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(p200300d82f30de008132f6dccba29134.dip0.t-ipconnect.de. [2003:d8:2f30:de00:8132:f6dc:cba2:9134]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3df4fd372ccsm11959114f8f.32.2025.09.05.23.56.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 05 Sep 2025 23:56:53 -0700 (PDT) Message-ID: <85e760cf-b994-40db-8d13-221feee55c60@redhat.com> Date: Sat, 6 Sep 2025 08:56:48 +0200 Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 19/37] mm/gup: remove record_subpages() To: John Hubbard , linux-kernel@vger.kernel.org Cc: Alexander Potapenko , Andrew Morton , Brendan Jackman , Christoph Lameter , Dennis Zhou , Dmitry Vyukov , dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, iommu@lists.linux.dev, io-uring@vger.kernel.org, Jason Gunthorpe , Jens Axboe , Johannes Weiner , kasan-dev@googlegroups.com, kvm@vger.kernel.org, "Liam R. Howlett" , Linus Torvalds , linux-arm-kernel@axis.com, linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, linux-ide@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mips@vger.kernel.org, linux-mmc@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, Lorenzo Stoakes , Marco Elver , Marek Szyprowski , Michal Hocko , Mike Rapoport , Muchun Song , netdev@vger.kernel.org, Oscar Salvador , Peter Xu , Robin Murphy , Suren Baghdasaryan , Tejun Heo , virtualization@lists.linux.dev, Vlastimil Babka , wireguard@lists.zx2c4.com, x86@kernel.org, Zi Yan References: <20250901150359.867252-1-david@redhat.com> <20250901150359.867252-20-david@redhat.com> <016307ba-427d-4646-8e4d-1ffefd2c1968@nvidia.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+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: <016307ba-427d-4646-8e4d-1ffefd2c1968@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 06.09.25 03:05, John Hubbard wrote: > On 9/1/25 8:03 AM, David Hildenbrand wrote: >> We can just cleanup the code by calculating the #refs earlier, >> so we can just inline what remains of record_subpages(). >> >> Calculate the number of references/pages ahead of times, and record them >> only once all our tests passed. >> >> Signed-off-by: David Hildenbrand >> --- >> mm/gup.c | 25 ++++++++----------------- >> 1 file changed, 8 insertions(+), 17 deletions(-) >> >> diff --git a/mm/gup.c b/mm/gup.c >> index c10cd969c1a3b..f0f4d1a68e094 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -484,19 +484,6 @@ static inline void mm_set_has_pinned_flag(struct mm_struct *mm) >> #ifdef CONFIG_MMU >> >> #ifdef CONFIG_HAVE_GUP_FAST >> -static int record_subpages(struct page *page, unsigned long sz, >> - unsigned long addr, unsigned long end, >> - struct page **pages) >> -{ >> - int nr; >> - >> - page += (addr & (sz - 1)) >> PAGE_SHIFT; >> - for (nr = 0; addr != end; nr++, addr += PAGE_SIZE) >> - pages[nr] = page++; >> - >> - return nr; >> -} >> - >> /** >> * try_grab_folio_fast() - Attempt to get or pin a folio in fast path. >> * @page: pointer to page to be grabbed >> @@ -2967,8 +2954,8 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr, >> if (pmd_special(orig)) >> return 0; >> >> - page = pmd_page(orig); >> - refs = record_subpages(page, PMD_SIZE, addr, end, pages + *nr); >> + refs = (end - addr) >> PAGE_SHIFT; >> + page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT); >> >> folio = try_grab_folio_fast(page, refs, flags); >> if (!folio) >> @@ -2989,6 +2976,8 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr, >> } >> >> *nr += refs; >> + for (; refs; refs--) >> + *(pages++) = page++; >> folio_set_referenced(folio); >> return 1; >> } >> @@ -3007,8 +2996,8 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr, >> if (pud_special(orig)) >> return 0; >> >> - page = pud_page(orig); >> - refs = record_subpages(page, PUD_SIZE, addr, end, pages + *nr); >> + refs = (end - addr) >> PAGE_SHIFT; >> + page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT); >> >> folio = try_grab_folio_fast(page, refs, flags); >> if (!folio) >> @@ -3030,6 +3019,8 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr, >> } >> >> *nr += refs; >> + for (; refs; refs--) >> + *(pages++) = page++; > > Hi David, Hi! > > Probably a similar sentiment as Lorenzo here...the above diffs make the code > *worse* to read. In fact, I recall adding record_subpages() here long ago, > specifically to help clarify what was going on. Well, there is a lot I dislike about record_subpages() to go back there. Starting with "as Willy keeps explaining, the concept of subpages do not exist and ending with "why do we fill out the array even on failure". :) > > Now it's been returned to it's original, cryptic form. > The code in the caller was so uncryptic that both me and Lorenzo missed that magical addition. :P > Just my take on it, for whatever that's worth. :) As always, appreciated. I could of course keep the simple loop in some "record_folio_pages" function and clean up what I dislike about record_subpages(). But I much rather want the call chain to be cleaned up instead, if possible. Roughly, what I am thinking (limiting it to pte+pmd case) about is the following: From d6d6d21dbf435d8030782a627175e36e6c7b2dfb Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Sat, 6 Sep 2025 08:33:42 +0200 Subject: [PATCH] tmp Signed-off-by: David Hildenbrand --- mm/gup.c | 79 ++++++++++++++++++++++++++------------------------------ 1 file changed, 36 insertions(+), 43 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index 22420f2069ee1..98907ead749c0 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2845,12 +2845,11 @@ static void __maybe_unused gup_fast_undo_dev_pagemap(int *nr, int nr_start, * also check pmd here to make sure pmd doesn't change (corresponds to * pmdp_collapse_flush() in the THP collapse code path). */ -static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, - unsigned long end, unsigned int flags, struct page **pages, - int *nr) +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) { struct dev_pagemap *pgmap = NULL; - int ret = 0; + unsigned long nr_pages = 0; pte_t *ptep, *ptem; ptem = ptep = pte_offset_map(&pmd, addr); @@ -2908,24 +2907,20 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, * details. */ if (flags & FOLL_PIN) { - ret = arch_make_folio_accessible(folio); - if (ret) { + if (arch_make_folio_accessible(folio)) { gup_put_folio(folio, 1, flags); goto pte_unmap; } } folio_set_referenced(folio); - pages[*nr] = page; - (*nr)++; + pages[nr_pages++] = page; } while (ptep++, addr += PAGE_SIZE, addr != end); - ret = 1; - pte_unmap: if (pgmap) put_dev_pagemap(pgmap); pte_unmap(ptem); - return ret; + return nr_pages; } #else @@ -2938,21 +2933,24 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, * get_user_pages_fast_only implementation that can pin pages. Thus it's still * useful to have gup_fast_pmd_leaf even if we can't operate on ptes. */ -static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, - unsigned long end, unsigned int flags, struct page **pages, - int *nr) +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) { return 0; } #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */ -static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr, - unsigned long end, unsigned int flags, struct page **pages, - int *nr) +static unsigned long gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr, + unsigned long end, unsigned int flags, struct page **pages) { + const unsigned long nr_pages = (end - addr) >> PAGE_SHIFT; struct page *page; struct folio *folio; - int refs; + unsigned long i; + + /* See gup_fast_pte_range() */ + if (pmd_protnone(orig)) + return 0; if (!pmd_access_permitted(orig, flags & FOLL_WRITE)) return 0; @@ -2960,33 +2958,30 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr, if (pmd_special(orig)) return 0; - refs = (end - addr) >> PAGE_SHIFT; page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT); - folio = try_grab_folio_fast(page, refs, flags); + folio = try_grab_folio_fast(page, nr_pages, flags); if (!folio) return 0; if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) { - gup_put_folio(folio, refs, flags); + gup_put_folio(folio, nr_pages, flags); return 0; } if (!gup_fast_folio_allowed(folio, flags)) { - gup_put_folio(folio, refs, flags); + gup_put_folio(folio, nr_pages, flags); return 0; } if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) { - gup_put_folio(folio, refs, flags); + gup_put_folio(folio, nr_pages, flags); return 0; } - pages += *nr; - *nr += refs; - for (; refs; refs--) + for (i = 0; i < nr_pages; i++) *(pages++) = page++; folio_set_referenced(folio); - return 1; + return nr_pages; } static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr, @@ -3033,11 +3028,11 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr, return 1; } -static int gup_fast_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, - unsigned long end, unsigned int flags, struct page **pages, - int *nr) +static unsigned long gup_fast_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, + unsigned long end, unsigned int flags, struct page **pages) { - unsigned long next; + unsigned long cur_nr_pages, next; + unsigned long nr_pages = 0; pmd_t *pmdp; pmdp = pmd_offset_lockless(pudp, pud, addr); @@ -3046,23 +3041,21 @@ static int gup_fast_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, next = pmd_addr_end(addr, end); if (!pmd_present(pmd)) - return 0; + break; - if (unlikely(pmd_leaf(pmd))) { - /* See gup_fast_pte_range() */ - if (pmd_protnone(pmd)) - return 0; + if (unlikely(pmd_leaf(pmd))) + cur_nr_pages = gup_fast_pmd_leaf(pmd, pmdp, addr, next, flags, pages); + else + cur_nr_pages = gup_fast_pte_range(pmd, pmdp, addr, next, flags, pages); - if (!gup_fast_pmd_leaf(pmd, pmdp, addr, next, flags, - pages, nr)) - return 0; + nr_pages += cur_nr_pages; + pages += cur_nr_pages; - } else if (!gup_fast_pte_range(pmd, pmdp, addr, next, flags, - pages, nr)) - return 0; + if (nr_pages != (next - addr) >> PAGE_SIZE) + break; } while (pmdp++, addr = next, addr != end); - return 1; + return nr_pages; } static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr, -- 2.50.1 Oh, I might even have found a bug moving away from that questionable "ret==1 means success" handling in gup_fast_pte_range()? Will have to double-check, but likely the following is the right thing to do. From 8f48b25ef93e7ef98611fd58ec89384ad5171782 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Sat, 6 Sep 2025 08:46:45 +0200 Subject: [PATCH] mm/gup: fix handling of errors from arch_make_folio_accessible() in follow_page_pte() In case we call arch_make_folio_accessible() and it fails, we would incorrectly return a value that is "!= 0" to the caller, indicating that we pinned all requested pages and that the caller can keep going. follow_page_pte() is not supposed to return error values, but instead 0 on failure and 1 on success. That is of course wrong, because the caller will just keep going pinning more pages. If we happen to pin a page afterwards, we're in trouble, because we essentially skipped some pages. Fixes: f28d43636d6f ("mm/gup/writeback: add callbacks for inaccessible pages") Signed-off-by: David Hildenbrand --- mm/gup.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index 22420f2069ee1..cff226ec0ee7d 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2908,8 +2908,7 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, * details. */ if (flags & FOLL_PIN) { - ret = arch_make_folio_accessible(folio); - if (ret) { + if (arch_make_folio_accessible(folio)) { gup_put_folio(folio, 1, flags); goto pte_unmap; } -- 2.50.1 -- Cheers David / dhildenb