On Tue, Jul 18, 2023 at 4:47 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote: > > > > 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. (+Hugh Dickins for vis) Yu, I am not familiar with the split_folio() case, so I am not sure it is the same exact race I stated above. Can you confirm whether or not doing folio_test_clear_mlocked() before folio_test_clear_lru() would fix the race you are referring to? IIUC, in this case, we make sure we clear PG_mlocked before we try to to clear PG_lru. If we fail to clear it, then someone else have the folio isolated after we clear PG_mlocked, so we can be sure that when they put the folio back it will be correctly made evictable. Is my understanding correct? If yes, I can add this fix to my next version of the RFC series to rework mlock_count. It would be a lot more complicated with the current implementation (as I stated in a previous email). > > > > > > > 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 > >>
