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xfs: recheck reflink / dirty page status before freeing CoW reservations 

Eryu Guan reported seeing occasional hangs when running generic/269 with
a new fsstress that supports clonerange/deduperange.  The cause of this
hang is an infinite loop when we convert the CoW fork extents from
unwritten to real just prior to writing the pages out; the infinite
loop happens because there's nothing in the CoW fork to convert, and so
it spins forever.

The fundamental issue here is that when we go to perform these CoW fork
conversions, we're supposed to have an extent waiting for us, but the
low space CoW reaper has snuck in and blown them away!  There are four
conditions that can dissuade the reaper from touching our file -- no
reflink iflag; dirty page cache; writeback in progress; or directio in
progress.  We check the four conditions prior to taking the locks, but
we neglect to recheck them once we have the locks, which is how we end
up whacking the writeback that's in progress.

Therefore, refactor the four checks into a helper function and call it
once again once we have the locks to make sure we really want to reap
the inode.  While we're at it, add an ASSERT for this weird condition so
that we'll fail noisily if we ever screw this up again.

Reported-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Tested-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
This commit is contained in:
Darrick J. Wong 2018-01-16 19:03:59 -08:00
parent a5f460b168
commit be78ff0e72
2 changed files with 52 additions and 23 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
fs/xfs/libxfs/xfs_bmap.c
View File

@ -4304,8 +4304,16 @@ xfs_bmapi_write(
while (bno < end && n < *nmap) {
bool need_alloc = false, wasdelay = false;
/* in hole or beyoned EOF? */
/* in hole or beyond EOF? */
if (eof || bma.got.br_startoff > bno) {
/*
* CoW fork conversions should /never/ hit EOF or
* holes. There should always be something for us
* to work on.
*/
ASSERT(!((flags & XFS_BMAPI_CONVERT) &&
(flags & XFS_BMAPI_COWFORK)));
if (flags & XFS_BMAPI_DELALLOC) {
/*
* For the COW fork we can reasonably get a

65
fs/xfs/xfs_icache.c
View File

@ -1654,6 +1654,39 @@ xfs_inode_clear_eofblocks_tag(
trace_xfs_perag_clear_eofblocks, XFS_ICI_EOFBLOCKS_TAG);
}
/*
* Set ourselves up to free CoW blocks from this file. If it's already clean
* then we can bail out quickly, but otherwise we must back off if the file
* is undergoing some kind of write.
*/
static bool
xfs_prep_free_cowblocks(
struct xfs_inode *ip,
struct xfs_ifork *ifp)
{
/*
* Just clear the tag if we have an empty cow fork or none at all. It's
* possible the inode was fully unshared since it was originally tagged.
*/
if (!xfs_is_reflink_inode(ip) || !ifp->if_bytes) {
trace_xfs_inode_free_cowblocks_invalid(ip);
xfs_inode_clear_cowblocks_tag(ip);
return false;
}
/*
* If the mapping is dirty or under writeback we cannot touch the
* CoW fork. Leave it alone if we're in the midst of a directio.
*/
if ((VFS_I(ip)->i_state & I_DIRTY_PAGES) ||
mapping_tagged(VFS_I(ip)->i_mapping, PAGECACHE_TAG_DIRTY) ||
mapping_tagged(VFS_I(ip)->i_mapping, PAGECACHE_TAG_WRITEBACK) ||
atomic_read(&VFS_I(ip)->i_dio_count))
return false;
return true;
}
/*
* Automatic CoW Reservation Freeing
*
@ -1672,29 +1705,12 @@ xfs_inode_free_cowblocks(
int flags,
void *args)
{
int ret;
struct xfs_eofblocks *eofb = args;
int match;
struct xfs_eofblocks *eofb = args;
struct xfs_ifork *ifp = XFS_IFORK_PTR(ip, XFS_COW_FORK);
int match;
int ret = 0;
/*
* Just clear the tag if we have an empty cow fork or none at all. It's
* possible the inode was fully unshared since it was originally tagged.
*/
if (!xfs_is_reflink_inode(ip) || !ifp->if_bytes) {
trace_xfs_inode_free_cowblocks_invalid(ip);
xfs_inode_clear_cowblocks_tag(ip);
return 0;
}
/*
* If the mapping is dirty or under writeback we cannot touch the
* CoW fork. Leave it alone if we're in the midst of a directio.
*/
if ((VFS_I(ip)->i_state & I_DIRTY_PAGES) ||
mapping_tagged(VFS_I(ip)->i_mapping, PAGECACHE_TAG_DIRTY) ||
mapping_tagged(VFS_I(ip)->i_mapping, PAGECACHE_TAG_WRITEBACK) ||
atomic_read(&VFS_I(ip)->i_dio_count))
if (!xfs_prep_free_cowblocks(ip, ifp))
return 0;
if (eofb) {
@ -1715,7 +1731,12 @@ xfs_inode_free_cowblocks(
xfs_ilock(ip, XFS_IOLOCK_EXCL);
xfs_ilock(ip, XFS_MMAPLOCK_EXCL);
ret = xfs_reflink_cancel_cow_range(ip, 0, NULLFILEOFF, false);
/*
* Check again, nobody else should be able to dirty blocks or change
* the reflink iflag now that we have the first two locks held.
*/
if (xfs_prep_free_cowblocks(ip, ifp))
ret = xfs_reflink_cancel_cow_range(ip, 0, NULLFILEOFF, false);
xfs_iunlock(ip, XFS_MMAPLOCK_EXCL);
xfs_iunlock(ip, XFS_IOLOCK_EXCL);