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

Qu Wenruo
2022-02-17T11:24:32Z

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.

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
>>