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If the AS_WRITEBACK_INDETERMINATE flag is set on the mappi= ng, 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 g= et_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= block CMA allocations, memory compaction, memory hotunplug, memory poiso= ning... ?! >>>> >>>> That sounds very bad. >>> >>> Yeah, these writeback folios become unmovable. It makes memory fragme= ntation >>> unrecoverable. I do not know why AS_WRITEBACK_INDETERMINATE is allowe= d, since >>> it is essentially a forever pin to writeback folios. Why not introduc= e a >>> retry and timeout mechanism instead of waiting for the writeback fore= ver? >> >> If there is no way around such indeterminate writebacks, to avoid frag= ment memory, >> these to-be-written-back folios should be migrated to a physically con= tiguous region. Either you have a preallocated region or get free pages f= rom MIGRATE_UNMOVABLE. > > But at what point? Before each writeback. And there should be a limit on the amount of unmov= able pages they can allocate. > > We surely don't want to make fuse consume only effectively-unmovable me= mory. Yes, that is undesirable, but the folio under writeback cannot be migrate= d, since migration needs to wait until its finish. Of course, the right way is to make writeback interruptible, so that migration can continue, but that routine might take a lot of effort I suppose. I admit my proposal is= more like a bandaid to minimize the memory fragmentation issue. -- Best Regards, Yan, Zi