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b=PJOwxP9FzjOl86X9NFuqd2aiJNaF8iCTgTaKo7xgxX/QyzJpD81FRZrQ00pX0Yzis 8ByDxuzFiszwHnWMP3tz+0nubZWpFI1oeVaqpqxnHmC47CJ6+1S0AImp1LLYYmyzjT CLPWJnT/RezJM5RB7Fva+S8bFuY7p81o/NzmH8HgY6QrX2VLrCiy//wlwWfb0hIZYU AGzjrCzNpXdd/lPFMVH7F1rIwxN4tF/gr/5bJKPG8gLEU3cOSlqaxr25EQ24ost70T pWWkBNhxTGMGZiPZr/fvts45XsnbITMiRGugctO9i4k0V+PQLfoebUapAYLGO20aRh IIh5SRg4L7ubw== Message-ID: Date: Mon, 12 Oct 2026 21:05:55 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: batch PTE-mapped large folios in gup_fast_pte_range() To: Rik van Riel , Yuan-Hao Hsu Cc: 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 References: <20260917083747.786-1-aa9736195201@gmail.com> <7dd4ec994502233cdc09fe5c4369adc89168d738.camel@surriel.com> <20261010195202.689-1-aa9736195201@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 90FC42000A X-Rspam-User: X-Stat-Signature: wda5wr7ntzybmx97boc9unzjszjs7sgg X-HE-Tag: 1791831963-991636 X-HE-Meta: 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 jGhgwhrU atwr4KzDlz+boR28OAQOTYNBoA7MulzmX0ojMq3O83TkLGJpXkJ0xifeVoXKDTgSi9ESOgUvTN04njvnGJjN2fORgoQYSDr4lzlHdKBNcMMcYNDHuP480FkjzpYMd92CoNEDknDNNtLR1cmTwC7itUbExICBOH34zUrXh8d3D0z2lSe42LQ4gcxjFol83McE5Vb4bq6ctm1VzDKfeK+wG0JlqdePhDVP1M2ZXBTBqkWfmGjfIdphDUE40x4NS+ZWvTJmuhjGvHP+0ci+uPAxfP4m7lufWt8rsxDKPSCHJNy+yZCAsrU/kUA/SocaEa/MSPNl42Cd0q+1cZh6jYB/i8ppZ+0SQAG3xL4IUMJPYNy0Pd87xPFhgQ7jZMCsirY+S+QiRyJlErqLOUV8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. -- Cheers, David