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[PATCH v5 2/6] xfs: push rounddown_pow_of_two() to after prealloc thrott

To: xfs@xxxxxxxxxxx
Subject: [PATCH v5 2/6] xfs: push rounddown_pow_of_two() to after prealloc throttle
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Mon, 18 Mar 2013 10:51:44 -0400
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1363618308-53594-1-git-send-email-bfoster@xxxxxxxxxx>
References: <1363618308-53594-1-git-send-email-bfoster@xxxxxxxxxx>
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@xxxxxxxxxx>
Reviewed-by: Ben Myers <bpm@xxxxxxx>
Reviewed-by: Mark Tinguely <tinguely@xxxxxxx>
---
 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 344557e..9072794 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
-- 
1.7.7.6

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