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

Filipe Manana
2022-02-17T12:33:43Z

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?

>
> 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
> >>