Re: [PATCH V12 04/14] xfs: Check for extent overflow when adding/removing dir entries

Chandan Babu R
2021-01-08T04:56:36Z

On Thu, 07 Jan 2021 17:17:13 -0800, Darrick J. Wong wrote:
> On Mon, Jan 04, 2021 at 04:01:10PM +0530, Chandan Babu R wrote:
> > Directory entry addition can cause the following,
> > 1. Data block can be added/removed.
> > A new extent can cause extent count to increase by 1.
> > 2. Free disk block can be added/removed.
> > Same behaviour as described above for Data block.
> > 3. Dabtree blocks.
> > XFS_DA_NODE_MAXDEPTH blocks can be added. Each of these
> > can be new extents. Hence extent count can increase by
> > XFS_DA_NODE_MAXDEPTH.
> >
> > Directory entry remove and rename (applicable only to the source
> > directory entry) operations are handled specially to allow them to
> > succeed in low extent count availability scenarios
> > i.e. xfs_bmap_del_extent_real() will now return -ENOSPC when a possible
> > extent count overflow is detected. -ENOSPC is already handled by higher
> > layers of XFS by letting,
> > 1. Empty Data/Free space index blocks to linger around until a future
> > remove operation frees them.
> > 2. Dabtree blocks would be swapped with the last block in the leaf space
> > followed by unmapping of the new last block.
> >
> > Also, Extent overflow check is performed for the target directory entry
> > of the rename operation only when the entry does not exist and a
> > non-zero space reservation is obtained successfully.
> >
> > Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
> > ---
> > fs/xfs/libxfs/xfs_bmap.c | 15 ++++++++++++
> > fs/xfs/libxfs/xfs_inode_fork.h | 13 ++++++++++
> > fs/xfs/xfs_inode.c | 45 ++++++++++++++++++++++++++++++++++
> > fs/xfs/xfs_symlink.c | 5 ++++
> > 4 files changed, 78 insertions(+)
> >
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index 32aeacf6f055..5fd804534e67 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -5151,6 +5151,21 @@ xfs_bmap_del_extent_real(
> > /*
> > * Deleting the middle of the extent.
> > */
> > +
> > + /*
> > + * For directories, -ENOSPC will be handled by higher layers of
> > + * XFS by letting the corresponding empty Data/Free blocks to
> > + * linger around until a future remove operation. Dabtree blocks
> > + * would be swapped with the last block in the leaf space and
> > + * then the new last block will be unmapped.
> > + */
> > + if (S_ISDIR(VFS_I(ip)->i_mode) &&
> > + whichfork == XFS_DATA_FORK &&
> > + xfs_iext_count_may_overflow(ip, whichfork, 1)) {
> > + error = -ENOSPC;
> > + goto done;
>
> Hmm... it strikes me as a little odd that we're checking file mode and
> fork type in the middle of the bmap code. However, I think it's the
> case that the only place where anyone would punch a hole in the /middle/
> of an extent is xattr trees and regular files, right? And both of those
> cases are checked before we end up in the bmap code, right?

Yes, your observation is correct. I will remove the file mode and fork type
checks.

>
> So we only really need this check to prevent extent count overflows when
> removing dirents from directories, like the comment says, and only
> because directories don't have a hard requirement that the bunmapi
> succeeds. And I think this logic covers xfs_remove too? That's a bit
> subtle, but as there's no extent count check in that function, there's
> not much to attach a comment to... :)

Yes, To provide more clarity, I should replace the above comment with
following,

/*
* -ENOSPC is returned since a directory entry remove operation must not fail
* due to low extent count availability. -ENOSPC will be handled by higher
* layers of XFS by letting the corresponding empty Data/Free blocks to linger
* around until a future remove operation. Dabtree blocks would be swapped with
* the last block in the leaf space and then the new last block will be
* unmapped.
*
* The above logic also applies to the source directory entry of a rename
* operation.
*/

>
> Hm. I think I'd like xfs_rename to get a brief comment that we're
> protected from extent count overflows in xfs_remove() by virtue of this
> "leave the dir block in place if we ENOSPC" capability:
>
> /*
> * NOTE: We don't need to check for extent overflows here
> * because the dir removename code will leave the dir block
> * in place if the extent count would overflow.
> */
> error = xfs_dir_removename(...);

Sure, I will add that.

>
> Do xattr trees also have the same ability? I think they do, at least
> for the dabtree part...?

The following code snippet from xfs_da_shrink_inode() does the special casing
only for the data fork i.e. for blocks holding directory entries.

for (;;) {
/*
* Remove extents. If we get ENOSPC for a dir we have to move
* the last block to the place we want to kill.
*/
error = xfs_bunmapi(tp, dp, dead_blkno, count,
xfs_bmapi_aflag(w), 0, &done);
if (error == -ENOSPC) {
if (w != XFS_DATA_FORK)
break;
error = xfs_da3_swap_lastblock(args, &dead_blkno,
&dead_buf);

So this facility is available only for directory entries.

Hence for xattrs, if we ever reach the extent count limit, the only way out is
to delete the corresponding file.

>
> I think I would've split this patch into three pieces:
>
> - create, link, and symlink in one patch (adding dirents),
> - the xfs_bmap_del_extent_real change and a comment for xfs_remove
> (removing dirents)
> - all the xfs_rename changes (adding and removing dirents)
>
> Though I dunno, this series is already 14 patches, and the part that I
> care most about is not leaving that subtlety in xfs_remove(). :)

I think you are right about that. I will split this patch according to what
you have mentioned above.

>
> Other than that, I follow the logic in this patch and will give it a
> testrun tonight.

