the cache on the final iput() to force recreation on the next
+ * 'fresh' open */
+ if (((xflags & FS_XFLAG_DAX) && !IS_DAX(inode)) ||
+ (!(xflags & FS_XFLAG_DAX) && IS_DAX(inode)))
+ inode->i_flags |= S_REVALIDATE;
}

static int
@@ -1320,13 +1323,6 @@ xfs_ioctl_setattr_dax_invalidate(
/* lock, flush and invalidate mapping in preparation for flag change */
xfs_ilock(ip, XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL);

- /* File size must be zero to avoid races with asynchronous page
- * faults */
- if (i_size_read(inode) > 0) {
- error = -EINVAL;
- goto out_unlock;
- }
-
error = filemap_write_and_wait(inode->i_mapping);
if (error)
goto out_unlock;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 0b4d8fc79e0f..4e9b7bf53c86 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1998,6 +1998,7 @@ struct super_operations {
#define S_ENCRYPTED 16384 /* Encrypted file (using fs/crypto/) */
#define S_CASEFOLD 32768 /* Casefolded file */
#define S_VERITY 65536 /* Verity file (using fs/verity/) */
+#define S_REVALIDATE 131072 /* Drop inode from cache on final put */

/*
* Note that nosuid etc flags are inode-specific: setting some file-system
@@ -2040,6 +2041,7 @@ static inline bool sb_rdonly(const struct super_block *sb) { return sb->s_flags
#define IS_ENCRYPTED(inode) ((inode)->i_flags & S_ENCRYPTED)
#define IS_CASEFOLDED(inode) ((inode)->i_flags & S_CASEFOLD)
#define IS_VERITY(inode) ((inode)->i_flags & S_VERITY)
+#define IS_REVALIDATE(inode) ((inode)->i_flags & S_REVALIDATE)

#define IS_WHITEOUT(inode) (S_ISCHR(inode->i_mode) && \
(inode)->i_rdev == WHITEOUT_DEV)
@@ -3027,7 +3029,8 @@ extern int inode_needs_sync(struct inode *inode);
extern int generic_delete_inode(struct inode *inode);
static inline int generic_drop_inode(struct inode *inode)
{
- return !inode->i_nlink || inode_unhashed(inode);
+ return !inode->i_nlink || inode_unhashed(inode) ||
+ IS_REVALIDATE(inode);
}

extern struct inode *ilookup5_nowait(struct super_block *sb,
--
2.20.1