xfs: resample the data fork mapping after cycling ILOCK

xfs_reflink_fill_{cow_hole,delalloc} are both presented with an inode,
a data fork mapping, and a cow fork mapping.  Unfortunately, these two
helpers cycle the ILOCK to grab a transaction, which means that the
mappings are stale as soon as we reacquire the ILOCK.  Currently we
refresh the cow fork mapping by re-calling xfs_find_trim_cow_extent, but
we don't refresh the data fork mapping beforehand, which means that the
xfs_bmap_trim_cow in that function queries the refcount btree about the
wrong physical blocks and returns an inaccurate value in *shared.

If *shared is now false, the directio write proceeds with a stale data
fork mapping.  Fix this by querying the data fork mapping if the
sequence counter changes across the ILOCK cycle.

Cc: hch@lst.de
Cc: stable@vger.kernel.org # v4.11
Fixes: 3c68d44a2b49a0 ("xfs: allocate direct I/O COW blocks in iomap_begin")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index a5c188b78138270095b92badbf7fda5f039d9de8..4801361366359b15936b6f3f497714e37294c7ba 100644 (file)
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -440,6 +440,7 @@ xfs_reflink_fill_cow_hole(
        struct xfs_mount        *mp = ip->i_mount;
        struct xfs_trans        *tp;
        xfs_filblks_t           resaligned;
+       unsigned int            seq_before = READ_ONCE(ip->i_df.if_seq);
        unsigned int            dblocks = 0, rblocks = 0;
        int                     nimaps;
        int                     error;
@@ -465,6 +466,22 @@ xfs_reflink_fill_cow_hole(
 
        *lockmode = XFS_ILOCK_EXCL;
 
+       /*
+        * The data fork mapping may have changed while we dropped the ILOCK
+        * (a racing O_DIRECT writer under IOLOCK_SHARED can complete a full
+        * CoW cycle including xfs_reflink_end_cow(), which remaps this offset
+        * and drops the refcount of the old shared block).  Re-read it so the
+        * shared-status recheck below and the caller's in-place iomap both
+        * operate on the current mapping rather than a stale physical block.
+        */
+       if (seq_before != READ_ONCE(ip->i_df.if_seq)) {
+               nimaps = 1;
+               error = xfs_bmapi_read(ip, imap->br_startoff,
+                               imap->br_blockcount, imap, &nimaps, 0);
+               if (error)
+                       goto out_trans_cancel;
+       }
+
        error = xfs_find_trim_cow_extent(ip, imap, cmap, shared, &found);
        if (error || !*shared)
                goto out_trans_cancel;
@@ -511,6 +528,8 @@ xfs_reflink_fill_delalloc(
        bool                    found;
 
        do {
+               unsigned int    seq_before = READ_ONCE(ip->i_df.if_seq);
+
                xfs_iunlock(ip, *lockmode);
                *lockmode = 0;
 
@@ -521,6 +540,23 @@ xfs_reflink_fill_delalloc(
 
                *lockmode = XFS_ILOCK_EXCL;
 
+               /*
+                * The data fork mapping may have changed while we dropped the
+                * ILOCK (a racing O_DIRECT writer under IOLOCK_SHARED can
+                * complete a full CoW cycle including xfs_reflink_end_cow(),
+                * which remaps this offset and drops the refcount of the old
+                * shared block).  Re-read it so the shared-status recheck
+                * below and the caller's in-place iomap both operate on the
+                * current mapping rather than a stale physical block.
+                */
+               if (seq_before != READ_ONCE(ip->i_df.if_seq)) {
+                       nimaps = 1;
+                       error = xfs_bmapi_read(ip, imap->br_startoff,
+                                       imap->br_blockcount, imap, &nimaps, 0);
+                       if (error)
+                               goto out_trans_cancel;
+               }
+
                error = xfs_find_trim_cow_extent(ip, imap, cmap, shared,
                                &found);
                if (error || !*shared)