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(p200300d82f4d0e00029859cc25140052.dip0.t-ipconnect.de. [2003:d8:2f4d:e00:298:59cc:2514:52]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45dda2021dfsm19061165e9.24.2025.09.05.04.38.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 05 Sep 2025 04:38:34 -0700 (PDT) Message-ID: <9fe9f8c7-f59d-4a4b-9668-d3cd2c5a5fc9@redhat.com> Date: Fri, 5 Sep 2025 13:38:32 +0200 Precedence: bulk X-Mailing-List: netdev@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: Lorenzo Stoakes Cc: linux-kernel@vger.kernel.org, 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 , John Hubbard , 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, 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> <5090355d-546a-4d06-99e1-064354d156b5@redhat.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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 05.09.25 13:34, Lorenzo Stoakes wrote: > On Fri, Sep 05, 2025 at 08:41:23AM +0200, David Hildenbrand wrote: >> On 01.09.25 17:03, 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 > > So strange I thought I looked at this...! > >>> --- >>> 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++; >>> folio_set_referenced(folio); >>> return 1; >>> } >> >> Okay, this code is nasty. We should rework this code to just return the nr and receive a the proper >> pages pointer, getting rid of the "*nr" parameter. >> >> For the time being, the following should do the trick: >> >> commit bfd07c995814354f6b66c5b6a72e96a7aa9fb73b (HEAD -> nth_page) >> Author: David Hildenbrand >> Date: Fri Sep 5 08:38:43 2025 +0200 >> >> fixup: mm/gup: remove record_subpages() >> pages is not adjusted by the caller, but idnexed by existing *nr. >> Signed-off-by: David Hildenbrand >> >> diff --git a/mm/gup.c b/mm/gup.c >> index 010fe56f6e132..22420f2069ee1 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -2981,6 +2981,7 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr, >> return 0; >> } >> + pages += *nr; >> *nr += refs; >> for (; refs; refs--) >> *(pages++) = page++; >> @@ -3024,6 +3025,7 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr, >> return 0; >> } >> + pages += *nr; >> *nr += refs; >> for (; refs; refs--) >> *(pages++) = page++; > > This looks correct. > > But. > > This is VERY nasty. Before we'd call record_subpages() with pages + *nr, where > it was clear we were offsetting by this, now we're making things imo way more > confusing. > > This makes me less in love with this approach to be honest. > > But perhaps it's the least worst thing for now until we can do a bigger > refactor... > > So since this seems correct to me, and for the sake of moving things forward > (was this one patch dropped from mm-new or does mm-new just have an old version? > Confused): > > Reviewed-by: Lorenzo Stoakes > > For this patch obviously with the fix applied. > > But can we PLEASE revisit this :) Yeah, I already asked someone internally if he would have time to do some refactorings in mm/gup.c. If that won't work out I shall do it at some point (and the same time reworking follow_page_mask() to just consume the array as well like gup does) -- Cheers David / dhildenb