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(was Re: [PATCH v3 6/7] mm/swap, shmem: use unified swapin helper for shmem) To: "Huang, Ying" Cc: Minchan Kim , Kairui Song , linux-mm@kvack.org, Kairui Song , Andrew Morton , Hugh Dickins , Johannes Weiner , Matthew Wilcox , Michal Hocko , Yosry Ahmed , David Hildenbrand , linux-kernel@vger.kernel.org, Yu Zhao Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 473D1180004 X-Stat-Signature: 3opentuu5cih7gca9y1sken47ej53gnd X-Rspam-User: X-HE-Tag: 1706744300-838957 X-HE-Meta: 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 kjE3uCwN laZa1wNByRe0KMiUKAH0+qZW653djJDa4rO4QiP8pbQlZ97DABgV+xF1IDZjh3b3ap/PExuSS0dmoGC4gSgf4Ie5C4VGUnN56qJtSPE0NO+B6pliCavjgQMZW7nOKTW/gCUBPMISFWTnwnnyR1SZQ4AcYF3pDkThxEjslETMZutP8uGBs8j9KP+ZSy/SCvvTK0ljJIgR5aLML2ox1JygE6gWJI3b2zgDcoFdETpO2TADOvM3VoMUDXGbr+fsw7FnIkaT6pY2f28otWW4kyTarMtQdNOQ3LG+DldiVFOh+P0lL4fKCYNWO+PsT/tTCLGpxGCkmTp60ru5+TLA= 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 Tue, Jan 30, 2024 at 6:53=E2=80=AFPM Huang, Ying = wrote: > > Hi, Minchan, > > When I review the patchset from Kairui, I checked the code to skip swap > cache in do_swap_page() for swap device with SWP_SYNCHRONOUS_IO. Is the > following race possible? Where a page is swapped out to a swap device > with SWP_SYNCHRONOUS_IO and the swap count is 1. Then 2 threads of the > process runs on CPU0 and CPU1 as below. CPU0 is running do_swap_page(). > > CPU0 CPU1 > ---- ---- > swap_cache_get_folio() > check sync io and swap count > alloc folio > swap_readpage() > folio_lock_or_retry() > swap in the swap entry > write page > swap out to same swap entry > pte_offset_map_lock() > check pte_same() > swap_free() <-- new content lost! > set_pte_at() <-- stale page! > folio_unlock() > pte_unmap_unlock() Yes, that path looks possible but hard to hit due to the requirement of swap in and swap out in a short window. I have the similar question on the previous zswap rb tree to xarray discussion regarding deleting an entry where the entry might change due to swap in then swap out. Chris