Hi David, On Thu, Feb 15, 2024 at 09:03:28PM +0100, David Hildenbrand wrote: < snip > > > > > > > We would detect later, that the PTE changed, but we would temporarily > > > mess with that swap slot that we might no longer "own". > > > > > > I was thinking about alternatives, it's tricky because of the concurrent > > > MADV_DONTNEED possibility. Something with another fake-swap entry type > > > (similar to migration entries) might work, but would require more changes. > > > > Yeah, in the long term I also think more work is needed for the swap subsystem. > > > > In my opinion, for this particular issue, or, for cache bypassed > > swapin, a new swap map value similar to SWAP_MAP_BAD/SWAP_MAP_SHMEM > > might be needed, that may even help to simplify the swap count release > > routine for cache bypassed swapin, and improve the performance. > > The question is if we really want to track that in the swapcache and not > rather in the page table. > > Imagine the following: > > (1) allocate the folio and lock it (we do that already) > > (2) take the page table lock. If the PTE is still the same, insert a new > "swapin_in_process" fake swp entry that references the locked folio. > > (3) read the folio from swap. This will unlock the folio IIUC. (we do that > already) > > (4) relock the folio. (we do that already, might not want to fail) > > (4) take the PTE lock. If the PTE did not change, turn it into a present PTE > entry. Otherwise, cleanup. > > > Any concurrent swap-in users would spot the new "swapin_in_process" fake swp > entry and wait for the page lock (just like we do with migration entries). > > Zap code would mostly only clear the "swapin_in_process" fake swp entry and > leave the cleanup to (4) above. Fortunately, concurrent fork() is impossible > as that cannot race with page faults. > > There might be one minor thing to optimize with the folio lock above. But in > essence, it would work just like migration entries, just that they are > installed only while we actually do read the content from disk etc. That's a great idea. I was thinking to have the synchronization in the page table but couldn't reach to the other non_swap_entry idea. Only concern of the approach is that it would be harder to have the fix in the stable tree. If there isn't strong objection, I prefer the Kairui's orginal solution(with some tweak of scheduler if it's necessary) first and then pursue your idea on latest tree.
