[PATCH 1/6] ext2 balloc: fix _with_rsv freeze

[Hugh Dickins]

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.

Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>
---
But the bigger question is, why does the my_rsv case come here to
find_next_usable_block at all?  Doesn't its 64-bit boundary limit, and its
memscan, blithely ignore what the reservation prepared?  It's messy too,
the complement of the memscan being that "i < 7" loop over in
ext2_try_to_allocate.  I think this ought to be cleaned up,
in ext2+reservations and ext3 and ext4.

 fs/ext2/balloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 2.6.19-rc6-mm2/fs/ext2/balloc.c	2006-11-24 08:18:02.000000000 +0000
+++ linux/fs/ext2/balloc.c	2006-11-27 19:28:41.000000000 +0000
@@ -570,7 +570,7 @@ find_next_usable_block(int start, struct
 		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 >= here)
-
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