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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fb497aea6sm231420515e9.4.2025.10.14.02.26.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Oct 2025 02:26:07 -0700 (PDT) Message-ID: <90ed950a-c3bb-46d5-91f9-338f5ca15af6@redhat.com> Date: Tue, 14 Oct 2025 11:26:06 +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] ksm: use range-walk function to jump over holes in scan_get_next_rmap_item To: Pedro Demarchi Gomes , Andrew Morton , craftfever@murena.io Cc: Xu Xin , Chengming Zhou , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20251014055828.124522-1-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: <20251014055828.124522-1-pedrodemargomes@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 14.10.25 07:58, 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. > > To evaluate this change, I created a test that maps a 1 TB virtual area > where only the first and last 10 MB are populated with identical data. > With this patch applied, KSM scanned and merged the region approximately > seven times faster. > > This problem was previously discussed in [1]. > > [1] https://lore.kernel.org/linux-mm/423de7a3-1c62-4e72-8e79-19a6413e420c@redhat.com/ > > Signed-off-by: Pedro Demarchi Gomes > --- > mm/ksm.c | 136 ++++++++++++++++++++++++++++++++----------------------- > 1 file changed, 79 insertions(+), 57 deletions(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 3aed0478fdce..584fd987e8ae 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -2455,15 +2455,80 @@ static bool should_skip_rmap_item(struct folio *folio, > return true; > } > > +struct ksm_walk_private { > + struct page *page; > + struct ksm_rmap_item *rmap_item; > + struct ksm_mm_slot *mm_slot; > +}; > + > +static int ksm_walk_test(unsigned long addr, unsigned long next, struct mm_walk *walk) > +{ > + struct vm_area_struct *vma = walk->vma; > + > + if (!vma || !(vma->vm_flags & VM_MERGEABLE)) The anon_vma check should go in here as well. How can we possibly get !vma? > + return 1; > + return 0; > +} > + > +static int ksm_pte_entry(pte_t *pte, 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 ksm_mm_slot *mm_slot = private->mm_slot; > + pte_t ptent = ptep_get(pte); > + struct page *page = pfn_to_online_page(pte_pfn(ptent)); Oh no. vm_normal_page() > + struct ksm_rmap_item *rmap_item; > + struct folio *folio; > + > + ksm_scan.address = addr; > + > + if (ksm_test_exit(mm)) > + return 1; > + > + if (!page) > + return 0; > + > + folio = page_folio(page); > + if (folio_is_zone_device(folio) || !folio_test_anon(folio)) > + return 0; > + > + folio_get(folio); > + > + flush_anon_page(vma, page, ksm_scan.address); > + flush_dcache_page(page); > + 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; > + > + if (should_skip_rmap_item(folio, rmap_item)) { > + folio_put(folio); > + return 0; > + } > + ksm_scan.address = end; > + private->page = page; > + } else > + folio_put(folio); > + You're under PTL, get_next_rmap_item() will perform an allocation, so that won't work. Observe how the original code worked around that by performing all magic outside of the PTL (folio_walk_end()). When you switch to .pmd_entry() (see below) you will be able to handle it. What you could also try doing is returing page+folio and letting the caller deal with everything starting at the flush_anon_page(). > + private->rmap_item = rmap_item; > + return 1; > +} > + > +struct mm_walk_ops walk_ops = { > + .pte_entry = ksm_pte_entry, > + .test_walk = ksm_walk_test, > + .walk_lock = PGWALK_RDLOCK, > +}; It's more complicated: you'd be remapping each PMD to be mapped by PTEs first, which is not what we want. You'll have to handle pmd_entry instead of pte_entry. > + > 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; > + int nid, ret; > > if (list_empty(&ksm_mm_head.slot.mm_node)) > return NULL; > @@ -2527,64 +2592,21 @@ 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; > - > - if (ksm_test_exit(mm)) > - break; > - > - 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); > - } > - > - if (tmp_page) { > - flush_anon_page(vma, tmp_page, ksm_scan.address); > - flush_dcache_page(tmp_page); > - 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; > - > - if (should_skip_rmap_item(folio, rmap_item)) { > - folio_put(folio); > - goto next_page; > - } > - > - ksm_scan.address += PAGE_SIZE; > - *page = tmp_page; > - } else { > - folio_put(folio); > - } > - mmap_read_unlock(mm); > - return rmap_item; > - } > -next_page: > - ksm_scan.address += PAGE_SIZE; > - cond_resched(); You're dropping all cond_resched(), which will be a problem. > - } > + struct ksm_walk_private walk_private = { > + .page = NULL, > + .rmap_item = NULL, > + .mm_slot = ksm_scan.mm_slot > + }; empty line missing > + ret = walk_page_range(mm, ksm_scan.address, -1, &walk_ops, (void *) &walk_private); > + *page = walk_private.page; > + if (ret) { > + mmap_read_unlock(mm); > + return walk_private.rmap_item; > } > > if (ksm_test_exit(mm)) { -- Cheers David / dhildenb