On Thu, May 21, 2020 at 07:53:09PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>
> When we're estimating a new speculative preallocation length for an
> extending write, we should walk backwards through the extent list to
> determine the number of number of blocks that are physically and
> logically contiguous with the write offset, and use that as an input to
> the preallocation size computation.
>
> This way, preallocation length is truly measured by the effectiveness of
> the allocator in giving us contiguous allocations without being
> influenced by the state of a given extent. This fixes both the problem
> where ZERO_RANGE within an EOF can reduce preallocation, and prevents
> the unnecessary shrinkage of preallocation when delalloc extents are
> turned into unwritten extents.
>
> This was found as a regression in xfs/014 after changing delalloc writes
> to create unwritten extents during writeback.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
> fs/xfs/xfs_iomap.c | 37 +++++++++++++++++++++++++------------
> 1 file changed, 25 insertions(+), 12 deletions(-)
>
>
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index ac970b13b1f8..6a308af93893 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -377,15 +377,17 @@ xfs_iomap_prealloc_size(
> loff_t count,
> struct xfs_iext_cursor *icur)
> {
> + struct xfs_iext_cursor ncur = *icur; /* struct copy */
> + struct xfs_bmbt_irec prev, got;
> struct xfs_mount *mp = ip->i_mount;
> struct xfs_ifork *ifp = XFS_IFORK_PTR(ip, whichfork);
> xfs_fileoff_t offset_fsb = XFS_B_TO_FSBT(mp, offset);
> - struct xfs_bmbt_irec prev;
> - int shift = 0;
> int64_t freesp;
> xfs_fsblock_t qblocks;
> - int qshift = 0;
> xfs_fsblock_t alloc_blocks = 0;
> + xfs_extlen_t plen;
> + int shift = 0;
> + int qshift = 0;
>
> if (offset + count <= XFS_ISIZE(ip))
> return 0;
> @@ -413,16 +415,27 @@ xfs_iomap_prealloc_size(
> * preallocation size.
> *
> * If the extent is a hole, then preallocation is essentially disabled.
> - * Otherwise we take the size of the preceding data extent as the basis
> - * for the preallocation size. If the size of the extent is greater than
> - * half the maximum extent length, then use the current offset as the
> - * basis. This ensures that for large files the preallocation size
> - * always extends to MAXEXTLEN rather than falling short due to things
> - * like stripe unit/width alignment of real extents.
> + * Otherwise we take the size of the preceding data extents as the basis
> + * for the preallocation size. Note that we don't care if the previous
> + * extents are written or not.
> + *
> + * If the size of the extents is greater than half the maximum extent
> + * length, then use the current offset as the basis. This ensures that
> + * for large files the preallocation size always extends to MAXEXTLEN
> + * rather than falling short due to things like stripe unit/width
> + * alignment of real extents.
> */
> - if (prev.br_blockcount <= (MAXEXTLEN >> 1))
> - alloc_blocks = prev.br_blockcount << 1;
> - else
> + plen = prev.br_blockcount;
If prev is initialized by peeking the previous extent, then it looks
like the first iteration of this loop compares the immediately previous
extent with itself..
> + while (xfs_iext_prev_extent(ifp, &ncur, &got)) {
> + if (plen > MAXEXTLEN / 2 ||
> + got.br_startoff + got.br_blockcount != prev.br_startoff ||
> + got.br_startblock + got.br_blockcount != prev.br_startblock)
We should probably check for nullstartblock (delalloc) extents
explicitly here rather than rely on the calculation to fail.
> + break;
> + plen += got.br_blockcount;
> + prev = got;
> + }
> + alloc_blocks = plen * 2;
Why do we replace the bit shifts with division/multiplication? I'd
prefer to see the former for obvious power of 2 operations, even if this
happens to be 32-bit arithmetic. I don't see any particular reason to
change it in this patch.
Brian
> + if (alloc_blocks > MAXEXTLEN)
> alloc_blocks = XFS_B_TO_FSB(mp, offset);
> if (!alloc_blocks)
> goto check_writeio;
>
