Re: [PATCH, RFC V2] ext4: flush delalloc blocks when space is low

[Eric Sandeen <sandeen@redhat.com>]

Jan Kara wrote:
...

>> +	/* try a sync to flush delalloc space & free resvd metadata */
>> +	if (!ext4_has_free_blocks(EXT4_SB(sb), 1) && dirtyblocks) {
>> +		if (!ext4_journal_current_handle()) {
>> +			down_read(&sb->s_umount);
>> +			sync_inodes_sb(sb);
>> +			up_read(&sb->s_umount);
>   ext4_should_retry_alloc() is called quite deep from the filesystem. In
> particular we can hold i_mutex of some inodes etc. So I'd almost bet
> that taking s_umount sem here violates lock ranking in some code paths
> (an easy check would be to enable lockdep and stress the filesystem a
> bit).
>   Also calling sync_inodes_sb() with i_mutex held just seems as a bad
> thing to do although I don't see where it could deadlock and so it's
> probably just a matter of taste...

Well, to be honest I agree with you ;)  It does still feel like a hack.

>   If we start writeback from ext4_nonda_switch as you do below, I think
> that we should get decent results even without synchronous writeback in
> the allocation path (maybe we'd need to tweak a bit the logic in
> ext4_nonda_switch to provide more time for writeback thread to catchup).

I think starting writeback helps a lot, but it seems that in the end we
still need a synchronous attempt when we hit a real enocpc... after I
finish dealing with this corruption thing I'll come back and look at this.

Maybe we should put the writeback in for now, and worry about the
synchronous sync-up later?

Thanks for the review,

-Eric

> 								Honza