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 BE5C34772A6 for ; Thu, 17 Sep 2026 08:42:25 +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=1789634555; cv=none; b=j7kpRaH20lPkvSJgvT4G0ib1MIv+pgA8nUSZF+Z3jyu5AbBtOom26WnDOVMLrjweNOTjmKIJcqaYxs6JGLkyeo3B29MwgYT8nkR8Rg7JV6YrTlgAmUYPzVHLOczctPHB3DTMtTAjRqZquY1Q7vTSe1tsSV4Wo2zCpY2/PvH76YE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789634555; c=relaxed/simple; bh=Tfmi36mRpCj6kQHIFuBvJmXvf3jhLdPG8xBzb//huEs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ZhtpxXZayNltHEjnsvmA3YGOlpSwHhbozcSLqYs0RwFxJx7g0tc8HfjzU35V0VS26Kbi53Evs7ez8IwyQzhBrzBRlfnDd6gp7mTqCf/bYX23AFEl96FboTm+Thq/+TX/34G42WkDV3Sf5J6cFv3CUcdGIyqrfcazegFI1Dzrors= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=VMg04wK9; 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="VMg04wK9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D08591F000FF; Thu, 17 Sep 2026 08:42:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789634544; bh=2vjVdEHW9kBktetbFrLAuLp8tP/fJwMsUdPrzrz9iTE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=VMg04wK9XgbrovCjHxg0w+TXe7CtSAOkA+NTfVubphgm5v0+sy00OPkVwFpB6FpHG N+eJJqQDflKg+id2871cDmMUo9huI7Zg6+kxK8vxWFC8GzoKK3JbAhnZrTYnpXb2K3 b11PM7oyD5fFZlByIhu1pXyaqN5Nl0Fe5NJ/cHQO+7pJxQfHMCf8vYoWctNmP2Qnsv 5SbPHvLkG+jIB7007QsPOIuoc42a241sDKeqqJm+mNuWKAEcZT5bWSo9SN3Q2K1ixS vSk6mOu1Kbcjg8Ewp198GbuSamVjpd03jQDi1vz5vq1H3S+flQcdtGksQ6VTi8aSTO K6YOolI/QzrPA== Message-ID: Date: Thu, 17 Sep 2026 10:42:16 +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: Yuan-Hao Hsu , Andrew Morton Cc: Jason Gunthorpe , John Hubbard , Peter Xu , Lorenzo Stoakes , liam@infradead.org, Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Rik van Riel , Aristeu Rozanski , Ryan Roberts , Dev Jain , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260917083747.786-1-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: 7bit On 9/17/26 10:37, Yuan-Hao Hsu wrote: > GUP-fast grabs a PTE-mapped large folio one page at a time. Every PTE > costs a try_grab_folio_fast() (a refcount cmpxchg, plus the pincount > atomic, a full barrier and a node stat update for FOLL_PIN), a > gup_fast_folio_allowed() and a folio_set_referenced(), so a 64 kB mTHP > pays sixteen of each and a PTE-mapped 2 MB THP pays 512. The PMD and > PUD leaf paths already take the whole range with one > try_grab_folio_fast() call, and the slow path is getting the same > treatment for PTEs in Rik's follow_page_mask() series. > > The users are the hot ones: iov_iter_extract_user_pages() pins every > O_DIRECT buffer through pin_user_pages_fast(), io_uring registers its > buffers through it, and so does RDMA memory registration. With mTHP > those buffers are PTE-mapped large folios. > > After the head page has been grabbed and verified as before, extend the > run to the following PTEs that map the next pages of the same folio > with the same protection bits, take the remaining references for the > run with try_grab_folio(), and fill pages[] from the run. The PTEs of > the run are read while the reference on the folio is held, so the folio > can neither be freed nor split underneath the scan; a THP collapse can > still detach the page table, so the pmd is checked again after the run > is grabbed and the run is dropped if it changed, leaving the next > iteration to hit the existing pmd check and bail out. A read-only run > of an anonymous folio stops at the first page that gup_must_unshare() > rejects, so the per-page exclusivity guarantee for FOLL_PIN is kept. > > folio_pte_batch_flags() does the scan. It reads the entries with > ptep_get(), which is not safe without the page table lock on > CONFIG_GUP_GET_PXX_LOW_HIGH (x86 PAE, mips32, sh) or with arm64 contpte, > and GUP-fast reads every PTE with ptep_get_lockless() for that reason. > Add FPB_LOCKLESS to make the helper do the same; it is inlined per call > site, so the other callers do not change. > > The order-0 path keeps its instruction sequence. The folio flags are > tested before folio_set_referenced() on purpose: a load of the flags > right after that locked instruction cost about 25% on order-0 pages in > the gup_test benchmark below, and reading them first brings that path > back to the baseline. > > gup_test ioctl over a 256 MB anonymous region, FOLL_WRITE, 65536 pages > per call, median of 15 runs, two baseline boots / one patched boot, on > an x86-64 VM: > > get_user_pages_fast() pin_user_pages_fast() > before after before after > 4 kB pages 849/789 us 783 us 1174/1138 us 1094 us > 64 kB mTHP 812/811 us 126 us 1215/1113 us 188 us > 1 MB mTHP 799/774 us 59 us 1185/1077 us 64 us > 2 MB THP, PTE-mapped 803/786 us 57 us 1159/1120 us 58 us > 2 MB THP, PMD-mapped 42/40 us 40 us 46/41 us 42 us > > fio O_DIRECT randread of a null_blk device (CPU bound) with the I/O > buffer in 64 kB mTHP, median of 15 five-second runs: > > bs=1M psync 126.2/125.6 GB/s -> 220.6 GB/s > bs=1M io_uring, iodepth 16 71.2/71.6 GB/s -> 112.1 GB/s > bs=64k io_uring, iodepth 16 50.2/50.7 GB/s -> 60.1 GB/s > > IORING_REGISTER_BUFFERS of 1 GB, median of 15: > > 64 kB mTHP 6179/5998 us -> 1599 us > 1 MB mTHP 5790/5801 us -> 893 us > 4 kB pages 7173/6741 us -> 6763 us > I think Rik was already working on this and sent some patches. Anyhow, there is quite some GUP review backlog I have t go through, so this will have to wait. -- Cheers, David