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