Thank you!

>
> --D
>
> > + }
> > +
> > old = got;
> >
> > got.br_blockcount = del->br_startoff - got.br_startoff;
> > diff --git a/fs/xfs/libxfs/xfs_inode_fork.h b/fs/xfs/libxfs/xfs_inode_fork.h
> > index bcac769a7df6..ea1a9dd8a763 100644
> > --- a/fs/xfs/libxfs/xfs_inode_fork.h
> > +++ b/fs/xfs/libxfs/xfs_inode_fork.h
> > @@ -47,6 +47,19 @@ struct xfs_ifork {
> > */
> > #define XFS_IEXT_PUNCH_HOLE_CNT (1)
> >
> > +/*
> > + * Directory entry addition can cause the following,
> > + * 1. Data block can be added/removed.
> > + * A new extent can cause extent count to increase by 1.
> > + * 2. Free disk block can be added/removed.
> > + * Same behaviour as described above for Data block.
> > + * 3. Dabtree blocks.
> > + * XFS_DA_NODE_MAXDEPTH blocks can be added. Each of these can be new
> > + * extents. Hence extent count can increase by XFS_DA_NODE_MAXDEPTH.
> > + */
> > +#define XFS_IEXT_DIR_MANIP_CNT(mp) \
> > + ((XFS_DA_NODE_MAXDEPTH + 1 + 1) * (mp)->m_dir_geo->fsbcount)
> > +
> > /*
> > * Fork handling.
> > */
> > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > index b7352bc4c815..0db21368c7e1 100644
> > --- a/fs/xfs/xfs_inode.c
> > +++ b/fs/xfs/xfs_inode.c
> > @@ -1042,6 +1042,11 @@ xfs_create(
> > if (error)
> > goto out_trans_cancel;
> >
> > + error = xfs_iext_count_may_overflow(dp, XFS_DATA_FORK,
> > + XFS_IEXT_DIR_MANIP_CNT(mp));
> > + if (error)
> > + goto out_trans_cancel;
> > +
> > /*
> > * A newly created regular or special file just has one directory
> > * entry pointing to them, but a directory also the "." entry
> > @@ -1258,6 +1263,11 @@ xfs_link(
> > xfs_trans_ijoin(tp, sip, XFS_ILOCK_EXCL);
> > xfs_trans_ijoin(tp, tdp, XFS_ILOCK_EXCL);
> >
> > + error = xfs_iext_count_may_overflow(tdp, XFS_DATA_FORK,
> > + XFS_IEXT_DIR_MANIP_CNT(mp));
> > + if (error)
> > + goto error_return;
> > +
> > /*
> > * If we are using project inheritance, we only allow hard link
> > * creation in our tree when the project IDs are the same; else
> > @@ -3106,6 +3116,35 @@ xfs_rename(
> > /*
> > * Check for expected errors before we dirty the transaction
> > * so we can return an error without a transaction abort.
> > + *
> > + * Extent count overflow check:
> > + *
> > + * From the perspective of src_dp, a rename operation is essentially a
> > + * directory entry remove operation. Hence the only place where we check
> > + * for extent count overflow for src_dp is in
> > + * xfs_bmap_del_extent_real(). xfs_bmap_del_extent_real() returns
> > + * -ENOSPC when it detects a possible extent count overflow and in
> > + * response, the higher layers of directory handling code do the
> > + * following:
> > + * 1. Data/Free blocks: XFS lets these blocks linger around until a
> > + * future remove operation removes them.
> > + * 2. Dabtree blocks: XFS swaps the blocks with the last block in the
> > + * Leaf space and unmaps the last block.
> > + *
> > + * For target_dp, there are two cases depending on whether the
> > + * destination directory entry exists or not.
> > + *
> > + * When destination directory entry does not exist (i.e. target_ip ==
> > + * NULL), extent count overflow check is performed only when transaction
> > + * has a non-zero sized space reservation associated with it. With a
> > + * zero-sized space reservation, XFS allows a rename operation to
> > + * continue only when the directory has sufficient free space in its
> > + * data/leaf/free space blocks to hold the new entry.
> > + *
> > + * When destination directory entry exists (i.e. target_ip != NULL), all
> > + * we need to do is change the inode number associated with the already
> > + * existing entry. Hence there is no need to perform an extent count
> > + * overflow check.
> > */
> > if (target_ip == NULL) {
> > /*
> > @@ -3116,6 +3155,12 @@ xfs_rename(
> > error = xfs_dir_canenter(tp, target_dp, target_name);
> > if (error)
> > goto out_trans_cancel;
> > + } else {
> > + error = xfs_iext_count_may_overflow(target_dp,
> > + XFS_DATA_FORK,
> > + XFS_IEXT_DIR_MANIP_CNT(mp));
> > + if (error)
> > + goto out_trans_cancel;
> > }
> > } else {
> > /*
> > diff --git a/fs/xfs/xfs_symlink.c b/fs/xfs/xfs_symlink.c
> > index 1f43fd7f3209..0b8136a32484 100644
> > --- a/fs/xfs/xfs_symlink.c
> > +++ b/fs/xfs/xfs_symlink.c
> > @@ -220,6 +220,11 @@ xfs_symlink(
> > if (error)
> > goto out_trans_cancel;
> >
> > + error = xfs_iext_count_may_overflow(dp, XFS_DATA_FORK,
> > + XFS_IEXT_DIR_MANIP_CNT(mp));
> > + if (error)
> > + goto out_trans_cancel;
> > +
> > /*
> > * Allocate an inode for the symlink.
> > */
>

--
chandan