Re: fsstress-induced corruption reproduced

Eric Sandeen
2010-01-04T23:08:57Z

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.

Of course with the above commit, we have no allocation context in
ext4_da_update_reserve_space... that's all long gone so we can't key
on that anymore.

However, I think the following change will fix it; I'll run it through
xfstests later on and be sure nothing else regresses.

-Eric

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5352db1..28cd8d8 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1257,9 +1257,10 @@ int ext4_get_blocks(handle_t *handle, struct
inode *inode, sector_t block,
* if the caller is from delayed allocation writeout path
* we have already reserved fs blocks for allocation
* let the underlying get_block() function know to
- * avoid double accounting
+ * avoid double accounting. Ditto for prealloc blocks.
*/
- if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE)
+ if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE ||
+ flags & EXT4_GET_BLOCKS_UNINIT_EXT)
EXT4_I(inode)->i_delalloc_reserved_flag = 1;
/*
* We need to check for EXT4 here because migrate

-Eric

> -Eric
>
>> I finally reproduced it; the problem happens when we fallocate() a
>> region of the file which we had recently written, and which is still in
>> the page cache marked as delayed allocation blocks. When we finally
>> write those blocks out, since they are marked BH_Delay,
>> ext4_get_blocks() calls ext4_da_update_reserve_space(), which ends up
>> bumping i_blocks a second time and charging the blocks against the
>> user's quota a second time. Oops.
>>
>> Fortunately the fsck problem is one that will be fixed with a preen (and
>> if quota is enabled, a quotacheck), so it's not super serious, but we
>> should fix it when we have a chance. If anyone has time to look at it,
>> please let me know. Otherwise, I'll put it on my todo list. I don't
>> consider seriously urgent since the case is highly unlikely to occur in
>> real life, and it doesn't have any security implications; the worst an
>> attacker could do is end up charging excesss quota to herself.
>>
>> I've included a simple reproduction case below; if you run this program,
>> it will create a file "test-file" in the current working directory which
>> will appear to be 32k, even though it is really only 16k long, and if
>> you then unmount the test file system and run e2fsck -p on it, you will get
>> the error message:
>>
>> Inode XXX, i_blocks is 64, should be 32. FIXED.
>>
>> - Ted
>>
>> #define _GNU_SOURCE
>>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <unistd.h>
>> #include <string.h>
>> #include <sys/types.h>
>> #include <fcntl.h>
>> #include <fcntl.h>
>>
>> #define BUFSIZE 1024
>>
>> int main(int argc, char **argv)
>> {
>> int i, fd, ret;
>> char buf[BUFSIZE];
>>
>> fd = open("test-file", O_RDWR|O_CREAT|O_TRUNC, 0644);
>> if (fd < 0) {
>> perror("open");
>> exit(1);
>> }
>> memset(&buf, 0, BUFSIZE);
>> for (i=0; i < 16; i++) {
>> ret = write(fd, &buf, BUFSIZE);
>> if (ret < 0) {
>> perror("write");
>> exit(1);
>> }
>> if (ret != BUFSIZE) {
>> fprintf(stderr, "Write return expected %d, got %d\n",
>> BUFSIZE, ret);
>> exit(1);
>> }
>> }
>> ret = fallocate(fd, 0, 0, 16384);
>> if (ret < 0) {
>> perror("fallocate");
>> exit(1);
>> }
>> ret = fsync(fd);
>> if (ret < 0) {
>> perror("fsync");
>> exit(1);
>> }
>> ret = close(fd);
>> if (ret < 0) {
>> perror("close");
>> exit(1);
>> }
>> exit(0);
>> }
>> --
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