From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DFFA0374185 for ; Thu, 9 Apr 2026 09:41:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775727712; cv=none; b=Gzethxq1lh6HQaI1P0Vmm6zWUz0NHfIjIa/tVkV4qoF+hK9c9epKLC8xEzwpZSxfRVB7clacDteqbrH535/2lz6EwaPWlSWZQToWUVVSsifC+iMN4fOI3EY1aFRXoQBdiXch2ERkM908ccwIKaWOFHEvROQyzgBtCk+f5kmZ/nY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775727712; c=relaxed/simple; bh=9OROoV0rWGe0sK3wBROT37WhokeK5lFEn2PwBB2BcG8=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=rAA5Oo/Gu/QL72aUH4TpiqZWvGfmX+z37Xpg/06sUivsSamuMpYlzko15eaheQQb82fm+akfqcyFSQPKwKZVfqxpl3NHZIwUoZ/RRRNQBNvSRRSaNEtizasVYYGQBHCB9V9N9Q/4a2M+KIV7Q2gYT+iphd8JMXuWq74WW1e14Z4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=CpdlWVOz; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="CpdlWVOz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D3ACAC4CEF7; Thu, 9 Apr 2026 09:41:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775727711; bh=9OROoV0rWGe0sK3wBROT37WhokeK5lFEn2PwBB2BcG8=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=CpdlWVOzX+elKep7Zbvhs6TzH27OkUDp245LGTRynhswGWCoBwmNL4vqlzB8dBsUH touwC30voN4iDgJjfzzJiherfnqelApY1d7oGWMqiGUsPiH1EYfq0Xu2BoPeeLbXIi EviFYCrrl5tJ8x+37mxe51x1xx/mKC9AaRveaDwRunYcS9cZfEIUellOaLLKeXKBST 0RV1Sc77RXZiz0QdnHqOJHbuDokRqEHV8U77MUp2zEmbcxKlJhIH7+FOTSTAdkzsdv Bi9PseMN7xC3itkBKy8BylIPMPkCJU9EdROqmMYUgqCYA9UJohN0z7RAupDUWI5n8A TUebnjf4vZIQg== Message-ID: Date: Thu, 9 Apr 2026 11:41:46 +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 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 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