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 9B0D841D103 for ; Thu, 17 Sep 2026 08:43:33 +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=1789634626; cv=none; b=t3eGhGbQddiXyyMhDVLX4j5MhpakWfNTJIpkM58Dz/byL3+AxjmQUq08uFPIDui/d0klb81LuYnER2hhSo2t4ZLGkpWiyRBSrLUSsQlxDN25qbUUsXCfeQi4HPCfUnMwMugN1aAHLGsHW/GdGQ8fZdNb10LHCwcf6es6JqIsoAM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789634626; c=relaxed/simple; bh=DZTC4uPqaHJj/OZS6NpKiXcAPICjbOhzo2h5GxYHiQI=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=tC04lOwVftsqh27UVSCbj/uo/EAlQ5ySHoI+p23EmDRwHtjzGyR2yT8bGoeWJ5bV9LLbNFli5juCASN3EjIu72wmryHhJt6YpVe43VPopRZ2RQ7nKB+FERVUYGqbsyHOTAwYWMUvVmym49ul7dClwf33hdjWqVRyn5erJMDc4fM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=cZ1Nv4E5; 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="cZ1Nv4E5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 505C11F000FF; Thu, 17 Sep 2026 08:43:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789634610; bh=AZdjSMK1O73M0g/SKYTQXMO6bB6h5wxyUE1M5wazgR0=; h=Date:Subject:From:To:Cc:References:In-Reply-To; b=cZ1Nv4E5M8ZqvrssciqIPmbcB5tnUu3LISQlyC31sZV4aLmai4oBlDr0ZCMJJtt3O Zwzp39lzTFzX94viFolQ+w6JHMMxiVd3yt/YwuJl15kd1pa3Y2XK36Uioz915XaIXy ADNAu+j61uQXJJogMbTMssodfeJEbzWjPHIaBa0SP5qkL8Okggq7YiVnjEXf6ZItyq KkS71wjnsZDMh+iXkUo4auLw3X08RPkjSfXECNvtxinNP60v6cSskmcff8CQujWKl3 WneAAh86Egy/phHyKYiGLo7lDuLPDfJ/qM2sax0hm/rPW3almwiNYYRM1OPwo03Lhf fD+pONFOrnWCQ== Message-ID: Date: Thu, 17 Sep 2026 10:43:22 +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() From: "David Hildenbrand (Arm)" 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> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/17/26 10:42, David Hildenbrand (Arm) wrote: > 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. > Ah, no, Rik looked at GUP not GUP-fast. So I'll have a look but it will take a while. -- Cheers, David