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h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=EucCbRvd8KoSEkBB6E0N+iAtZB854R2c9Agr4NE1ihGmQzd3iO3ZT9WsNHayT2nJB nX7jN2yfpviE7lXh91GBNjJbf9lTah3PoJrurIEBcOCXKguPqNxRpfwQ5hx7hSzVsa RkUPOU0qMgMtzUGIFCSu8q/y5EKM0x0VpResoxQU= Date: Tue, 27 Oct 2026 12:22:27 +0000 From: Yeoreum Yun To: "David Hildenbrand (Arm)" Cc: Rik van Riel , Yuan-Hao Hsu , Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu , Lorenzo Stoakes , liam@infradead.org, Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Aristeu Rozanski , Ryan Roberts , Dev Jain , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/gup: batch PTE-mapped large folios in gup_fast_pte_range() Message-ID: References: <20260917083747.786-1-aa9736195201@gmail.com> <7dd4ec994502233cdc09fe5c4369adc89168d738.camel@surriel.com> <20261010195202.689-1-aa9736195201@gmail.com> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Queue-Id: 4351820002 X-Stat-Signature: me76traj6ek6kiymrr3wudnzr3dauuub X-Rspamd-Server: rspam06 X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Rspam-User: X-HE-Tag: 1793103753-121891 X-HE-Meta: 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 DlA0VMif YrSVgpWVwvTRwQqTwepQgCvHdJD8ZgjSPjj65h0VrfxzaW+kQA7nKyCRe9y7o8vLWj7tehqEbEcGCSQjEYYkfMCpxuZGZKpUcfGGrQhf9EHlk2W8G9wl+u78tHEsN/9wWImwS2vqrwYQmc6snLNoZYOXpcR2AFALX7rUb3WVxVg3ZKXYevn0alatOSaE8zXVevrEJmh26B+oo2c4AJjdgm9xUd1qsrLUdscbR+j4AXOlzBspIVTzGcDcO2T1XqBg0G8fMIjV2+m2Py1HDIWaaWRGCMAcUvGxNGhU5EecTF4fWUhVjRbjKtIFowG1OhnXfP41N4BSRNLXtGg6qUaDNBBdUIS49fg8ac1Vj2exXXo+T+dCKhQY7OviyCA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Mon, Oct 12, 2026 at 09:05:55PM +0200, David Hildenbrand (Arm) wrote: > On 10/12/26 19:58, Rik van Riel wrote: > > On Sun, 2026-10-11 at 03:52 +0800, Yuan-Hao Hsu wrote: > >> On Wed, 07 Oct 2026 06:47:06 -0400, Rik van Riel wrote: > >>> Since this is lockless, the PTEs could change under > >>> us between when we walk them in folio_pte_batch_flags() > >>> and the batched try_grab_folio(). > >>> > >>> Is that race condition safe, or do we need to re-check > >>> afterward that: > >>> 1) the ptes still point at the same folio, and/or > >>> 2) the folio still spans the entire range, and was > >>>    not split before we grabbed the refcount? > >> > >> It is safe, because the batch only runs after the first page of the > >> run has gone through the unchanged per-page path: the folio reference > >> is already held (and for FOLL_PIN the _pincount already raised, with > >> the smp_mb__after_atomic() in try_grab_folio_fast()) before any of > >> the other ptes is read. > >> > >> 2) With that reference held the folio cannot be split: > > > > You are right. The page tables can still be remapped > > to point at something else after gup_fast_pte_range() > > grabs a reference to the folio, but that can happen > > no matter what precautions we take inside gup_fast, > > because it can happen after gup_fast has returned. > > > > I have no other concerns, so feel free to add my: > > > > Reviewed-by: Rik van Riel > > There are a bunch of things I don't like about this, including suspiciously LLM > -sloppy looking stuff like > > + if (!pte_write(pte)) { > + for (i = 1; i < nr; i++) { > + if (gup_must_unshare(NULL, flags, page + i)) > + break; > + } > + nr = i; > + } > > I'll find someone to polish this up. Thanks David, I'll post with a little bit fix in next rc-period. -- Sincerely, Yeoreum Yun