From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794305128; b=qfzwGt/WHxGUOXV3rG15Q2aCmYmQnb+yfiAGJx4mqc28CpyyKB5BRYwZ01pqfgJoJbk2 9zp8W7u6dMeKpS0XDTOq1pUNu7XNYy7+RPJoICuSamCBPK7f9V92YQmwDpGG9VBZeF4t0 UF/FuTTFDZbHzBPrI9ws/3juruXD+sWiV53u5eKcRMYrZ4QcTFS+OTgGjkBITu46KgdEL /iQ0ZjdvKR1lBMR4FUNk3cvdK9I6g/DSrSsYyxmSNCXIOuDnY62j3ZQXEyDQniInqJXwR qbkCFgg0PZLn2ya7gp2LRmy4yobdbe0ETrv//5oWyw/DEORbAmNsADOuBMp+GYLVDsw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794305128; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=n7JHToNvmvovrxsCT+NsYxHL5scA7i94JGOHZjSN1bs=; b=CO1fHrHDqlVOv8bctm7n1qABBv9X8Ld9r8JS/Q0RjLKARtZ2hME6v+PqNpYEpo3VX6OZ +akMEChCTFJYW/ku09bVv90a9yBg35or1Ae0y3lhCx4yPNw0ZgfhutoNZ3tlVvGNqtbUc uhiKDM/UaliCbV9auENIytpXNRAc4dFovnIgIsIgG7qxwxAtOK0CVidtQKptc7BlUxjiv Km5pNodEw7w/3iHVEmrr/0Sa7SD+UyJTM1GfTQWVYjhpF7URcyneKBCcfBnJwGJF88ih/ m2frblpYXKPYJnfw/JISYj3x2i/35Uo0Jk6NDNh01EalirCh3TFbR+Cm4euDSvi0akA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=FkxRLQKe; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 73F9D60A2603; Tue, 10 Nov 2026 10:05:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=FkxRLQKe Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7CD241F008A4; Tue, 10 Nov 2026 10:05:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794305128; bh=n7JHToNvmvovrxsCT+NsYxHL5scA7i94JGOHZjSN1bs=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=FkxRLQKe2exp2p0YBuRQCorQW8fK+JO16bZr31sSfbbg66nXkxqHsgponQcdBK0jX DavDBSPy277+yZXKBuTxwIBFw44WRdpUQ63FuWtJJLKGi7WUV5STiwvF1mgjz+D+s9 2R3Zw5dfR3kp3fgXtxCypQsKsIjuRwRiI2Q5KT525LpWnU9ICcu+uzDOiLYFpvxkAU vtn+DgLDVhMLMXx3dM1BGJ6IVMrSmMzlUlOag0e4HVtvDX8dhJHzkpfejT26Pf9LSJ odI8MM+xQgHW8NPsi+f9yRInvQzSygij6hL4uFUzd9HXeq1XBPygQHUrwSKyNM524Q LhEEWntTWrKqw== Date: Tue, 10 Nov 2026 10:05:20 +0000 From: "Lorenzo Stoakes (ARM)" To: "Xu Xin (vivo)" Cc: akpm@linux-foundation.org, chengming.zhou@linux.dev, corbet@lwn.net, david@kernel.org, link@vivo.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, odys.yuan@gmail.com, rppt@kernel.org, surenb@google.com, tujinjiang@huawei.com, vbabka@kernel.org, xiao.wang@vivo.com Subject: Re: [PATCH 2/4] ksm: skip rmap_item allocation for non-zero pages under only_merge_zero_pages Message-ID: References: <20261110074312.219847-1-xu.xin@linux.dev> <20261110074451.220524-1-xu.xin@linux.dev> <20261110074451.220524-2-xu.xin@linux.dev> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261110074451.220524-2-xu.xin@linux.dev> On Tue, Nov 10, 2026 at 03:44:49PM +0800, Xu Xin (vivo) wrote: > When only_merge_zero_pages is enabled, ksmd only ever merges empty > (zeroed) anonymous pages with the kernel zero page. A page that carries > data can never be merged, yet the current code still allocates an > rmap_item for it in scan_get_next_rmap_item() before cmp_and_merge_page() > decides to skip it. > > On workloads with a large amount of non-mergeable anonymous memory this > is a real cost: every scanned non-zero page pays for an rmap_item > allocation, and the rmap_items accumulate in the per-mm pending list. > For example, a MySQL instance with hundreds of gigabytes of anonymous > memory (mostly buffer pool pages that cannot be merged) ends up with > ksmd creating rmap_item metadata for each candidate page; the metadata > overhead can wipe out any gain from the zero-page merging and even turn > the net benefit negative. > > Add should_skip_non_zero_page(), called in scan_get_next_rmap_item() before > get_next_rmap_item(), which, when only_merge_zero_pages is enabled, > checks whether the candidate page is all zeros. A non-zero page is > skipped entirely: no rmap_item is allocated for it, and any stale > pending rmap_items below it are freed (as get_next_rmap_item() would). > The page is reported back to ksm_do_scan() as ERR_PTR(-EAGAIN), which > counts it as scanned and continues, so the pages_to_scan budget is > preserved. > > The all-zero check (pages_identical() against the kernel zero page) reads > the whole page. For a non-zero page this is a net win: the extra page read > is cheaper than the rmap_item allocation and the stable/unstable tree > operations it would otherwise trigger. > > A page that is zero now but is written before cmp_and_merge_page() runs > is still handled correctly, because the checksum-based decision in > cmp_and_merge_page() (added by the previous patch) is the authoritative > one. > Please re-write this in your own words. It's rambling on and never gets to the point of _what this patch does and why_. I'm not interested in