linux-ext4 - [PATCH 12/12] ext3 balloc: fix _with

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Date:	Tue, 28 Nov 2006 20:15:41 -0800
From:	Mingming Cao <cmm@...ibm.com>
To:	Andrew Morton <akpm@...l.org>, Hugh Dickins <hugh@...itas.com>
Cc:	Mel Gorman <mel@...net.ie>, "Martin J. Bligh" <mbligh@...igh.org>,
	linux-kernel@...r.kernel.org,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: [PATCH 12/12] ext3 balloc: fix _with_rsv freeze

------------------------------------------------------
Subject: ext2 balloc: fix _with_rsv freeze
From: Hugh Dickins <hugh@...itas.com>

After several days of testing ext2 with reservations, it got caught inside
ext2_try_to_allocate_with_rsv: alloc_new_reservation repeatedly succeeding on
the window [12cff,12d0e], ext2_try_to_allocate repeatedly failing to find the
free block guaranteed to be included (unless there's contention).

Fix the range to find_next_usable_block's memscan: the scan from "here"
(0xcfe) up to (but excluding) "maxblocks" (0xd0e) needs to scan 3 bytes not 2
(the relevant bytes of bitmap in this case being f7 df ff - none 00, but the
premature cutoff implying that the last was found 00).

Is this a problem for mainline ext2?  No, because the "size" in its memscan is
always EXT2_BLOCKS_PER_GROUP(sb), which mkfs.ext2 requires to be a multiple of
8.  Is this a problem for ext3 or ext4?  No, because they have an additional
extN_test_allocatable test which rescues them from the error.

--------------------------------------------------

Sync up a reservation fix from ext2 in ext4
Signed-off-by: Mingming Cao <cmm@...ibm.com>

---

 linux-2.6.19-rc5-cmm/fs/ext4/balloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN fs/ext4/balloc.c~ext4-balloc-fix-_with_rsv-freeze fs/ext4/balloc.c
--- linux-2.6.19-rc5/fs/ext4/balloc.c~ext4-balloc-fix-_with_rsv-freeze	2006-11-28 19:37:12.000000000 -0800
+++ linux-2.6.19-rc5-cmm/fs/ext4/balloc.c	2006-11-28 19:37:12.000000000 -0800
@@ -747,7 +747,7 @@ find_next_usable_block(ext4_grpblk_t sta
 		here = 0;

 	p = ((char *)bh->b_data) + (here >> 3);
-	r = memscan(p, 0, (maxblocks - here + 7) >> 3);
+	r = memscan(p, 0, ((maxblocks + 7) >> 3 - (here >> 3));
 	next = (r - ((char *)bh->b_data)) << 3;

 	if (next < maxblocks && next >= start && ext4_test_allocatable(next, bh))

_

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