Re: fsstress-induced corruption reproduced

Eric Sandeen
2010-01-05T23:38:01Z

Aneesh Kumar K.V wrote:
> On Mon, Jan 04, 2010 at 05:08:55PM -0600, Eric Sandeen wrote:
>
>> Eric Sandeen wrote:
>>
>>> Theodore Ts'o wrote:
>>>
>>>> One of the things which has been annoying me for a while now is a
>>>> hard-to-reproduce xfsqa failure in test #13 (fsstress), which causes the
>>>> a test failure because the file system found to be inconsistent:
>>>>
>>>> Inode NNN, i_blocks is X, should be Y.
>>>>
>>> Interesting, this apparently has gotten much worse since 2.6.32.
>>>
>>> I wrote an xfstests reproducer, and couldn't hit it on .32; hit it right
>>> off on 2.6.33-rc2.
>>>
>>> Probably should find out why ;) I'll go take a look.
>>>
>> commit d21cd8f163ac44b15c465aab7306db931c606908
>> Author: Dmitry Monakhov <dmonakhov@openvz.org>
>> Date: Thu Dec 10 03:31:45 2009 +0000
>>
>> ext4: Fix potential quota deadlock
>>
>> seems to be the culprit.
>>
>> (unfortunately this means that the error we saw before is something
>> -else- to be fixed, yet) Anyway ...
>>
>> This is because we used to do this in ext4_mb_mark_diskspace_used() :
>>
>> /*
>> * Now reduce the dirty block count also. Should not go negative
>> */
>> if (!(ac->ac_flags & EXT4_MB_DELALLOC_RESERVED))
>> /* release all the reserved blocks if non delalloc */
>> percpu_counter_sub(&sbi->s_dirtyblocks_counter,
>> reserv_blks);
>> else {
>> percpu_counter_sub(&sbi->s_dirtyblocks_counter,
>> ac->ac_b_ex.fe_len);
>> /* convert reserved quota blocks to real quota blocks */
>> vfs_dq_claim_block(ac->ac_inode, ac->ac_b_ex.fe_len);
>> }
>>
>> i.e. the vfs_dq_claim_block was conditional based on
>> EXT4_MB_DELALLOC_RESERVED... and the testcase did not go that way,
>> because we had already preallocated the blocks.
>>
>> But with the above quota deadlock commit it's not unconditional
>> anymore in ext4_da_update_reserve_space and we always call
>> vfs_dq_claim_block which over-accounts.
>>
>>
>
> It is still conditional right ? We call ext4_da_update_reserve_space
> only if EXT4_GET_BLOCKS_UPDATE_RESERVE_SPACE is set . That will
> happen only in case of delayed allocation. I guess the problem is
> same as what Ted stated. But i am not sure why we are able to reproduce
> it much easily on 2.6.33-rc2.
>
>
Maybe something like this works:

Index: linux-2.6/fs/ext4/inode.c
===================================================================
--- linux-2.6.orig/fs/ext4/inode.c
+++ linux-2.6/fs/ext4/inode.c
@@ -1203,6 +1203,7 @@ int ext4_get_blocks(handle_t *handle, st
int flags)
{
int retval;
+ int was_unwritten;

clear_buffer_mapped(bh);
clear_buffer_unwritten(bh);
@@ -1253,9 +1254,13 @@ int ext4_get_blocks(handle_t *handle, st
* part of the uninitialized extent to be an initialized
* extent. This is because we need to avoid the combination
* of BH_Unwritten and BH_Mapped flags being simultaneously
- * set on the buffer_head.
+ * set on the buffer_head. However, if it was unwritten we
+ * don't want to update reserved space later.
*/
- clear_buffer_unwritten(bh);
+ if (buffer_unwritten(bh)) {
+ was_unwritten = 1;
+ clear_buffer_unwritten(bh);
+ }

/*
* New blocks allocate and/or writing to uninitialized extent
@@ -1301,7 +1306,8 @@ int ext4_get_blocks(handle_t *handle, st
* Update reserved blocks/metadata blocks after successful
* block allocation which had been deferred till now.
*/
- if ((retval > 0) && (flags & EXT4_GET_BLOCKS_UPDATE_RESERVE_SPACE))
+ if ((retval > 0) && !was_unwritten &&
+ (flags & EXT4_GET_BLOCKS_UPDATE_RESERVE_SPACE))
ext4_da_update_reserve_space(inode, retval);

up_write((&EXT4_I(inode)->i_data_sem));

but that might leave the previous reservations hanging around from
prior to the fallocate ...

-Eric