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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770375671; bh=OfuemE8NvbpkvzumGm/QBmZRuLBhKVNLRdNOqH8+xw8=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=JqYRZLZo2NU/xYu+jVFba4I8F8wjgB2QbQ6NFqY0InIReMZq6P7/iW5j2DGhbDvuH Bl62/oPvl6dTHhbcwtXCYfTNKTMY4ONgcEhGwculNZ/YnqMN2Qm+KCVzidmmhfszPQ cc0+VP4/wJpJYdZ0dUNs9rhA1LO11ZeFCl6b/CmD0IJlcDj/Ia7Q6FIKc+9niic8K5 pKFVCnn5oeYnEs0qX/t8GypgfhImI//1SySOv4MpR/pDYMy5D1GdbAfO0TmVcXiBgy mn+BxdSngydqDTzj8V+GEHUeTe3LPcLD00TIqHoIw6+4IahFAOWOoOzY/7LIIWhcsx J4uDDyzcwt1GQ== Message-ID: <9f75b988-9729-452b-beb5-deab0718faa8@kernel.org> Date: Fri, 6 Feb 2026 12:01:08 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range To: xu.xin16@zte.com.cn, akpm@linux-foundation.org Cc: chengming.zhou@linux.dev, hughd@google.com, wang.yaxin@zte.com.cn, yang.yang29@zte.com.cn, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260206180132364R_lYZujwOYfhazgKmxArZ@zte.com.cn> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 1398B12001B X-Stat-Signature: iatncrt3n4nd3mtyhry7spp4qd14ufo8 X-HE-Tag: 1770375672-900595 X-HE-Meta: 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 m8GuNWVq aEVs7 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/6/26 11:01, xu.xin16@zte.com.cn wrote: > From: xu xin > > Problem > ======= > When available memory is extremely tight, causing KSM pages to be swapped > out, or when there is significant memory fragmentation and THP triggers > memory compaction, the system will invoke the rmap_walk_ksm function to > perform reverse mapping. However, we observed that this function becomes > particularly time-consuming when a large number of VMAs (e.g., 20,000) > share the same anon_vma. Through debug trace analysis, we found that most > of the latency 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. > > Root Reaon s/Reaon/Reason/ or better "Cause" > ========== > 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 pgoff_start and pgoff_end 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 > ======== > In fact, we can significantly improve performance by passing a more precise > range based on the given addr. Since the original pages merged by KSM > correspond to anonymous VMAs, the page offset can be calculated as > pgoff = address >> PAGE_SHIFT. Therefore, we can optimize the call by > defining: > > pgoff_start = rmap_item->address >> PAGE_SHIFT; > > since KSM folios are always order-0, so folio_nr_pages(KSM folio) is always 1, > so the line: > > "pgoff_end = pgoff_start + folio_nr_pages(folio) - 1;" > > becomes directly: > > "pgoff_end = pgoff_start;" > > Performance > =========== > In our real embedded Linux environment, the measured metrcis were as follows: > > 1) Time_ms: Max time for holding anon_vma lock in a single rmap_walk_ksm. > 2) Nr_iteration_total: The max times of iterations in a loop of anon_vma_interval_tree_foreach > 3) Skip_addr_out_of_range: The max times of skipping due to the first check (vma->vm_start > and vma->vm_end) in a loop of anon_vma_interval_tree_foreach. > 4) Skip_mm_mismatch: The max times of skipping due to the second check (rmap_item->mm == vma->vm_mm) > in a loop of anon_vma_interval_tree_foreach. > > The result is as follows: > > Time_ms Nr_iteration_total Skip_addr_out_of_range Skip_mm_mismatch > Before patched: 228.65 22169 22168 0 > After pacthed: 0.396 3 0 2 s/pacthed/patched/ But I would just call it "Before" and "After". > > The referenced reproducer of rmap_walk_ksm can be found at: > https://lore.kernel.org/all/20260206151424734QIyWL_pA-1QeJPbJlUxsO@zte.com.cn/ > > Signed-off-by: xu xin Did you accidentally drop a Co-developed-by: Wang Yaxin ? > --- > mm/ksm.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 950e122bcbf4..54f72e92b7f3 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -3170,6 +3170,9 @@ 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 pgoff_t pgoff_start = rmap_item->address >> PAGE_SHIFT; > + /* KSM folios are always order-0 normal pages */ > + const pgoff_t pgoff_end = pgoff_start; Maybe simply const pgoff_t pgoff = rmap_item->address >> PAGE_SHIFT; and drop pgoff_end? Then you simply pass pgoff as start and end below. You could add the KSM folio comment above the anon_vma_interval_tree_foreach. If the tools/testing/selftests/mm/rmap.c selftests keeps passing rmap_walk_ksm() should be working as expected. Did you run it to make sure? -- Cheers, David