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b=CpdlWVOzX+elKep7Zbvhs6TzH27OkUDp245LGTRynhswGWCoBwmNL4vqlzB8dBsUH touwC30voN4iDgJjfzzJiherfnqelApY1d7oGWMqiGUsPiH1EYfq0Xu2BoPeeLbXIi EviFYCrrl5tJ8x+37mxe51x1xx/mKC9AaRveaDwRunYcS9cZfEIUellOaLLKeXKBST 0RV1Sc77RXZiz0QdnHqOJHbuDokRqEHV8U77MUp2zEmbcxKlJhIH7+FOTSTAdkzsdv Bi9PseMN7xC3itkBKy8BylIPMPkCJU9EdROqmMYUgqCYA9UJohN0z7RAupDUWI5n8A TUebnjf4vZIQg== Message-ID: Date: Thu, 9 Apr 2026 11:41:46 +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 From: "David Hildenbrand (Arm)" 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> <015c3268-9c95-4314-b28d-c5e33eb2fb86@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 5C2FB4000F X-Stat-Signature: ak7hjzu95kmj6ikakhq5of48jog58gyp X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1775727712-649984 X-HE-Meta: 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 mP/n24ir JUco5TrVtmYAuU/4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/9/26 11:37, David Hildenbrand (Arm) wrote: > On 4/9/26 11:18, Lorenzo Stoakes wrote: >> On Wed, Apr 08, 2026 at 02:57:10PM +0200, David Hildenbrand (Arm) wrote: >>> >>> 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. As we're using a dedicate kmem cache it might "only" add 8 bytes, not sure. Still an undesired increase given that we need that for each entry in the stable/unstable tree. -- Cheers, David