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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 = -EBUSY; >>>>> goto out; >>>> >>>> Ehm, doesn't this mean that any fuse user can essentially completely block >>>> CMA allocations, memory compaction, memory hotunplug, memory poisoning... ?! >>>> >>>> That sounds very bad. >>> >>> The page under writeback are already unmovable while they are under >>> writeback. This patch is only making potentially unrelated tasks to >>> synchronously wait on writeback completion for such pages which in worst >>> case can be indefinite. This actually is solving an isolation issue on a >>> multi-tenant machine. >>> >> Are you sure, because I read in the cover letter: >> >> "In the current FUSE writeback design (see commit 3be5a52b30aa ("fuse: >> support writable mmap"))), a temp page is allocated for every dirty >> page to be written back, the contents of the dirty page are copied over to >> the temp page, and the temp page gets handed to the server to write back. >> This is done so that writeback may be immediately cleared on the dirty >> page," >> >> Which to me means that they are immediately movable again? > > Oh sorry, my mistake, yes this will become an isolation issue with the > removal of the temp page in-between which this series is doing. I think > the tradeoff is between extra memory plus slow write performance versus > temporary unmovable memory. No, the tradeoff is slow FUSE performance vs whole system slowdown due to memory fragmentation. AS_WRITEBACK_INDETERMINATE indicates it is not temporary. -- Best Regards, Yan, Zi