From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 28C97421255 for ; Mon, 12 Oct 2026 19:06:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791831964; cv=none; b=nHESxc5bsrKqP66BiIiiJLab7Sxef5/O7bRSfO/OI2SeHyM4VSK+8TI1SKVmVbCGD80mhfYzRFxIAzc4+aXPCfyL0qsRheDOoDti2trsel6pDD8T+p4q9Wd4MzlG0c7gqS+6r38WeFHuCfqjseA0/sDgmvi3pC36iNd0yNWcmlU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791831964; c=relaxed/simple; bh=bDN5pxww94Il7uHAoByuOSUi1Eh2s835x7lS1Tnmgwc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=h2LqMLdqvlk892DB3dZgTlHd3ZV+hYPjGZwJIFYBjp7wNtrbywhLv4mWsnfcwYLAdlHLmvCMn7nFpzUc9bdTsZm3G+iJmQ2fIbwFmHRtLPxwmg2YzatHzpDobufZAy0dF0kXySKqFv8ZftB5EjgJvhbx6RwdSXqEav8p0qm9GuI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PJOwxP9F; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="PJOwxP9F" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 318391F0089B; Mon, 12 Oct 2026 19:05:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791831962; bh=3UVup6GJ/cJrIO5b7tQA3kWhFihBuF5irLWCeT7h/2o=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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