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b=XPkMRWPzKndUdWW0Tvg+MS0bDZYX40MnzKGYhZGFwdjr1/LLTVPwvqg0KiaKKDgeK 2wvVHsSiwQ7OQR6CroHP3dhgmxVFFJFqlqr30cPlAjtpdPAlZE1Of9vuknCdi1B9AI /IgOLtbqPmLKLrnBfV1oWtICCIBaPJdHVqm3blN8VX+cT9AaJi5wx3a0BaAb+X8r7Z Dv9fQk6YngQobS6xXdsW+hc+Q8uc7qVRS/Erux2faipZ8UtX4+MLOZ5LlKV82ASTCQ YpTwp/RA1DFJGO/qlhsWS8io/FgjmOGlxHBs3vdAaAWC46mPZT9j6j3lvNLa0jh9s5 aw8hin45iKgNA== Message-ID: <015c3268-9c95-4314-b28d-c5e33eb2fb86@kernel.org> Date: Thu, 9 Apr 2026 11:37:10 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range To: Lorenzo Stoakes Cc: xu.xin16@zte.com.cn, hughd@google.com, akpm@linux-foundation.org, chengming.zhou@linux.dev, wang.yaxin@zte.com.cn, yang.yang29@zte.com.cn, michel@lespinasse.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <9950c6c1-f960-58c0-4312-e4f5ac122043@google.com> <20260407142141059pWDasxUAknP5rqvAMl28K@zte.com.cn> <8332aedb-e499-4789-8f46-832df8d60224@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: 9aa7999nyg4mddrhekwkmpnrs7ka3tki X-Rspamd-Queue-Id: 8C26F80010 X-Rspamd-Server: rspam09 X-HE-Tag: 1775727437-333653 X-HE-Meta: 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 JEJjDMgM X20nHieYMBl4DQOT57Gd22jcwk/z0EumBIMw6Wpn3QrFEydYvNlNMrxZOShd0bwkpIJHYDyTV0AFpXy9JYYwsLxxRCVy7RzJkgD1gDlZuNukTejm3UBePQQH6u1j8ZeavvHsh5y5rrGLK1gJ7RcAxwkRxfoUm6c5ftvVXfZ7dzZt/hN56gUKofSZcodvqUZ2OLnKS3jlRxhBJhihkRMS0J454fdvlo/46bKbhB/xp3W4vI8j96Y+4GOrzz75F1mc6m72F107kUnCHULlC+bliyws1x32evnlTmPUvoHhz+sMc+1Pxfq7yLmlkMw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/9/26 11:18, Lorenzo Stoakes wrote: > On Wed, Apr 08, 2026 at 02:57:10PM +0200, David Hildenbrand (Arm) wrote: >> On 4/7/26 11:36, Lorenzo Stoakes (Oracle) wrote: >>> >>> Hmm yeah, we mark them unmergeable but don't update the VMA flags (since using >>> &dummy), so they can just be merged later right? >>> >>> And then the: >>> >>> void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc) >>> { >>> ... >>> const pgoff_t pgoff = rmap_item->address >> PAGE_SHIFT; >>> ... >>> anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root, >>> pgoff, pgoff) { >>> ... >>> } >>> ... >>> } >>> >>> Would _assume_ that folio->pgoff == addr >> PAGE_SHIFT, which will no longer be >>> the case here? >> >> I'm wondering whether we could figure the pgoff out, somehow, so we >> wouldn't have to store it elsewhere. >> >> What we need is essentially what __folio_set_anon() would have done for >> the original folio we replaced. >> >> folio->index = linear_page_index(vma, address); >> >> Could we obtain that from the anon_vma assigned to our rmap_item? >> >> pgoff_t pgoff; >> >> pgoff = (rmap_item->address - anon_vma->vma->vm_start) >> PAGE_SHIFT; >> pgoff += anon_vma->vma->vm_pgoff; > > anon_vma doesn't have a vma field :) it has anon_vma->rb_root which maps to all > 'related' VMAs. Right, anon_vma_chain has. Dammit. > > And we're already looking at what might be covered by the anon_vma by > invoking anon_vma_interval_tree_foreach() on anon_vma->rb_root in [0, > ULONG_MAX). > >> >> It would be the same adjustment everywhere we look in child processes, >> because the moment they would mremap() would be where we would have >> unshared. >> >> Just a thought after reading avc_start_pgoff ... > > One interesting thing here is in the anon_vma_interval_tree_foreach() loop > we check: > > if (addr < vma->vm_start || addr >= vma->vm_end) > continue; > > Which is the same as saying 'hey we are ignoring remaps'. > > But... if _we_ got remapped previously (the unsharing is only temporary), > then we'd _still_ have an anon_vma with an old index != addr >> PAGE_SHIFT, > and would still not be able to figure out the correct pgoff after sharing. > > I wonder if we could just store the pgoff in the rmap_item though? That's what I said elsewhere and what I was trying to avoid here. It's 64bytes, and adding a new item will increase it to 96 bytes IIUC. -- Cheers, David