Re: [QUESTION] BUG_ON in ext4_mb_simple_scan_group

zhangyi (F)
2020-04-17T02:06:40Z

Hi, Ritesh

On 2020/4/17 2:33, Ritesh Harjani wrote:
> Hello Kun,
>
> On 4/16/20 7:49 PM, yangerkun wrote:
>> Nowadays, we trigger the a bug that has been reported before[1](trigger the bug with read block bitmap error before). After search the patch,
>> I found some related patch which has not been included in our kernel.
>>
>> eb5760863fc2 ext4: mark block bitmap corrupted when found instead of BUGON
>> 736dedbb1a7d ext4: mark block bitmap corrupted when found
>> 206f6d552d0c ext4: mark inode bitmap corrupted when found
>> db79e6d1fb1f ext4: add new ext4_mark_group_bitmap_corrupted() helper
>> 0db9fdeb347c ext4: fix wrong return value in ext4_read_inode_bitmap()
>
> I see that you anyways have figured all these patches out.
>
>>
>> Maybe this patch can fix the problem, but I am a little confused with
>> the explain from Ted described in the mail:
>>
>>  > What probably happened is that the page containing actual allocation
>>  > bitmap was pushed out of memory due to memory pressure.  However, the
>>  > buddy bitmap was still cached in memory.  That's actually quite
>>  > possible since the buddy bitmap will often be referenced more
>>  > frequently than the allocation bitmap (for example, while searching
>>  > for free space of a specific size, and then having that block group
>>  > skipped when it's not available).
>>
>>  > Since there was an I/O error reading the allocation bitmap, the buffer
>>  > is not valid.  So it's not surprising that the BUG_ON(k >= max) is
>>  > getting triggered.
>
> @Others, please correct me if I am wrong here.
>
> So just as a small summary. Ext4 maintains an inode (we call it as
> buddy cache inode which is sbi->s_buddy_cache) which stores the block
> bitmap and buddy information for every block group. So we require 2
> blocks for every block group to store both of this info in it.
>
> So what generally happens is whenever there is a request to block
> allocation, this(buddy and block bitmap information is loaded from the
> disk into the page cache.
>
> When someone does the block allocation these pages get loaded into the
> page cache. And it will be there until these pages are getting heavily
> used (that's coz of page eviction algo in mm).
> But in case when the memory pressure is high, these pages may get
> written out and eventually getting evicted from the page cache.
> Now if any of this page is not present in the page cache we go and try
> to read it from the disk. (I think that's the job of
> ext4_mb_load_buddy_gfp()).
>
> So let's say while reading this page from disk we get an I/O error,
> so this means, as Ted explained, that the buffer which was not properly
> read and hence it is not uptodate (and so we also don't set buffer
> verified bit).
> And in that case we should mark that block group corrupted. So that next
> time, ext4_mb_good_group() does not allow us to do allocation from that
> block group. I think some of the patches which you pointed add the logic
> into the mballoc. So that we don't hit that bug_on().
>
> {...
> [Addition info - not related to your issue though]
> So this could also be an e.g. where the grp->bb_free may not be uptodate
> for a block group of which bitmap was not properly loaded.
> ...}
>
>
>>
>> (Our machine: x86, 4K page size, 4K block size)
>>
>> After check the related code, we found that once we get a IO error from ext4_wait_block_bitmap, ext4_mb_init_cache will return directly with a error number, so the latter ext4_mb_simple_scan_group may never been called! So any other scene will trigger this BUG_ON?
>
> Sorry that's not what I see in latest upstream kernel.
> I am not sure which kernel version you are checking this on.
> Check the latest upstream kernel and compare with it.
>
>

Thanks for your reply.

We check the upstream kernel 5.7-rc1, on our machine which has 4K page size
and 4K block size, if the ext4_wait_block_bitmap() invoked from
ext4_mb_init_cache() return -EIO, the 'err' variable will be set and the
subsequent loop will be jumped out due to '!buffer_verified[group - first_group]
&& blocks_per_page == 1', so the -EIO error number will return by
ext4_mb_load_buddy() and there is no chance to invoke ext4_mb_simple_scan_group()
and trigger BUG_ON().

static int ext4_mb_init_cache(struct page *page, char *incore, gfp_t gfp)
{
...
/* wait for I/O completion */
for (i = 0, group = first_group; i < groups_per_page; i++, group++) {
...
err2 = ext4_wait_block_bitmap(sb, group, bh[i]);
if (!err)
err = err2; <------ set -EIO here
}

first_block = page->index * blocks_per_page;
for (i = 0; i < blocks_per_page; i++) {
group = (first_block + i) >> 1;
...
if (!buffer_verified(bh[group - first_group]))
/* Skip faulty bitmaps */
continue;<----- blocks_per_page == 1, we jump out here
err = 0; <---- never excute
...
out:
...
return err;
}

static noinline_for_stack int
ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
{
...
err = ext4_mb_load_buddy(sb, group, &e4b);
if (err)
goto out; <--- return here
...
if (cr == 0)
ext4_mb_simple_scan_group(ac, &e4b); <--- never invoke
...
}

We also find that ext4_group_info:bb_counters and the corresponding buddy bit map
are updated or initialized at the same time, so even if we encounter page miss and
forget to mark that block group corrupted due to IO failure, it seems that it also
could not trigger this inconsistency. Am I missing something ?

Thanks,
Yi.

>> -----
>> [1] https://www.spinics.net/lists/linux-ext4/msg60329.html
>>
>
>
> .