[PATCH v3 2/7] xfs: push rounddown_pow_of_two() to after prealloc throttle

Brian Foster
2013-02-19T16:35:00Z

The round down occurs towards the beginning of the function. Push
it down after throttling has occurred. This is to support adding
further transformations to 'alloc_blocks' that might not preserve
power-of-two alignment (and thus could lead to rounding down
multiple times).

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
---
fs/xfs/xfs_iomap.c | 19 +++++++++++++++++--
1 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index d914419..daa08f6 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -389,8 +389,15 @@ xfs_iomap_prealloc_size(
if (!alloc_blocks)
goto check_writeio;

- alloc_blocks = XFS_FILEOFF_MIN(MAXEXTLEN,
- rounddown_pow_of_two(alloc_blocks));
+ /*
+ * MAXEXTLEN is not a power of two value but we round the prealloc down
+ * to the nearest power of two value after throttling. To prevent the
+ * round down from unconditionally reducing the maximum supported prealloc
+ * size, we round up first, apply appropriate throttling, round down and
+ * cap the value to MAXEXTLEN.
+ */
+ alloc_blocks = XFS_FILEOFF_MIN(roundup_pow_of_two(MAXEXTLEN),
+ alloc_blocks);

xfs_icsb_sync_counters(mp, XFS_ICSB_LAZY_COUNT);
freesp = mp->m_sb.sb_fdblocks;
@@ -407,6 +414,14 @@ xfs_iomap_prealloc_size(
}
if (shift)
alloc_blocks >>= shift;
+ /*
+ * rounddown_pow_of_two() returns an undefined result if we pass in
+ * alloc_blocks = 0.
+ */
+ if (alloc_blocks)
+ alloc_blocks = rounddown_pow_of_two(alloc_blocks);
+ if (alloc_blocks > MAXEXTLEN)
+ alloc_blocks = MAXEXTLEN;

/*
* If we are still trying to allocate more space than is