On 7/19/23 06:48, Yosry Ahmed wrote: > On Sun, Jul 16, 2023 at 6:58 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote: >> >> >> >> On 7/17/23 08:35, Yu Zhao wrote: >>> On Sun, Jul 16, 2023 at 6:00 PM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote: >>>> >>>> On 7/15/2023 2:06 PM, Yu Zhao wrote: >>>>> There is a problem here that I didn't have the time to elaborate: we >>>>> can't mlock() a folio that is within the range but not fully mapped >>>>> because this folio can be on the deferred split queue. When the split >>>>> happens, those unmapped folios (not mapped by this vma but are mapped >>>>> into other vmas) will be stranded on the unevictable lru. >>>> >>>> This should be fine unless I missed something. During large folio split, >>>> the unmap_folio() will be migrate(anon)/unmap(file) folio. Folio will be >>>> munlocked in unmap_folio(). So the head/tail pages will be evictable always. >>> >>> It's close but not entirely accurate: munlock can fail on isolated folios. >> Yes. The munlock just clear PG_mlocked bit but with PG_unevictable left. >> >> Could this also happen against normal 4K page? I mean when user try to munlock >> a normal 4K page and this 4K page is isolated. So it become unevictable page? > > Looks like it can be possible. If cpu 1 is in __munlock_folio() and > cpu 2 is isolating the folio for any purpose: > > cpu1 cpu2 > isolate folio > folio_test_clear_lru() // 0 > putback folio // add > to unevictable list > folio_test_clear_mlocked() Yes. Yu showed this sequence to me in another email. I thought the putback_lru() could correct the none-mlocked but unevictable folio. But it doesn't because of this race. > > > The page would be stranded on the unevictable list in this case, no? > Maybe we should only try to isolate the page (clear PG_lru) after we > possibly clear PG_mlocked? In this case if we fail to isolate we know > for sure that whoever has the page isolated will observe that > PG_mlocked is clear and correctly make the page evictable. > > This probably would be complicated with the current implementation, as > we first need to decrement mlock_count to determine if we want to > clear PG_mlocked, and to do so we need to isolate the page as > mlock_count overlays page->lru. With the proposal in [1] to rework > mlock_count, it might be much simpler as far as I can tell. I intend > to refresh this proposal soon-ish. > > [1]https://lore.kernel.org/lkml/20230618065719.1363271-1-yosryahmed@xxxxxxxxxx/ > >> >> >> Regards >> Yin, Fengwei >>
