Re: [PATCH v2] btrfs: remove an ASSERT() which can cause false alert

Qu Wenruo
2022-02-17T13:00:16Z

On 2022/2/17 20:32, Filipe Manana wrote:
> On Thu, Feb 17, 2022 at 11:24 AM Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
>>
>>
>>
>> On 2022/2/17 19:14, Filipe Manana wrote:
>>> On Thu, Feb 17, 2022 at 05:49:34PM +0800, Qu Wenruo wrote:
>>>> [BUG]
>>>> Su reported that with his VM running on an M1 mac, it has a high chance
>>>> to trigger the following ASSERT() with scrub and balance test cases:
>>>>
>>>> ASSERT(cache->start == chunk_offset);
>>>> ret = scrub_chunk(sctx, cache, scrub_dev, found_key.offset,
>>>> dev_extent_len);
>>>>
>>>> [CAUSE]
>>>> The ASSERT() is making sure that the block group cache (@cache) and the
>>>> chunk offset from the device extent match.
>>>>
>>>> But the truth is, block group caches and dev extent items are deleted at
>>>> different timing.
>>>>
>>>> For block group caches and items, after btrfs_remove_block_group() they
>>>> will be marked deleted, but the device extent and the chunk item is
>>>> still in dev tree and chunk tree respectively.
>>>
>>> This is not correct, what happens is:
>>>
>>> 1) btrfs_remove_chunk() will remove the device extent items and the chunk
>>> item;
>>>
>>> 2) It then calls btrfs_remove_block_group(). This will not remove the
>>> extent map if the block group was frozen (due to trim, and scrub
>>> itself). But it will remove the block group (struct btrfs_block_group)
>>> from the red black tree of block groups, and mark the block group
>>> as deleted (set struct btrfs_block_group::removed to 1).
>>>
>>> If the extent map was not removed, meaning the block group is frozen,
>>> then no one will be able to create a new block group with the same logical
>>> address before the block group is unfrozen (by someone who is holding
>>> a reference on it). So a lookup on the red black tree will return NULL
>>> for the logical address until the block group is unfrozen and its
>>> logical address is reused for a new block group.
>>>
>>> So the ordering of what you are saying is reversed - the device extent
>>> and chunk items are removed before marking the block group as deleted.
>>>
>>>>
>>>> The device extents and chunk items are only deleted in
>>>> btrfs_remove_chunk(), which happens either a btrfs_delete_unused_bgs()
>>>> or btrfs_relocate_chunk().
>>>
>>> This is also confusing because it gives a wrong idea of the ordering.
>>> The device extents and chunk items are removed by btrfs_remove_chunk(),
>>> which happens before marking a block group as deleted.
>>>
>>>>
>>>> This timing difference leaves a window where we can have a mismatch
>>>> between block group cache and device extent tree, and triggering the
>>>> ASSERT().
>>>
>>> I would like to see more details on this. The sequence of steps between
>>> two tasks that result in this assertion being triggered.
>>>
>>>>
>>>> [FIX]
>>>>
>>>> - Remove the ASSERT()
>>>>
>>>> - Add a quick exit if our found bg doesn't match @chunk_offset
>>>
>>> This shouldn't happen. I would like to know the sequence of steps
>>> between 2 (or more) tasks that leads to that.
>>>
>>> We are getting the block group with btrfs_lookup_block_group() at
>>> scrub_enumerate_chunks(), that calls block_group_cache_tree_search()
>>> with the "contains" argument set to 1, meaning it can return a
>>> block group that contains the given bytenr but does not start at
>>> that bytenr necessarily.
>>>
>>> This gives me the idea a small block group was deleted and then a
>>> new one was allocated which starts at a lower logical offset and
>>> includes "chunk_offset" (ends beyond that).
>>>
>>> We should probably be using btrfs_lookup_first_block_group() at
>>> scrub_enumerate_chunks(), so it looks for a block group that starts
>>> exactly at the given logical address.
>>>
>>> But anyway, as I read this and see the patch's diff, this feels a lot
>>> like a "bail out if something unexpected happens but we don't know
>>> exactly why/how that is possible".
>>
>> You're completely correct.
>>
>> Unfortunately I have no way to reproduce the bug on x86_64 VMs, and all
>> my ARM VMs are too slow to reproduce.
>>
>> Furthermore, due to some unrelated bugs, v5.17-rc based kernels all
>> failed to boot inside the VMs on Apple M1.
>>
>> So unfortunately I don't have extra details for now.
>>
>> Will find a way to reproduce first, then update the patch with better
>> comments.
>
> Was it triggered with some specific test case from fstests?

