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If the AS_WRITEBACK_INDETERMINATE flag is set on the mapping= , the >>> writeback may take an indeterminate amount of time to complete, and >>> waits may get stuck. >>> >>> Signed-off-by: Joanne Koong >>> Reviewed-by: Shakeel Butt >>> --- >>> mm/migrate.c | 5 ++++- >>> 1 file changed, 4 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/migrate.c b/mm/migrate.c >>> index df91248755e4..fe73284e5246 100644 >>> --- a/mm/migrate.c >>> +++ b/mm/migrate.c >>> @@ -1260,7 +1260,10 @@ static int migrate_folio_unmap(new_folio_t get= _new_folio, >>> */ >>> switch (mode) { >>> case MIGRATE_SYNC: >>> - break; >>> + if (!src->mapping || >>> + !mapping_writeback_indeterminate(src->mapping)) >>> + break; >>> + fallthrough; >>> default: >>> rc =3D -EBUSY; >>> goto out; >> >> Ehm, doesn't this mean that any fuse user can essentially completely b= lock CMA allocations, memory compaction, memory hotunplug, memory poisoni= ng... ?! >> >> That sounds very bad. > > Yeah, these writeback folios become unmovable. It makes memory fragment= ation > unrecoverable. I do not know why AS_WRITEBACK_INDETERMINATE is allowed,= since > it is essentially a forever pin to writeback folios. Why not introduce = a > retry and timeout mechanism instead of waiting for the writeback foreve= r? If there is no way around such indeterminate writebacks, to avoid fragmen= t memory, these to-be-written-back folios should be migrated to a physically contig= uous region. Either you have a preallocated region or get free pages from= MIGRATE_UNMOVABLE. -- Best Regards, Yan, Zi