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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fb483bcf9sm255457535e9.6.2025.10.14.08.59.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Oct 2025 08:59:45 -0700 (PDT) Message-ID: <77b69bcb-6df0-4c3a-bb7c-a003fd51d292@redhat.com> Date: Tue, 14 Oct 2025 17:59:45 +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 v2] ksm: use range-walk function to jump over holes in scan_get_next_rmap_item To: Pedro Demarchi Gomes , Andrew Morton Cc: Xu Xin , Chengming Zhou , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20251014151126.87589-1-pedrodemargomes@gmail.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: <20251014151126.87589-1-pedrodemargomes@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 14.10.25 17:11, Pedro Demarchi Gomes wrote: > Currently, scan_get_next_rmap_item() walks every page address in a VMA > to locate mergeable pages. This becomes highly inefficient when scanning > large virtual memory areas that contain mostly unmapped regions. > > This patch replaces the per-address lookup with a range walk using > walk_page_range(). The range walker allows KSM to skip over entire > unmapped holes in a VMA, avoiding unnecessary lookups. > This problem was previously discussed in [1]. > > Changes since v1 [2]: > - Use pmd_entry to walk page range > - Use cond_resched inside pmd_entry() > - walk_page_range returns page+folio > > [1] https://lore.kernel.org/linux-mm/423de7a3-1c62-4e72-8e79-19a6413e420c@redhat.com/ > [2] https://lore.kernel.org/linux-mm/20251014055828.124522-1-pedrodemargomes@gmail.com/ > > Signed-off-by: Pedro Demarchi Gomes > --- [...] > + > +static int ksm_pmd_entry(pmd_t *pmd, unsigned long addr, > + unsigned long end, struct mm_walk *walk) > +{ > + struct mm_struct *mm = walk->mm; > + struct vm_area_struct *vma = walk->vma; > + struct ksm_walk_private *private = (struct ksm_walk_private *) walk->private; > + struct folio *folio; > + pte_t *start_pte, *pte, ptent; > + spinlock_t *ptl; > + int ret = 0; > + > + start_pte = pte = pte_offset_map_lock(mm, pmd, addr, &ptl); > + if (!start_pte) { > + ksm_scan.address = end; > + return 0; > + } Please take more time to understand the details. If there is a THP there you actually have to find the relevant page. > + > + for (; addr < end; pte++, addr += PAGE_SIZE) { > + ptent = ptep_get(pte); > + struct page *page = vm_normal_page(vma, addr, ptent); > + ksm_scan.address = addr; Updating that value from in here is a bit nasty. I wonder if you should rather make the function also return the address of the found page as well. In the caller, if we don't find any page, there is no need to update the address from this function I guess. We iterated the complete MM space in that case. > + > + if (ksm_test_exit(mm)) { > + ret = 1; > + break; > + } > + > + if (!page) > + continue; > + > + folio = page_folio(page); > + if (folio_is_zone_device(folio) || !folio_test_anon(folio)) > + continue; > + > + ret = 1; > + folio_get(folio); > + private->page = page; > + private->folio = folio; > + private->vma = vma; > + break; > + } > + pte_unmap_unlock(start_pte, ptl); > + > + cond_resched(); > + return ret; > +} > + > +struct mm_walk_ops walk_ops = { > + .pmd_entry = ksm_pmd_entry, > + .test_walk = ksm_walk_test, > + .walk_lock = PGWALK_RDLOCK, > +}; > + > static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page) > { > struct mm_struct *mm; > struct ksm_mm_slot *mm_slot; > struct mm_slot *slot; > - struct vm_area_struct *vma; > struct ksm_rmap_item *rmap_item; > - struct vma_iterator vmi; > int nid; > > if (list_empty(&ksm_mm_head.slot.mm_node)) > @@ -2527,64 +2595,40 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page) > > slot = &mm_slot->slot; > mm = slot->mm; > - vma_iter_init(&vmi, mm, ksm_scan.address); > > mmap_read_lock(mm); > if (ksm_test_exit(mm)) > goto no_vmas; > > - for_each_vma(vmi, vma) { > - if (!(vma->vm_flags & VM_MERGEABLE)) > - continue; > - if (ksm_scan.address < vma->vm_start) > - ksm_scan.address = vma->vm_start; > - if (!vma->anon_vma) > - ksm_scan.address = vma->vm_end; > - > - while (ksm_scan.address < vma->vm_end) { > - struct page *tmp_page = NULL; > - struct folio_walk fw; > - struct folio *folio; > +get_page: > + struct ksm_walk_private walk_private = { > + .page = NULL, > + .folio = NULL, > + .vma = NULL > + }; > > - if (ksm_test_exit(mm)) > - break; > + walk_page_range(mm, ksm_scan.address, -1, &walk_ops, (void *) &walk_private); > + if (walk_private.page) { > + flush_anon_page(walk_private.vma, walk_private.page, ksm_scan.address); > + flush_dcache_page(walk_private.page); Keep working on the folio please. > + rmap_item = get_next_rmap_item(mm_slot, > + ksm_scan.rmap_list, ksm_scan.address); > + if (rmap_item) { > + ksm_scan.rmap_list = > + &rmap_item->rmap_list; > > - folio = folio_walk_start(&fw, vma, ksm_scan.address, 0); > - if (folio) { > - if (!folio_is_zone_device(folio) && > - folio_test_anon(folio)) { > - folio_get(folio); > - tmp_page = fw.page; > - } > - folio_walk_end(&fw, vma); > + ksm_scan.address += PAGE_SIZE; > + if (should_skip_rmap_item(walk_private.folio, rmap_item)) { > + folio_put(walk_private.folio); > + goto get_page; Can you make that a while() loop to avoid the label? -- Cheers David / dhildenb