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[2003:d8:2f3f:4b00:ee13:8c22:5cc5:d169]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429952df5c9sm16339288f8f.41.2025.10.27.14.40.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 Oct 2025 14:40:50 -0700 (PDT) Message-ID: <726dcb51-82a7-49a7-a8e5-49bc3eb05dcf@redhat.com> Date: Mon, 27 Oct 2025 22:40:49 +0100 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 v4 2/2] mm: Optimize mremap() by PTE batching To: Dev Jain , akpm@linux-foundation.org Cc: Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, vbabka@suse.cz, jannh@google.com, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, peterx@redhat.com, ryan.roberts@arm.com, mingo@kernel.org, libang.li@antgroup.com, maobibo@loongson.cn, zhengqi.arch@bytedance.com, baohua@kernel.org, anshuman.khandual@arm.com, willy@infradead.org, ioworker0@gmail.com, yang@os.amperecomputing.com, baolin.wang@linux.alibaba.com, ziy@nvidia.com, hughd@google.com References: <20250610035043.75448-1-dev.jain@arm.com> <20250610035043.75448-3-dev.jain@arm.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: <20250610035043.75448-3-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10.06.25 05:50, Dev Jain wrote: > Use folio_pte_batch() to optimize move_ptes(). On arm64, if the ptes > are painted with the contig bit, then ptep_get() will iterate through all 16 > entries to collect a/d bits. Hence this optimization will result in a 16x > reduction in the number of ptep_get() calls. Next, ptep_get_and_clear() > will eventually call contpte_try_unfold() on every contig block, thus > flushing the TLB for the complete large folio range. Instead, use > get_and_clear_full_ptes() so as to elide TLBIs on each contig block, and only > do them on the starting and ending contig block. > > For split folios, there will be no pte batching; nr_ptes will be 1. For > pagetable splitting, the ptes will still point to the same large folio; > for arm64, this results in the optimization described above, and for other > arches (including the general case), a minor improvement is expected due to > a reduction in the number of function calls. > > Signed-off-by: Dev Jain > --- > mm/mremap.c | 39 ++++++++++++++++++++++++++++++++------- > 1 file changed, 32 insertions(+), 7 deletions(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index 180b12225368..18b215521ada 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -170,6 +170,23 @@ static pte_t move_soft_dirty_pte(pte_t pte) > return pte; > } > > +static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr, > + pte_t *ptep, pte_t pte, int max_nr) > +{ > + const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; > + struct folio *folio; > + > + if (max_nr == 1) > + return 1; > + > + folio = vm_normal_folio(vma, addr, pte); > + if (!folio || !folio_test_large(folio)) > + return 1; > + > + return folio_pte_batch(folio, addr, ptep, pte, max_nr, flags, NULL, > + NULL, NULL); > +} Dev, I think there is another bug hiding in here. That function ignores the writable bit, which is not what you need here, in particular for anonymous folios in some cases. Later set_ptes() could end up marking ptes writable that were not writable before, which is bad (at least for anonymous folios, maybe also for pagecache folios). I think you really must respect the writable bit through something like FPB_RESPECT_WRITE. I patched out the "pte_batch_hint(ptep, pte) == 1" check we have upstream to make it reproduce on x86_64, but the following reproducer should likely reproduce on aarch64 without further kernel modifications. # ./mremap BUG: Memory modified #define _GNU_SOURCE #include #include #include #include #include #include #include #include #include #include #include static size_t pagesize; static size_t thpsize; static int pagemap_fd; static uint64_t pagemap_get_entry(int fd, char *start) { const unsigned long pfn = (unsigned long)start / getpagesize(); uint64_t entry; int ret; ret = pread(fd, &entry, sizeof(entry), pfn * sizeof(entry)); if (ret != sizeof(entry)) { perror("reading pagemap failed"); exit(-1); } return entry; } static bool pagemap_is_populated(int fd, char *start) { return pagemap_get_entry(fd, start) & ((1ULL << 62) | (1ULL << 63)); } unsigned long pagemap_get_pfn(int fd, char *start) { uint64_t entry = pagemap_get_entry(fd, start); /* If present (63th bit), PFN is at bit 0 -- 54. */ if (entry & (1ULL << 63)) return entry & 0x007fffffffffffffull; return -1ul; } int main(void) { char *mem, *mmap_mem, *tmp, *mremap_mem = MAP_FAILED; size_t size, mmap_size; int ret; pagesize = getpagesize(); thpsize = 2 * 1024 * 1024ul; pagemap_fd = open("/proc/self/pagemap", O_RDONLY); if (pagemap_fd < 0) { perror("opening pagemap failed"); return -1; } /* For alignment purposes, we need twice the thp size. */ mmap_size = 2 * thpsize; mmap_mem = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); if (mmap_mem == MAP_FAILED) { perror("mmap() failed"); return -1; } /* We need a THP-aligned memory area. */ mem = (char *)(((uintptr_t)mmap_mem + thpsize) & ~(thpsize - 1)); ret = madvise(mem, thpsize, MADV_HUGEPAGE); if (ret) { perror("MADV_HUGEPAGE failed"); return -1; } /* * Try to populate a THP. Touch the first sub-page and test if we get * another sub-page populated automatically. */ mem[0] = 0; if (!pagemap_is_populated(pagemap_fd, mem + pagesize)) { perror("Did not get a THP populated"); return -1; } /* Share only the first page of the THP. */ if (madvise(mem, pagesize, MADV_DONTFORK)) { perror("MADV_DONTFORK failed"); return -1; } ret = fork(); if (ret < 0) { perror("fork() failed"); return -1; } else if (!ret) { while (true) { char c = *((volatile char *)(mem + pagesize)); if (c) { fprintf(stderr, "BUG: Memory modified\n"); exit(-2); } } } /* Merge VMAs again. */ if (madvise(mem, pagesize, MADV_DOFORK)) { perror("MADV_DONTFORK failed"); return -1; } /* Mremap multiple pages. */ mremap_mem = mmap(NULL, 2 * pagesize, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); if (mem == MAP_FAILED) { perror("mmap() failed"); return -1; } tmp = mremap(mem, 2 * pagesize, 2 * pagesize, MREMAP_MAYMOVE | MREMAP_FIXED, mremap_mem); if (tmp != mremap_mem) { perror("mremap() failed"); return -1; } /* Write into both pages. The child should never see these updates. */ memset(mremap_mem, 1, 2 * pagesize); pause(); } -- Cheers David / dhildenb