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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , "Matthew Wilcox (Oracle)" , Hugh Dickins , Chris Mason , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, Sashiko Subject: Re: [PATCH v2 1/2] mm/memory: recheck PG_uptodate after ->page_mkwrite() Message-ID: References: <20261109120403.82361-1-kas@kernel.org> <20261109120403.82361-2-kas@kernel.org> X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Nov 09, 2026 at 04:11:40PM +0000, Lorenzo Stoakes (ARM) wrote: > On Mon, Nov 09, 2026 at 12:04:01PM +0000, Kiryl Shutsemau (Meta) wrote: > > do_shared_fault() unlocks the folio around ->page_mkwrite(). Writeback > > can lock it in that window, fail, and clear PG_uptodate with the folio > > left in the page cache. ecryptfs_writepages() does this when encryption > > fails. filemap_page_mkwrite() relocks the folio and checks only > > ->mapping, so the fault maps a folio that is not uptodate: > > > > CPU0 CPU1 > > do_shared_fault() > > folio_unlock() > > ecryptfs_writepages() > > folio_clear_uptodate() > > filemap_page_mkwrite() > > folio_lock() > > finish_fault() > > > > The next read(2) or fault on the folio reads it in again from the file, > > over anything stored through the mapping in the meantime. Those stores > > are lost. > > > > Recheck PG_uptodate once ->page_mkwrite() returns with the folio locked, > > and retry the fault if it is clear, the same way filemap_fault() retries > > on a cached folio that lost PG_uptodate. Skip folios outside the page > > cache: driver pages from ->fault() never get PG_uptodate and would > > retry forever. > > > > The check stays out of do_page_mkwrite(): wp_page_shared() calls it on > > a folio that is still mapped, and a retry there would fault on the same > > PTE forever. > > > > Reported-by: Sashiko > > Closes: https://lore.kernel.org/sashiko-outbox-202966@kernel.org/ > > No Fixes? Sorry I may have missed context from v1 here :) The window is from 2007, when __do_fault() started to drop the folio lock around ->page_mkwrite(): Fixes: 6967614761fd ("ocfs2: release page lock before calling ->page_mkwrite") Not sure how useful such Fixes is. It predates any possible stable tree. > > --- > > mm/memory.c | 22 ++++++++++++++++++++++ > > 1 file changed, 22 insertions(+) > > > > diff --git a/mm/memory.c b/mm/memory.c > > index fe72e66c9c2e..cc1d0b7d15c7 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -5613,6 +5613,18 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf) > > return VM_FAULT_OOM; > > } > > > > +/* > > + * A page cache folio that is not uptodate holds data that was never read in > > + * or cleared, and mapping it leaks stale memory to userspace. > > + */ > > +static bool file_folio_not_uptodate(struct folio *folio) > > +{ > > + if (likely(folio_test_uptodate(folio))) > > + return false; > > + /* Driver pages from ->fault() never get PG_uptodate */ > > I guess you mean drivers that return non-page cache folios from ->fault()? Yes. How about: /* * ->fault() can also return driver memory that is not in the page * cache. It never gets PG_uptodate. */ I would rather avoid "rmappable". These folios do get folio_add_file_rmap_ptes() in set_pte_range(), and the word reads like PG_large_rmappable. As you said, DAX never reaches the check, so "page cache" is accurate. > > + return !folio_test_anon(folio) && folio->mapping; > > Technically folio_test_anon() can never return true based on where this is > called from right? Could this just be a: > > VM_WARN_ON_ONCE(folio_test_anon(folio)); > /* ... comment */ > return folio->mapping; > > Instead? Right, all callers pass a file folio. I will make it an assert, but at the top of the helper, so it checks every call and not only the rare !uptodate one. > > @@ -6163,6 +6175,16 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf) > > folio_put(folio); > > return tmp; > > } > > + > > + /* > > + * The folio was unlocked for ->page_mkwrite(). If it lost > > + * PG_uptodate meanwhile, retry the fault to read it in again. > > + */ > > + if (unlikely(file_folio_not_uptodate(folio))) { > > As Pedro said, I think this unlikely() is probably unnecessary given the true > return case is implicitly unlikely. As I told Pedro, I prefer it at the call site. That is where the reader needs to see that the retry is the rare path. But if you insist... -- Kiryl Shutsemau / Kirill A. Shutemov