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Inserting Ivan Orlov's syzbot fix commit 2ce0bdfe
("mm: khugepaged: fix kernel BUG in hpage_collapse_scan_file()")
ahead of Jiaqi Yan's and David Stevens's commits
12904d95 ("mm/khugepaged: recover from poisoned file-backed memory")
cae106dd ("mm/khugepaged: refactor collapse_file control flow")
ac492b9c ("mm/khugepaged: skip shmem with userfaultfd")
(all of which restructure collapse_file()) did not work out well.

xfstests generic/086 on huge tmpfs (with accelerated khugepaged) freezes
(if not on the first attempt, then the 2nd or 3rd) in find_lock_entries()
while doing drop_caches: the file's xarray seems to have been corrupted,
with find_get_entry() returning nonsense which makes no progress.

Bisection led to ac492b9c; and diff against earlier working linux-next
suggested that it's probably down to an errant xas_store(), which does not
belong with the later changes (and nor does the positioning of warnings). 
The later changes...