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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775728514; bh=7Jqv0DZIrzFjI5F0XxPVosyqI7IXF3zzlfSfOKKQ134=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=mXfStgu/3TLzTJSJjLl8FnH2jzRgL7wFbNyTxVRAwxXcBCOJJ80al87IknCfF0YG3 bpgBRvPZ7GuNHwdIltxZhZ9X/8zy5b8VG6o+ludtnWSQQgv6U0ws5i7M/Kaa8nyjol orwEkErAbxpVLwYkk8bKGvKwe/M+cJLWtFrgFjuDWroZINYZ8SKNKYEJkaodZSeTW6 ZUFLypObeT0ujyGXbFc+Z/MXRHp5LNNxRXzz5rlQ6ho4Y2+xiHOz517EOQoV1CIwA+ hPicY2nRCoqew0KAKDO8IWaiCEG0yHyjCuqIy+GjffWdqiETSCciyRvmoIvn8ZNO1R Jx5KErSlJXTRQ== Message-ID: <5401c1d2-5f42-4288-9dad-2b9768b579c7@kernel.org> Date: Thu, 9 Apr 2026 11:55:10 +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 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: BF492160003 X-Stat-Signature: km9rca1z9woe73iefn9z6d1nqhpzs46m X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1775728515-166345 X-HE-Meta: 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 71aM5LjK TYxnAWG+azuwepsHT6ILJ02SjB+lvfrO3xvP7CZ2EJanMdoLDXoTtGbpIeDSrZ1S0mKTxE+U5dSkel2/cVpxx5M8+QsCNuSKhlRsdvE4Vw6p5R+HINMoL7rqX2xuMGuVjWwUUbKU+90alS+K3RAClPVydOa594bVb9KNRFNn6M1SoeQD7I4xEgIm13libutVewC7en/b8cVmZy8Tk44mczg97K0ULViBxa6Kol7YdfGpjQ/GxtfD7ZdJKtDi6i1H9eP/EVoHd/vTcJ96YqtASikxWJehGLYMCpoj778svOBTQWxoV5+uy4ejbuQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/9/26 11:41, David Hildenbrand (Arm) wrote: > On 4/9/26 11:37, David Hildenbrand (Arm) wrote: >> On 4/9/26 11:18, Lorenzo Stoakes wrote: >>> >>> 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). >>> >>> >>> 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. > Hmm, maybe we could do the following. I think the other members are only relevant for the unstable tree. diff --git a/mm/ksm.c b/mm/ksm.c index 7d5b76478f0b..0c6bfed280f7 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -191,12 +191,13 @@ struct ksm_stable_node { * @nid: NUMA node id of unstable tree in which linked (may not match page) * @mm: the memory structure this rmap_item is pointing into * @address: the virtual address this rmap_item tracks (+ flags in low bits) - * @oldchecksum: previous checksum of the page at that virtual address + * @oldchecksum: previous checksum of the page at that virtual address (unstable tree) * @node: rb node of this rmap_item in the unstable tree * @head: pointer to stable_node heading this list in the stable tree * @hlist: link into hlist of rmap_items hanging off that stable_node - * @age: number of scan iterations since creation - * @remaining_skips: how many scans to skip + * @age: number of scan iterations since creation (unstable tree) + * @remaining_skips: how many scans to skip (unstable tree) + * @pgoff: pgoff into @anon_vma where the page is mapped (stable tree) */ struct ksm_rmap_item { struct ksm_rmap_item *rmap_list; @@ -208,9 +209,14 @@ struct ksm_rmap_item { }; struct mm_struct *mm; unsigned long address; /* + low bits used for flags below */ - unsigned int oldchecksum; /* when unstable */ - rmap_age_t age; - rmap_age_t remaining_skips; + union { + struct { + unsigned int oldchecksum; + rmap_age_t age; + rmap_age_t remaining_skips; + }; + pgoff_t pgoff; + }; union { struct rb_node node; /* when node of unstable tree */ struct { /* when listed from stable tree */ @@ -1600,6 +1606,7 @@ static int try_to_merge_with_ksm_page(struct ksm_rmap_item *rmap_item, /* Must get reference to anon_vma while still holding mmap_lock */ rmap_item->anon_vma = vma->anon_vma; + rmap_item->pgoff = linear_page_index(vma, rmap_item->address); get_anon_vma(vma->anon_vma); out: mmap_read_unlock(mm); -- 2.43.0 -- Cheers, David