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commit 8310d48b upstream.

In commit 19be0eaf ("mm: remove gup_flags FOLL_WRITE games from
__get_user_pages()"), the mm code was changed from unsetting FOLL_WRITE
after a COW was resolved to setting the (newly introduced) FOLL_COW
instead.  Simultaneously, the check in gup.c was updated to still allow
writes with FOLL_FORCE set if FOLL_COW had also been set.

However, a similar check in huge_memory.c was forgotten.  As a result,
remote memory writes to ro regions of memory backed by transparent huge
pages cause an infinite loop in the kernel (handle_mm_fault sets
FOLL_COW and returns 0 causing a retry, but follow_trans_huge_pmd bails
out immidiately because `(flags & FOLL_WRITE) && !pmd_write(*pmd)` is
true.

While in this state the process is stil SIGKILLable, but little else
works (e.g.  no ptrace attach, no other signals).  This is easily
reproduced with the following code (assuming thp are set to always):

    #i...