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smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PB6NWsR4; 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="PB6NWsR4" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 702151F00893; Tue, 9 Jun 2026 08:13:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780992828; bh=F3/jNBu5N7OCPMcWMcRwgTuxydIWRqWnD8HUI8iOmjs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=PB6NWsR4+oVPktwfSl2DPOt9yWl9YbtkJepNVYv6U7Wjormv0lqP1TJEto6Ojbd8e F8+uBW9J43tW8SI4U62b+6gEkza+rScAxWTLZbJ970WWqbjDcFwEDVS0lmbTyV/Goz CMVlyoEz+l9WK8j9o6W6ME9gY6eMVVn5YIIukI7T+mUL3TZs+FlVszUuvYtEE7lrA2 a5xVkN2hKvFI7UKhH93mv/WT5DXi706A+Ro0syhpvXHuMwDiBKQBFs/Bz10A0eegAC tcQ3y/AEa/o/p5rwIFarRgGv4n9wk9RupiImfc3gP+dg37RmbobFwYh3X5GC4KGFfJ TJm6NmAUjb9zQ== Message-ID: Date: Tue, 9 Jun 2026 10:13:45 +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 2/3] ksm: Optimize rmap_walk_ksm by passing a suitable page index 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: <20260609124703736MQ6Jkpc_7wR7HaeDDPk6k@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: 8bit On 6/9/26 06:47, xu.xin16@zte.com.cn wrote: > From: xu xin > > User impact / Why this matters to Linux users > ============================================= > When a system runs with KSM enabled and memory becomes tight, KSM pages > may be swapped out or migrated. The kernel then performs a reverse map > walk by rmap_walk_ksm to locate all page table entries that reference > these pages. If A large number of unrelated VMAs can attach to a single > anon_vma related with this KSM page, then rmap_walk might be severe > performance bottleneck. In our embedded test environment, we observed > ~20,000 VMAs sharing one anon_vma without any fork – purely from VMA > splits， which cause 200~700ms duration of rmap_walk_ksm. > > When one of those VMAs mapped a KSM page, then this KSM page's rmapping > will become bottleneck with hold its anon_vma lock for a long time. The > anon_vma lock is not only used by KSM; it is a core lock protecting the > VMA interval tree and is acquired by many critical memory operations: > > • Page faults: do_anonymous_page(), do_wp_page() (during COW) > • Memory reclaim: try_to_unmap() > • Page migration & compaction: migrate_pages(), compact_zone() > • mlock / munlock: mlock_fixup() > • Process exit: exit_mmap() (tearing down VMAs) > • Cgroup memory accounting: mem_cgroup_move_charge() > > If one thread holds the anon_vma lock for hundreds of milliseconds > because of an inefficient KSM rmap walk, any other thread that tries to > acquire the same lock (e.g., an application taking a page fault, kswapd > reclaiming pages, or a migration thread) will block. This leads to > stalled application threads, increased latency spikes, and in extreme > cases container timeouts or watchdog triggers. > > This patch reduces the worst-case anon_vma lock hold time during KSM > rmap walk from >500 ms to <1 ms, thereby almost eliminating this > source of lock contention and improving system responsiveness under > memory pressure. > > Real-world examples: > ==================== > - JVM / Go runtime: These use mmap for heap regions and later call > mprotect(PROT_NONE) for garbage collection barriers or guard pages, > splitting the original VMA into thousands of small pieces over time. > > - Database engines (MySQL, PostgreSQL): Large shared memory buffers > or anonymous mappings are managed with madvise(MADV_DONTNEED) to > release specific pages, which also splits VMAs. > > * Why the benchmark numbers are realistic: We observed ~20,000 VMAs The "*" at the start looks odd (the list above used "-", and I assume this should be separate from the list). > sharing one anon_vma on a production system running a Java application > with KSM enabled. The lock hold time before the patch was measured at > 228 ms (max) during rmap walks triggered by memory compaction and page > migration. The benchmark reproduces that VMA count and lock‑hold > behavior in a controlled environment. > > Root Cause > ========== > Through local debugging trace analysis, we found that most of the latency > of rmap_walk_ksm occurs within anon_vma_interval_tree_foreach(), leading > to an excessively long hold time on the anon_vma lock (even reaching 500ms > or more), which in turn causes upper-layer applications (waiting for the > anon_vma lock) to be blocked for extended periods. > > Further investigation revealed that 99.9% of iterations inside the > anon_vma_interval_tree_foreach loop are skipped due to the first check > "if (addr < vma->vm_start || addr >= vma->vm_end)), indicating that a large > number of loop iterations are ineffective. This inefficiency arises because > the start page index and the end page index parameters passed to > anon_vma_interval_tree_foreach span the entire address space from 0 to > ULONG_MAX, resulting in very poor loop efficiency. > > Solution > ======== > We cannot rely solely on anon_vma to locate all PTEs mapping this page > but also need to have the original page's linear_page_index. Since the > implementation of anon_vma_interval_tree_foreach — it essentially > iterates to find a suitable VMA such that the provided page index falls > within the candidate's vm_pgoff range. > > vm_pgoff <= original linear page offset <= (vm_pgoff + vma_pages(v) - 1) > > Fortunately, we have already linear_page_index. in ksm_rmap_item in the Misplaced "." > previos patch of series, so that we use it to get the index to accelerate > the searching. Avoid talking a about "previous patch" in a series. Something like: "Fortunately, an earlier commit introduced the linear_page_index to struct ksm_rmap_item, allowing for optimizing the RMAP walk." > > Test results > ============ > A rmap testbench can be obtained with two Out-Of-Tree patches at [1][2]. > After applying the OOT patches and building rmap_benchmark from: > tools/testing/rmap/rmap_benchmark.c, we can start the performance test. > > The testing result in QEMU is shown as follows: > > KSM rmapping Maximum duration Average duration > > Before: 705.12 ms (705119858 ns) 532.04 ms (532041586 ns) > After: 1.67 ms (1665917 ns) 1.44 ms (1443784 ns) > > [1] https://lore.kernel.org/all/202605301703094695zmVgcSC27BNR0rH0N8_x@zte.com.cn > [2] https://lore.kernel.org/all/20260530170404509QpJmBtpSjn3uQHeVKA2iA@zte.com.cn/ > > Co-developed-by: Wang Yaxin > Signed-off-by: Wang Yaxin > Signed-off-by: xu xin > --- > mm/ksm.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index e0ba29e3c0a4..9e1879d96751 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -3208,6 +3208,7 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc) > hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) { > /* Ignore the stable/unstable/sqnr flags */ > const unsigned long addr = rmap_item->address & PAGE_MASK; > + const unsigned long index = rmap_item->linear_page_index; remove_rmap_item_from_tree() does the hlist_del(&rmap_item->hlist); once we clear STABLE_FLAG + rmap_item->linear_page_index. So, this should always be valid (just like rmap_item->anon_vma). Good. > struct anon_vma *anon_vma = rmap_item->anon_vma; > struct anon_vma_chain *vmac; > struct vm_area_struct *vma; > @@ -3221,8 +3222,12 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc) > anon_vma_lock_read(anon_vma); > } > > + /* > + * Currently KSM folios are order-0 normal pages, so the end > + * page's index should be the same as the start page's index. Maybe best to say "Currently, KSM folios are always small folios, so it's sufficient to search for a single page." I don't even want to imagine how large-folio support could look like, and how it interacts with rmap_items :/ Let's hope we'll never have to go there, and if so, this code here will be the least of our concerns. Do we want to explain a bit how this works and on which properties this relies on? "We can simply use the linear_page_index of the de-duplicated anonymous page that we remembered in the rmap_item while de-duplicating. Note that mremap() always de-duplicates KSM folios: so if there was mremap() in our parent or our child, we wouldn't have the KSM folio mapped in these processes anymore." > + */ > anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root, > - 0, ULONG_MAX) { > + index, index) { > > cond_resched(); > vma = vmac->vma; I hope we're not missing some other weird corner cases. Sashiko seems to be happy, but I don't trust that ;) Hoping other people can also give this another look before we move this upstream. Acked-by: David Hildenbrand (Arm) -- Cheers, David