BUG_ON(!mapping_empty(&inode->i_data))

[Hugh Dickins]

Running my usual tmpfs kernel builds swapping load, on Sunday's rc4-mm1
mmotm (I never got to try rc3-mm1 but presume it behaved the same way),
I hit clear_inode()'s BUG_ON(!mapping_empty(&inode->i_data)); on two
machines, within an hour or few, repeatably though not to order.

The stack backtrace has always been clear_inode < ext4_clear_inode <
ext4_evict_inode < evict < dispose_list < prune_icache_sb <
super_cache_scan < do_shrink_slab < shrink_slab_memcg < shrink_slab <
shrink_node_memgs < shrink_node < balance_pgdat < kswapd.

ext4 is the disk filesystem I read the source to build from, and also
the filesystem I use on a loop device on a tmpfs file: I have not tried
with other filesystems, nor checked whether perhaps it happens always on
the loop one or always on the disk one.  I have not seen it happen with
tmpfs - probably because its inodes cannot be evicted by the shrinker
anyway; I have not seen it happen when "rm -rf" evicts ext4 or tmpfs
inodes (but suspect that may be down to timing, or less pressure).
I doubt it's a matter of filesystem: think it's an XArray thing.

Whenever I've looked at the XArray nodes involved, the root node
(shift 6) contained one or three (adjacent) pointers to empty shift
0 nodes, which each had offset and parent and array correctly set.
Is there some way in which empty nodes can get left behind, and so
fail eviction's mapping_empty() check?

I did wonder whether some might get left behind if xas_alloc() fails
(though probably the tree here is too shallow to show that).  Printks
showed that occasionally xas_alloc() did fail while testing (maybe at
memcg limit), but there was no correlation with the BUG_ONs.

I did wonder whether this is a long-standing issue, which your new
BUG_ON is the first to detect: so tried 5.12-rc5 clear_inode() with
a BUG_ON(!xa_empty(&inode->i_data.i_pages)) after its nrpages and
nrexceptional BUG_ONs.  The result there surprised me: I expected
it to behave the same way, but it hits that BUG_ON in a minute or
so, instead of an hour or so.  Was there a fix you made somewhere,
to avoid the BUG_ON(!mapping_empty) most of the time? but needs
more work. I looked around a little, but didn't find any.

I had hoped to work this out myself, and save us both some writing:
but better hand over to you, in the hope that you'll quickly guess
what's up, then I can try patches. I do like the no-nrexceptionals
series, but there's something still to be fixed.

Hugh