From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793725265; b=vECWTyaWqXSEtPZXb8KSzg6Drw0h+GcYTItOoNqcOIt5tLDECdoAi/9HG/ds5g9j60Ve FjOrcTTEZp3WtuwbogWl1NW9TViEOqO9Fysr+FhMzTuWJI9oNju7a7XovvG9yL+PVW2d1 VFIdT6I1OKRE5DAW6tFrgd0zQ9KFRedPR/oJwDDBNzlrsUXNl2OBWgIySzjgT85pm9jLU X1/pQQI3uj+/oRl+12VfRin63jQWVuciaa4aIeYURSaRm0cgJj8qk4jotKDq5D+eafoaf qK9h1DWxqVaUMsOJoRu53SbCLXBRSoJMvKyCf7DDPrNoEmCNsSkB/wG5YJTHcKx2KiQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793725265; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=wqUvTXfukWXNzOCriNVHhFLZ147TTmxYwIoKaQEo0dQ=; b=ukZ7a+mhYdWTq9YdEIgTrIXReMEAw3ATMWai4StfIVz26pYOsgsFHFeGl8oNOqeSfX/X mJjEtUWVsKIIHmOaOIAvGS26JQMJ2EJyKDN8FbZohMR2fY8rvLTnUJ9VV+aJyYdLLvAQK BcLzpR7pymRIgSowBF4XbU8mmtiSv9NDKg+7/Tvfq6R9lC7wdWXofSjstnuRGbVceLwrM 0WWhVGek4b/eq2D14TbnhOSbss8dTTE3PmvVPTXLQTC3mOBzhJ1Ywp2W98bI3Yh8ehq1/ xeOTrtasgr6Pjnl+4p32je0czhrPS/o9acdYob7VH0lxFsYi3A+wiQ7dFvMo8XKzCIw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=IuGtwCYS; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C054C606A324 for ; Tue, 3 Nov 2026 17:01:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=IuGtwCYS Received: by smtp.kernel.org (Postfix) with ESMTPSA id EF1A41F000C1; Tue, 3 Nov 2026 17:00:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793725264; bh=wqUvTXfukWXNzOCriNVHhFLZ147TTmxYwIoKaQEo0dQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=IuGtwCYS8X5aygLJmD5gCSCrg4nEfgzGqpox6wW6mjjUVvWzLybK67dvHxyGOqdEH yeU5ZpJeMqVbVO03qEMAxxIOlobDgpA/jxoT4cpv1JkQPeioCLBIM43pC+Tls5I9wp v2y73whOt5ZWNRVIMFC4rbNrEfZu50SkCCKFHANhUXGWHKlLSn4/EQj4VaDavZFiPs oB8HOPcNvHfi0JokBTShLxYv8Li3O6kkPi94E2duvwn6AvmFq63TYtR0Xwap9PGWIC 3Jtpp5fHN0DXSL8l1VjX+sYw43NzlHlINi2M1zH8czg55i8ahvmATgu78aZPkcZ1e7 2mWeWLfA4ahuw== Message-ID: <0f7a599b-0b22-442b-bfb3-435a85abd550@kernel.org> Date: Tue, 3 Nov 2026 18:00:56 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 4/4] mm: gup: batch PTE-mapped large folios in gup_fast_pte_range() To: Yeoreum Yun , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Jason Gunthorpe , John Hubbard , Peter Xu , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Andrew Morton , Rik van Riel , Yuan-Hao Hsu References: <20261102-refactor_gup_fast-v2-0-bc1ac018c834@arm.com> <20261102-refactor_gup_fast-v2-4-bc1ac018c834@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20261102-refactor_gup_fast-v2-4-bc1ac018c834@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/2/26 12:30, Yeoreum Yun 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 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 collapsed, freed nor split underneath the scan. > 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 > > Signed-off-by: Yuan-Hao Hsu > Signed-off-by: Yeoreum Yun > --- > mm/gup.c | 46 +++++++++++++++++++++++++++++----------------- > 1 file changed, 29 insertions(+), 17 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 49681f63950ec..94a9dc45e5953 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2901,7 +2901,8 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, > unsigned long addr, unsigned long end, > unsigned int flags, struct page **pages) > { > - unsigned long nr_pages = 0; > + unsigned long nr_pages = 0, nr_batch; > + struct folio *folio = NULL; > pte_t *ptep, *ptem; > > ptem = ptep = pte_offset_map(&pmd, addr); > @@ -2910,7 +2911,9 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, > do { > pte_t pte = ptep_get_lockless(ptep); > struct page *page; > - struct folio *folio; > + unsigned long i; > + > + nr_batch = 1; > > /* > * Always fallback to ordinary GUP on PROT_NONE-mapped pages: > @@ -2937,19 +2940,21 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, > goto pte_unmap; > > if (unlikely(pmd_val(pmd) != pmd_val(pmdp_get_lockless(pmdp))) || > - unlikely(pte_val(pte) != pte_val(ptep_get_lockless(ptep)))) { > - gup_put_folio(folio, 1, flags); > + unlikely(pte_val(pte) != pte_val(ptep_get_lockless(ptep)))) > goto pte_unmap; > - } As discussed, we should really add some more details why each of these comparisons is required. Let's do that in a separate patch. (I recall the pte check is required for concurrent PTE changes like unmap/protection etc and the pmd check is for THP collapse on architectures without IPI during TLB flushes) > > - if (!gup_fast_folio_allowed(folio, flags)) { > - gup_put_folio(folio, 1, flags); > + if (!gup_fast_folio_allowed(folio, flags)) > goto pte_unmap; > - } > > - if (!pte_write(pte) && gup_must_unshare(NULL, flags, folio, page)) { > - gup_put_folio(folio, 1, flags); > - goto pte_unmap; > + if (folio_test_large(folio)) > + nr_batch = folio_pte_batch_flags(folio, NULL, ptep, &pte, > + (end - addr) >> PAGE_SHIFT, > + FPB_LOCKLESS | FPB_RESPECT_WRITE); Batching after the checks above, when we already stabilized the folio by grabbing a reference should indeed be correct. Concurrent collapse should already see the raised reference and back off. > + > + if (!pte_write(pte)) { > + nr_batch = gup_unshared_pages(NULL, flags, folio, page, nr_batch); > + if (nr_batch == 0) !nr_batch > + goto pte_unmap; > } > > /* > @@ -2958,16 +2963,23 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, > * see Documentation/core-api/pin_user_pages.rst for > * details. > */ > - if ((flags & FOLL_PIN) && arch_make_folio_accessible(folio)) { > - gup_put_folio(folio, 1, flags); > + if ((flags & FOLL_PIN) && arch_make_folio_accessible(folio)) > goto pte_unmap; The label is now slightly misnamed. > - } > + > + if (nr_batch > 1 && try_grab_folio(folio, nr_batch - 1, flags)) > + nr_batch = 1; > + > folio_set_referenced(folio); > - pages[nr_pages] = page; > - nr_pages++; > - } while (ptep++, addr += PAGE_SIZE, addr != end); > + for (i = 0; i < nr_batch; i++) { > + pages[nr_pages + i] = page + i; > + } No need for {}. Rik will replace that with a proper helper later. > + nr_pages += nr_batch; > + folio = NULL; > + } while (ptep += nr_batch, addr += nr_batch * PAGE_SIZE, addr != end); > > pte_unmap: > + if (folio) > + gup_put_folio(folio, 1, flags); > pte_unmap(ptem); > return nr_pages; > } > -- Cheers, David