reading a novel to understand the code :) It seems very LLM generated, LLMs are _terrible_ at writing commit messages and commits, please re-write all of that. > Signed-off-by: Xu Xin (vivo) > Co-developed-by: Jinjiang Tu > Signed-off-by: Jinjiang Tu Why are this and the other patches in-reply-to 1/4? That's not how you send a series in mm. > --- > mm/ksm.c | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 82 insertions(+) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 49f3728a3d03..e641c37ee096 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -2535,6 +2535,69 @@ static bool should_skip_rmap_item(struct folio *folio, > return true; > } > > +/* > + * Determines whether a candidate page should be skipped entirely when > + * only_merge_zero_pages is enabled, before an rmap_item is allocated for > + * it. > + * > + * A page that is not all zeros carries data and can never be merged with > + * the kernel zero page, so there is no point in allocating an rmap_item > + * for it. Skipping it here avoids the rmap_item metadata cost that full > + * KSM would otherwise pay for every non-mergeable page. > + * > + * @rmap_list: head of the per-mm pending rmap_item list > + * @addr: address of the candidate page > + * @folio: folio containing the candidate page > + * @page: the candidate page > + * > + * Returns true if the page should be skipped (and any stale pending > + * rmap_items below @addr are freed), false if it should be processed. > + */ This is a really weird mix of kdoc style + not kdoc style while not being a kdoc, for a static function? > +static bool should_skip_non_zero_page(struct ksm_rmap_item **rmap_list, > + unsigned long addr, struct folio *folio, > + struct page *page) > +{ > + struct ksm_rmap_item *rmap_item; > + > + if (!ksm_only_merge_zero_pages) > + return false; > + > + /* > + * Pages already merged by KSM are handled by cmp_and_merge_page(); > + * do not skip them here. > + */ > + if (folio_test_ksm(folio)) > + return false; > + > + /* > + * An all-zero page is exactly what we want to merge with the kernel > + * zero page; process it. > + */ > + if (pages_identical(page, ZERO_PAGE(0))) > + return false; > + > + /* > + * The page carries data: free any pending rmap_items that are below > + * it (they are stale, as get_next_rmap_item() would), and skip it so > + * no rmap_item is allocated for it. > + * > + * Compare the masked address, unlike get_next_rmap_item() which > + * compares the raw rmap_item->address (which may carry the > + * UNSTABLE/STABLE_FLAG bits). That is intentional: a pending > + * rmap_item at the same address as a non-zero page is stale and must > + * be freed here, not returned for processing. > + */ We don't need a prose version of the code, and it reads like AI slop, we don't want AI slop comments, thanks. > + while (*rmap_list) { > + rmap_item = *rmap_list; Why is rmap_item function-scoped when it could be block-scoped here... > + if ((rmap_item->address & PAGE_MASK) > addr) > + break; > + *rmap_list = rmap_item->rmap_list; > + remove_rmap_item_from_tree(rmap_item); > + free_rmap_item(rmap_item); > + } This code is very ugly. Newline... > + return true; > +} > + > struct ksm_next_page_arg { > struct folio *folio; > struct page *page; > @@ -2737,6 +2800,16 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page) > if (tmp_page) { > flush_anon_page(vma, tmp_page, ksm_scan.address); > flush_dcache_page(tmp_page); > + > + if (should_skip_non_zero_page(ksm_scan.rmap_list, > + ksm_scan.address, folio, > + tmp_page)) { > + folio_put(folio); > + ksm_scan.address += PAGE_SIZE; > + mmap_read_unlock(mm); > + return ERR_PTR(-EAGAIN); > + } > + I don't love just shoving more logic into an already massive function. No to this. At some point technical debt has to be paid, clean up this code first please. > rmap_item = get_next_rmap_item(mm_slot, > ksm_scan.rmap_list, ksm_scan.address); > if (rmap_item) { > @@ -2839,6 +2912,15 @@ static void ksm_do_scan(unsigned int scan_npages) > rmap_item = scan_get_next_rmap_item(&page); > if (!rmap_item) > return; > + if (rmap_item == ERR_PTR(-EAGAIN)) { > + /* > + * A non-zero page was skipped under > + * only_merge_zero_pages: it was examined but not > + * merged, so count it as scanned and keep going. > + */ Similarly schloppy. > + ksm_pages_scanned++; > + continue; > + } > cmp_and_merge_page(page, rmap_item); > put_page(page); > ksm_pages_scanned++; > -- > 2.34.1 > All in all adding far too much complexity in a cycle which is fix-focused... -- Cheers, Lorenzo