On 11/16/10 7:01 AM, Jan Kara wrote:
> On Tue 16-11-10 22:00:58, Nick Piggin wrote:
>> I saw a lock order warning on ext4 trigger. This should solve it.
>> Raciness shouldn't matter much, because writeback can stop just
>> after we make the test and return anyway (so the API is racy anyway).
> Hmm, for now the fix is OK. Ultimately, we probably want to call
> writeback_inodes_sb() directly from all the callers. They all just want to
> reduce uncertainty of delayed allocation reservations by writing delayed
> data and actually wait for some of the writeback to happen before they
> retry again the allocation.
For ext4, at least, it's just best-effort. We're not actually out of
space yet when this starts pushing. But it helps us avoid enospc:
commit c8afb44682fcef6273e8b8eb19fab13ddd05b386
Author: Eric Sandeen <sandeen@xxxxxxxxxx>
Date: Wed Dec 23 07:58:12 2009 -0500
ext4: flush delalloc blocks when space is low
Creating many small files in rapid succession on a small
filesystem can lead to spurious ENOSPC; on a 104MB filesystem:
for i in `seq 1 22500`; do
echo -n > $SCRATCH_MNT/$i
echo XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX > $SCRATCH_MNT/$i
done
leads to ENOSPC even though after a sync, 40% of the fs is free
again.
<snip>
We don't need it to be synchronous - in fact I didn't think it was ...
ext4 should probably use btrfs's new variant and just get rid of the
one I put in, for a very large system/filesystem it could end up doing
a rather insane amount of IO when the fs starts to get full.
as for the locking problems ... sorry about that!
-Eric
> Although the callers generally cannot get umount_sem because they hold
> other locks, they have the superblock well pinned so grabbing umount_sem
> makes sense mostly to make assertions happy. But as I'm thinking about it,
> trylock *is* maybe the right answer to this anyway...
>
> So
> Acked-by: Jan Kara <jack@xxxxxxx>
>
> Honza
>>
>> Signed-off-by: Nick Piggin <npiggin@xxxxxxxxx>
>>
>> Index: linux-2.6/fs/fs-writeback.c
>> ===================================================================
>> --- linux-2.6.orig/fs/fs-writeback.c 2010-11-16 21:44:32.000000000 +1100
>> +++ linux-2.6/fs/fs-writeback.c 2010-11-16 21:49:37.000000000 +1100
>> @@ -1125,16 +1125,20 @@ EXPORT_SYMBOL(writeback_inodes_sb);
>> *
>> * Invoke writeback_inodes_sb if no writeback is currently underway.
>> * Returns 1 if writeback was started, 0 if not.
>> + *
>> + * May be called inside i_lock. May not start writeback if locks cannot
>> + * be acquired.
>> */
>> int writeback_inodes_sb_if_idle(struct super_block *sb)
>> {
>> if (!writeback_in_progress(sb->s_bdi)) {
>> - down_read(&sb->s_umount);
>> - writeback_inodes_sb(sb);
>> - up_read(&sb->s_umount);
>> - return 1;
>> - } else
>> - return 0;
>> + if (down_read_trylock(&sb->s_umount)) {
>> + writeback_inodes_sb(sb);
>> + up_read(&sb->s_umount);
>> + return 1;
>> + }
>> + }
>> + return 0;
>> }
>> EXPORT_SYMBOL(writeback_inodes_sb_if_idle);
>>
>> @@ -1145,17 +1149,21 @@ EXPORT_SYMBOL(writeback_inodes_sb_if_idl
>> *
>> * Invoke writeback_inodes_sb if no writeback is currently underway.
>> * Returns 1 if writeback was started, 0 if not.
>> + *
>> + * May be called inside i_lock. May not start writeback if locks cannot
>> + * be acquired.
>> */
>> int writeback_inodes_sb_nr_if_idle(struct super_block *sb,
>> unsigned long nr)
>> {
>> if (!writeback_in_progress(sb->s_bdi)) {
>> - down_read(&sb->s_umount);
>> - writeback_inodes_sb_nr(sb, nr);
>> - up_read(&sb->s_umount);
>> - return 1;
>> - } else
>> - return 0;
>> + if (down_read_trylock(&sb->s_umount)) {
>> + writeback_inodes_sb_nr(sb, nr);
>> + up_read(&sb->s_umount);
>> + return 1;
>> + }
>> + }
>> + return 0;
>> }
>> EXPORT_SYMBOL(writeback_inodes_sb_nr_if_idle);
>>
>> --
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