Yes, from a regular run on fstests.

But that was on an older branch (some older misc-next, which has the
patch but still based on v5.16).

The v5.17-rc based branch will not boot inside the ARM vm at all, and no
early boot messages after the EFI routine.

Will attack the boot problem first.

Thanks,
Qu
>
>>
>> Thanks,
>> Qu
>>
>>>
>>>>
>>>> - Add a comment on the existing checks in scrub_chunk()
>>>> This is the ultimate safenet, as it will iterate through all the stripes
>>>> of the found chunk.
>>>> And only scrub the stripe if it matches both device and physical
>>>> offset.
>>>>
>>>> So even by some how we got a dev extent which is no longer owned
>>>> by the target chunk, we will just skip this chunk completely, without
>>>> any consequence.
>>>>
>>>> Reported-by: Su Yue <l@damenly.su>
>>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>>> ---
>>>> Changelog:
>>>> v2:
>>>> - Add a new quick exit with extra comments
>>>>
>>>> - Add a new comment in the existing safenet in scrub_chunk()
>>>> ---
>>>> fs/btrfs/scrub.c | 21 ++++++++++++++++++++-
>>>> 1 file changed, 20 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
>>>> index 11089568b287..1c3ed4720ddd 100644
>>>> --- a/fs/btrfs/scrub.c
>>>> +++ b/fs/btrfs/scrub.c
>>>> @@ -3578,6 +3578,14 @@ static noinline_for_stack int scrub_chunk(struct scrub_ctx *sctx,
>>>> goto out;
>>>>
>>>> map = em->map_lookup;
>>>> +
>>>> + /*
>>>> + * Due to the delayed modification of device tree, this chunk
>>>> + * may not own this dev extent.
>>>
>>> This is confusing. What delayed modification, what do you mean by it
>>> exactly? Same below, with more details.
>>>
>>>> + *
>>>> + * So we need to iterate through all stripes, and only scrub
>>>> + * the stripe which matches both device and physical offset.
>>>> + */
>>>> for (i = 0; i < map->num_stripes; ++i) {
>>>> if (map->stripes[i].dev->bdev == scrub_dev->bdev &&
>>>> map->stripes[i].physical == dev_offset) {
>>>> @@ -3699,6 +3707,18 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
>>>> if (!cache)
>>>> goto skip;
>>>>
>>>> + /*
>>>> + * Since dev extents modification is delayed, it's possible
>>>> + * we got a bg which doesn't really own this dev extent.
>>>
>>> Same as before. Too confusing, what is delayed dev extent modification?
>>>
>>> Once added, device extents can only be deleted, they are never modified.
>>> I think you are referring to the deletions and the fact we use the commit
>>> roots to find extents, but that should be a more clear comment, without
>>> such level of ambiguity.
>>>
>>> Thanks.
>>>
>>>> + *
>>>> + * Here just do a quick skip, scrub_chunk() has a more
>>>> + * comprehensive check in it.
>>>> + */
>>>> + if (cache->start != chunk_offset) {
>>>> + btrfs_put_block_group(cache);
>>>> + goto skip;
>>>> + }
>>>> +
>>>> if (sctx->is_dev_replace && btrfs_is_zoned(fs_info)) {
>>>> spin_lock(&cache->lock);
>>>> if (!cache->to_copy) {
>>>> @@ -3822,7 +3842,6 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
>>>> dev_replace->item_needs_writeback = 1;
>>>> up_write(&dev_replace->rwsem);
>>>>
>>>> - ASSERT(cache->start == chunk_offset);
>>>> ret = scrub_chunk(sctx, cache, scrub_dev, found_key.offset,
>>>> dev_extent_len);
>>>>
>>>> --
>>>> 2.35.1
>>>>