From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 250AA3806AD for ; Tue, 9 Jun 2026 07:44:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780991088; cv=none; b=tw+cCjsTyKrzAElAT7OM2i3vadjiEkW0k1XyFmbd6BjTB25e7AfwEVAG2Y7ARZuhPxpaKtUw2FuXeWKnCFkErxWqub1LRpvWCHZIydrNP3s6XXtwCa29KeMfZWylyQPJWSDo8GTvXHPqpUhQsETa3Uh4uydEoFWeNojwP52tZH8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780991088; c=relaxed/simple; bh=vuGTkAbua7AQQw6C7ODqSYBSzLiRs8/N1NAwFmkJ3Pg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=NIVaf1ybNOUG3oTk8XPSDe8Lo94UjXcXGMIrMHkfqvtCFxlGLB/jT+78AN+ypNHRgugTQOE/W1d2w1uBwy6oXJi4ZzSISyqDRy3m8fazX74+BqVaxReyvJnqjilKllVZfemxPG7KloxtVyiZAJoR93DTyiYNrZq0gakgNMEjdFA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=XDopFgC1; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="XDopFgC1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8B8021F00893; Tue, 9 Jun 2026 07:44:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780991080; bh=M2jMGukV8yyAtn/4N/ehPeGeMg24FUo6LtfBjjS6QVk=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=XDopFgC1HfBlX5774qviVQckb2+Afen8JWH9vOCweIzsxR8dUgxIy0ZnE3zE5oOt7 DrBi3SnhxooN9BjVxFZTaUcc92Fk7Ul0VF++nAAnwzZQJiQFz5VVbUb7CJPrpdfdN6 fIZczgpMWNRzs+owIV6oig3qLEOVO5U6JbQY0d+AGGt9MTsw8K9WE+ZL1ooCWLg/kK /uQlogS/9aI3bLqcZKxpNddmOqraUpa17+U6s4S6xAKnfPdm1h3ZweYAE2nbvUVgrz hVGLJOsA2QSjs1PRXvw/56GoYa3w8/wuIafVI/9IpZA3x4HoYTcY+LeO6pL6kVqCyp xqkynbAPM6iiw== Message-ID: <84886648-8cbb-49db-a816-de3e0d359ed0@kernel.org> Date: Tue, 9 Jun 2026 09:44:37 +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 v8 1/3] ksm: add linear_page_index into ksm_rmap_item To: xu.xin16@zte.com.cn, akpm@linux-foundation.org Cc: chengming.zhou@linux.dev, hughd@google.com, wang.yaxin@zte.com.cn, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ljs@kernel.org References: <20260609124031750t-yHeQFCJs4gAeOmsvEJx@zte.com.cn> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/9/26 06:40, xu.xin16@zte.com.cn wrote: > From: xu xin > > As preparation for KSM rmap optimizations, let's track the original > linear_page_index() of a de-duplicated page in its ksm_rmap_item, so we can > efficiently search for the page in an address space, avoiding scanning the > entire address space. This was previously discussed in [1, 2]. > > To avoid growing ksm_rmap_item, let's squeeze it into the existing > structure by overlying some members (oldchecksum, age, remaining_skips) > that are only relevant while on the unstable tree. The new entry will > only be relevant for entries in the stable tree. > > However, as the age information is read by should_skip_rmap_item() with the > smart-scanning approach even while we have an entry in the stable tree, but > the page changes (no longer a KSM page, for example due to COW), we have to > change the handling there a bit. > > We'll calculate the linear page index in try_to_merge_with_ksm_page(), when > adding it to the stable tree, and reset the index (to reset overlayed data) > when removing an item from the stable tree -- in > remove_rmap_item_from_tree(), remove_node_from_stable_tree() and > break_cow(). > > To be specially clarified, the reason for resetting the stored index at > break_cow() is: > > - When a page successfully becomes a KSM page (i.e., after > stable_tree_append() sets STABLE_FLAG), both anon_vma and the index are > stored and remain valid. > > - However, during the merging process, there are several failure paths > where we already prepared an rmap item to be added to the stable tree, > but must revert that as some part of the merge process failed. Examples > include: > * The second call to try_to_merge_with_ksm_page() fails in > try_to_merge_two_pages(). > * stable_tree_insert() fails in cmp_and_merge_page(). > In such cases, break_cow() is invoked to break the COW mapping and > discard the KSM state. > > Currently, break_cow() already contains a put_anon_vma(rmap_item->anon_vma) > to release the reference taken during the aborted merge. Because the index > is logically paired with anon_vma (both are only meaningful when the > rmap_item is in a stable state), it must also be cleared (or reset) in > break_cow() to avoid leaving