The actual logic looks good, but I think the new helper and another
third set of comment explaining what is going on makes this area even
more confusing. What about something like this instead?
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index bb590a267a7f9..26f9874361cd3 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -352,22 +352,10 @@ xfs_quota_calc_throttle(
}
/*
- * If we are doing a write at the end of the file and there are no allocations
- * past this one, then extend the allocation out to the file system's write
- * iosize.
- *
* If we don't have a user specified preallocation size, dynamically increase
* the preallocation size as the size of the file grows. Cap the maximum size
* at a single extent or less if the filesystem is near full. The closer the
- * filesystem is to full, the smaller the maximum prealocation.
- *
- * As an exception we don't do any preallocation at all if the file is smaller
- * than the minimum preallocation and we are using the default dynamic
- * preallocation scheme, as it is likely this is the only write to the file that
- * is going to be done.
- *
- * We clean up any extra space left over when the file is closed in
- * xfs_inactive().
+ * filesystem is to full, the smaller the maximum preallocation.
*/
STATIC xfs_fsblock_t
xfs_iomap_prealloc_size(
@@ -380,52 +368,58 @@ xfs_iomap_prealloc_size(
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;
+ struct xfs_iext_cursor ncur = *icur;
+ struct xfs_bmbt_irec prev, got;
int shift = 0;
int64_t freesp;
xfs_fsblock_t qblocks;
int qshift = 0;
- xfs_fsblock_t alloc_blocks = 0;
+ xfs_fsblock_t alloc_blocks;
+ xfs_extlen_t plen;
- if (offset + count <= XFS_ISIZE(ip))
- return 0;
-
- if (!(mp->m_flags & XFS_MOUNT_ALLOCSIZE) &&
- (XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_allocsize_blocks)))
+ /*
+ * As an exception we don't do any preallocation at all if the file is
+ * smaller than the minimum preallocation and we are using the default
+ * dynamic preallocation scheme, as it is likely this is the only write
+ * to the file that is going to be done.
+ */
+ if (XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_allocsize_blocks))
return 0;
/*
- * If an explicit allocsize is set, the file is small, or we
- * are writing behind a hole, then use the minimum prealloc:
+ * Otherwise use the minimum prealloca size for small files, or if we
+ * are writing right after a hole.
*/
- if ((mp->m_flags & XFS_MOUNT_ALLOCSIZE) ||
- XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_dalign) ||
- !xfs_iext_peek_prev_extent(ifp, icur, &prev) ||
+ if (XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_dalign) ||
+ !xfs_iext_prev_extent(ifp, &ncur, &prev) ||
prev.br_startoff + prev.br_blockcount < offset_fsb)
return mp->m_allocsize_blocks;
/*
- * Determine the initial size of the preallocation. We are beyond the
- * current EOF here, but we need to take into account whether this is
- * a sparse write or an extending write when determining the
- * preallocation size. Hence we need to look up the extent that ends
- * at the current write offset and use the result to determine the
- * 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.
+ * Take the size of the contiguous 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 (prev.br_blockcount <= (MAXEXTLEN >> 1))
- alloc_blocks = prev.br_blockcount << 1;
- else
+ plen = prev.br_blockcount;
+ 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)
+ break;
+ plen += got.br_blockcount;
+ prev = got;
+ }
+
+ /*
+ * 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.
+ */
+ alloc_blocks = plen * 2;
+ if (alloc_blocks > MAXEXTLEN)
alloc_blocks = XFS_B_TO_FSB(mp, offset);
- if (!alloc_blocks)
- goto check_writeio;
qblocks = alloc_blocks;
/*
@@ -494,7 +488,6 @@ xfs_iomap_prealloc_size(
*/
while (alloc_blocks && alloc_blocks >= freesp)
alloc_blocks >>= 4;
-check_writeio:
if (alloc_blocks < mp->m_allocsize_blocks)
alloc_blocks = mp->m_allocsize_blocks;
trace_xfs_iomap_prealloc_size(ip, alloc_blocks, shift,
@@ -961,9 +954,16 @@ xfs_buffered_write_iomap_begin(
if (error)
goto out_unlock;
- if (eof) {
- prealloc_blocks = xfs_iomap_prealloc_size(ip, allocfork, offset,
- count, &icur);
+ if (eof && offset + count > XFS_ISIZE(ip)) {
+ /*
+ * Determine the initial size of the preallocation.
+ * We clean up any extra preallocation when the file is closed.
+ */
+ if (mp->m_flags & XFS_MOUNT_ALLOCSIZE)
+ prealloc_blocks = mp->m_allocsize_blocks;
+ else
+ prealloc_blocks = xfs_iomap_prealloc_size(ip, allocfork,
+ offset, count, &icur);
if (prealloc_blocks) {
xfs_extlen_t align;
xfs_off_t end_offset;
