From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta1.migadu.com (out-183.mta1.migadu.com [95.215.58.183]) (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 CFBD622F01 for ; Sun, 8 Feb 2026 04:06:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770523590; cv=none; b=OaOmLoqsCwsp9mS1v0RFwy+L1C50R5imEbW/tik8hOdSz4/PKr9y5fSVi3oRLSLL38RGC5hS4AmSceCjZsTvvsyCBTOAX7kfDHNDbwhNn3TSgH20ASVnDFPnnl10Ls6jGrx6Zk4E32l/bhjHQU+CC1J8cCm7M4vRJfNBiLCDDoA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770523590; c=relaxed/simple; bh=eSoLf7PkeLQVVm5+g3TXwDeP5ASA6WzA+5N7rambOGo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ud8vdj9tOBJ1ddvamjsnAcaOeMUSJnVTQ3ODEVW5WtwCT/icjtBf6qUY4Tqyzj0Tb+8jA169vNvR+kz+zMnrZRO3sOEHAHjLXvRtipPog6umRxpoo01gjBi8y2lh1pb+URzlZ65dlY8TSWcNtCWVZvY3RAYfq3KfR1T8Z1VwoBo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=MOWZ+ZGR; arc=none smtp.client-ip=95.215.58.183 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="MOWZ+ZGR" Message-ID: <8e552bda-7d34-477a-8574-25a9183e6a92@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1770523587; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=fnYuG1+aCX2u+rotI13J0ci1rd1Dt/+JxIlZC/9lbAM=; b=MOWZ+ZGRVIxwhBvWwqXiE1GX/wBAEPf8RKNCrjsekGsiUVwx1BvcGW/Rf8KVK4sQqLsGWl H/G5nwkddSNnc5LLFS3yF5B/X2Ya5M/cdciXEQ2qV9SJYFPk0XaPspy40oRv3TNwtoM84r r2tcaqmb7PY7g/HC6qunK/ko60lzqQk= Date: Sun, 8 Feb 2026 12:06:20 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH mm-new v7 4/5] mm: khugepaged: skip lazy-free folios Content-Language: en-US To: "David Hildenbrand (Arm)" , Barry Song <21cnbao@gmail.com>, Vernon Yang Cc: akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, dev.jain@arm.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang References: <20260207081613.588598-1-vernon2gm@gmail.com> <20260207081613.588598-5-vernon2gm@gmail.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2026/2/8 05:38, David Hildenbrand (Arm) wrote: > On 2/7/26 14:51, Lance Yang wrote: >> >> >> On 2026/2/7 16:34, Barry Song wrote: >>> On Sat, Feb 7, 2026 at 4:16 PM Vernon Yang wrote: >>>> >>>> From: Vernon Yang >>>> >>>> For example, create three task: hot1 -> cold -> hot2. After all three >>>> task are created, each allocate memory 128MB. the hot1/hot2 task >>>> continuously access 128 MB memory, while the cold task only accesses >>>> its memory briefly and then call madvise(MADV_FREE). However, >>>> khugepaged >>>> still prioritizes scanning the cold task and only scans the hot2 task >>>> after completing the scan of the cold task. >>>> >>>> And if we collapse with a lazyfree page, that content will never be >>>> none >>>> and the deferred shrinker cannot reclaim them. >>>> >>>> So if the user has explicitly informed us via MADV_FREE that this >>>> memory >>>> will be freed, it is appropriate for khugepaged to skip it only, >>>> thereby >>>> avoiding unnecessary scan and collapse operations to reducing CPU >>>> wastage. >>>> >>>> Here are the performance test results: >>>> (Throughput bigger is better, other smaller is better) >>>> >>>> Testing on x86_64 machine: >>>> >>>> | task hot2           | without patch | with patch    |  delta  | >>>> |---------------------|---------------|---------------|---------| >>>> | total accesses time |  3.14 sec     |  2.93 sec     | -6.69%  | >>>> | cycles per access   |  4.96         |  2.21         | -55.44% | >>>> | Throughput          |  104.38 M/sec |  111.89 M/sec | +7.19%  | >>>> | dTLB-load-misses    |  284814532    |  69597236     | -75.56% | >>>> >>>> Testing on qemu-system-x86_64 -enable-kvm: >>>> >>>> | task hot2           | without patch | with patch    |  delta  | >>>> |---------------------|---------------|---------------|---------| >>>> | total accesses time |  3.35 sec     |  2.96 sec     | -11.64% | >>>> | cycles per access   |  7.29         |  2.07         | -71.60% | >>>> | Throughput          |  97.67 M/sec  |  110.77 M/sec | +13.41% | >>>> | dTLB-load-misses    |  241600871    |  3216108      | -98.67% | >>>> >>>> Signed-off-by: Vernon Yang >>>> Acked-by: David Hildenbrand (arm) >>>> Reviewed-by: Lance Yang >>>> --- >>>>   include/trace/events/huge_memory.h |  1 + >>>>   mm/khugepaged.c                    | 13 +++++++++++++ >>>>   2 files changed, 14 insertions(+) >>>> >>>> diff --git a/include/trace/events/huge_memory.h b/include/trace/ >>>> events/huge_memory.h >>>> index 384e29f6bef0..bcdc57eea270 100644 >>>> --- a/include/trace/events/huge_memory.h >>>> +++ b/include/trace/events/huge_memory.h >>>> @@ -25,6 +25,7 @@ >>>>          EM( SCAN_PAGE_LRU, "page_not_in_lru")              \ >>>>          EM( SCAN_PAGE_LOCK, "page_locked")                  \ >>>>          EM( SCAN_PAGE_ANON, "page_not_anon")                \ >>>> +       EM( SCAN_PAGE_LAZYFREE, "page_lazyfree")                \ >>>>          EM( SCAN_PAGE_COMPOUND, "page_compound")                \ >>>>          EM( SCAN_ANY_PROCESS, "no_process_for_page")          \ >>>>          EM( SCAN_VMA_NULL, "vma_null")                     \ >>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>> index 8b68ae3bc2c5..0d160e612e16 100644 >>>> --- a/mm/khugepaged.c >>>> +++ b/mm/khugepaged.c >>>> @@ -46,6 +46,7 @@ enum scan_result { >>>>          SCAN_PAGE_LRU, >>>>          SCAN_PAGE_LOCK, >>>>          SCAN_PAGE_ANON, >>>> +       SCAN_PAGE_LAZYFREE, >>>>          SCAN_PAGE_COMPOUND, >>>>          SCAN_ANY_PROCESS, >>>>          SCAN_VMA_NULL, >>>> @@ -583,6 +584,12 @@ static enum scan_result >>>> __collapse_huge_page_isolate(struct vm_area_struct *vma, >>>>                  folio = page_folio(page); >>>>                  VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio); >>>> >>>> +               if (cc->is_khugepaged && !pte_dirty(pteval) && >>>> +                   folio_test_lazyfree(folio)) { >>> >>> We have two corner cases here: >> >> Good catch! >> >>> >>> 1. Even if a lazyfree folio is dirty, if the VMA has the VM_DROPPABLE >>> flag, >>> a lazyfree folio may still be dropped, even when its PTE is dirty. > > Good point! > >> >> Right. When the VMA has VM_DROPPABLE, we would drop the lazyfree folio >> regardless of whether it (or the PTE) is dirty in try_to_unmap_one(). >> >> So, IMHO, we could go with: >> >> cc->is_khugepaged && folio_test_lazyfree(folio) && >>      (!pte_dirty(pteval) || (vma->vm_flags & VM_DROPPABLE)) > > Hm. In a VM_DROPPABLE mapping all folios should be marked as lazy-free > (see folio_add_new_anon_rmap()). Ah, I missed that apparently :) > The new (collapse) folio will also be marked lazy (due to > folio_add_new_anon_rmap()) free and can just get dropped any time. > > So likely we should just not skip collapse for lazyfree folios in > VM_DROPPABLE mappings? > > if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) && >     folio_test_lazyfree(folio) && !pte_dirty(pteval)) { >     ... > } Yep. That should be doing the trick. Thanks!