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k7eW2MX664o9P4aEkqSxkTVtoSXYg6lqFVk7/DOSeRGahblCnzbDuG8SkmCQ1Fc9zg76Nw/Zq1cennm9NLwcxde+h3z5B/z7tmFoMYTEDZOG/ciaQgMIxSsRJVU4I5q1ijwyhrvdwFUs28wDZwdnlqyBMj0oNbvinDkVyUIgSKfHHL7HebUI8vlmYqNgdmVIocuFRJlmGMy3XYj9sM2BbqMYjV3ZnTQsYs3kZz6vSorqOkUjk2SDEw+b+yV8tlKl0SISne/AQvXuGREYPvG1pISZX05o3qbRlgK58AoC4n1MMmF463J/ffUldaSm58l7sE3ApqxZQ3bhtWcue/bFyntfwAwzzHDtvLTECCp8dOvqAIUvwr/bwNrRx1TyjDuJj9j+tNitUho1g2T66REVWCAZ1K+vaYn4665zZDPvj/lNaezR6vqXs3W3H1w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 19 Dec 2024, at 11:09, Bernd Schubert wrote: > On 12/19/24 17:02, Zi Yan wrote: >> On 19 Dec 2024, at 11:00, Zi Yan wrote: >>> On 19 Dec 2024, at 10:56, Bernd Schubert wrote: >>> >>>> On 12/19/24 16:55, Zi Yan wrote: >>>>> On 19 Dec 2024, at 10:53, Shakeel Butt wrote: >>>>> >>>>>> On Thu, Dec 19, 2024 at 04:47:18PM +0100, David Hildenbrand wrote:= >>>>>>> On 19.12.24 16:43, Shakeel Butt wrote: >>>>>>>> On Thu, Dec 19, 2024 at 02:05:04PM +0100, David Hildenbrand wrot= e: >>>>>>>>> On 23.11.24 00:23, Joanne Koong wrote: >>>>>>>>>> For migrations called in MIGRATE_SYNC mode, skip migrating the= folio if >>>>>>>>>> it is under writeback and has the AS_WRITEBACK_INDETERMINATE f= lag set on its >>>>>>>>>> mapping. 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_foli= o_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 compl= etely block >>>>>>>>> CMA allocations, memory compaction, memory hotunplug, memory po= isoning... ?! >>>>>>>>> >>>>>>>>> That sounds very bad. >>>>>>>> >>>>>>>> The page under writeback are already unmovable while they are un= der >>>>>>>> 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 is= sue 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 dir= ty >>>>>>> page to be written back, the contents of the dirty page are copie= d over to >>>>>>> the temp page, and the temp page gets handed to the server to wri= te 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 v= ersus >>>>>> temporary unmovable memory. >>>>> >>>>> No, the tradeoff is slow FUSE performance vs whole system slowdown = due to >>>>> memory fragmentation. AS_WRITEBACK_INDETERMINATE indicates it is no= t >>>>> temporary. >>>> >>>> Is there is a difference between FUSE TMP page being unmovable and >>>> AS_WRITEBACK_INDETERMINATE folios/pages being unmovable? >> >> (Fix my response location) >> >> Both are unmovable, but you can control where FUSE TMP page >> can come from to avoid spread across the entire memory space. For exam= ple, >> allocate a contiguous region as a TMP page pool. > > Wouldn't it make sense to have that for fuse writeback pages as well? > Fuse tries to limit dirty pages anyway. Can fuse constraint the location of writeback pages? Something like what I proposed[1], migrating pages to a location before their writeback? Will= that be a performance concern? In terms of the number of dirty pages, you only need one page out of 512 pages to prevent 2MB THP from allocation. For CMA allocation, one unmovab= le page can kill one contiguous range. What is the limit of fuse dirty pages= ? [1] https://lore.kernel.org/linux-mm/90C41581-179F-40B6-9801-9C9DBBEB1AF4= @nvidia.com/ -- Best Regards, Yan, Zi