stale linear_page_index values that could > confuse subsequent rmap walks or scanning logic. > > [1] https://lore.kernel.org/all/adTPQSb-qSSHviJN@lucifer/ > [2] https://lore.kernel.org/all/202604091806051535BJWZ_FTtdIm3Snk24ei_@zte.com.cn/ [...] > +/* > + * break_cow: actively break COW, replacing the KSM page by a fresh anonymous > + * page. This is called when rmap_item has not yet become stable, but page > + * has been merged. > + */ > static void break_cow(struct ksm_rmap_item *rmap_item) > { > struct mm_struct *mm = rmap_item->mm; > @@ -787,6 +798,11 @@ static void break_cow(struct ksm_rmap_item *rmap_item) > * to undo, we also need to drop a reference to the anon_vma. > */ > put_anon_vma(rmap_item->anon_vma); > + /* > + * Reset linear_page_index that might overlay age-related > + * information. (it's still unstable node) > + */ > + rmap_item->linear_page_index = 0; Sashiko comments that, on 32bit, it is not overlaying age-related information. So setting it to 0 won't clear the age. Which is what we document with the "might". On 32bit, it simply behaves the way it was before (no reset of age-related information here). We could move oldchecksum below remaining_skips to clear age-related information consistently. Not sure whether that is really worth it. I would even say that we can just stop supporting KSM on 32bit completely. > > mmap_read_lock(mm); > vma = find_mergeable_vma(mm, addr); > @@ -899,6 +915,8 @@ static void remove_node_from_stable_tree(struct ksm_stable_node *stable_node) > VM_BUG_ON(stable_node->rmap_hlist_len <= 0); > stable_node->rmap_hlist_len--; > put_anon_vma(rmap_item->anon_vma); > + /* Reset linear_page_index that might overlay age-related information. */ > + rmap_item->linear_page_index = 0; > rmap_item->address &= PAGE_MASK; > cond_resched(); > } > @@ -1052,6 +1070,8 @@ static void remove_rmap_item_from_tree(struct ksm_rmap_item *rmap_item) > stable_node->rmap_hlist_len--; > > put_anon_vma(rmap_item->anon_vma); > + /* Reset linear_page_index that might overlay age-related information. */ > + rmap_item->linear_page_index = 0; > rmap_item->head = NULL; > rmap_item->address &= PAGE_MASK; > > @@ -1598,8 +1618,16 @@ static int try_to_merge_with_ksm_page(struct ksm_rmap_item *rmap_item, > /* Unstable nid is in union with stable anon_vma: remove first */ > remove_rmap_item_from_tree(rmap_item); > > - /* Must get reference to anon_vma while still holding mmap_lock */ > + /* > + * Must get reference to anon_vma while still holding mmap_lock. I think this sentence should go, and instead ... > + * Must can only reference the VMA while still holding the mmap This one should become: "We can consider the VMA only while still holding the mmap lock, so ... > + * lock, so reference the anon_vma and calculate the linear page > + * index early, before stable_tree_append(). If anything goes > + * wrong that prevents the rmap_item from being added to the > + * stable_tree, break_cow() will clean it up. > + */ > rmap_item->anon_vma = vma->anon_vma; > + rmap_item->linear_page_index = linear_page_index(vma, rmap_item->address); > get_anon_vma(vma->anon_vma); > out: > mmap_read_unlock(mm); > @@ -2458,6 +2486,13 @@ static bool should_skip_rmap_item(struct folio *folio, > if (folio_test_ksm(folio)) > return false; Thinking about the overlay once more, I'm trying to assess what it means when we clear rmap_item->checksum. We'd do that now in: (a) remove_node_from_stable_tree(): We had a stable node -> KSM page, but something changed. (b) remove_rmap_item_from_tree(): Same as (a) (c) break_cow(): we have to unshare, either because insertion into the stable tree failed, or because we would have only a single PTE mapping the KSM folio after a failed merge. For (c), I guess if we'd have to, we could remember the checksum while processing the rmap_item. Clearing rmap_item->checksum implies that cmp_and_merge_page() would refuse to merge one round. Having rmap_item->checksum cleared is just like allocating a fresh rmap_item. So it will fix itself up during the next scan. So my best guess is that this is alright. Hoping for no surprises in corner cases Acked-by: David Hildenbrand (Arm) -- Cheers, David