From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-187.mta1.migadu.com (out-187.mta1.migadu.com [95.215.58.187]) (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 D8280199FB0 for ; Sat, 7 Feb 2026 13:51:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.187 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770472310; cv=none; b=mIeN7wwcekAfugFgwg0V4xWtO+Xi+FdZzKt/C7I0KdDJqF3aEXbettxBIi0CtWLbMxDUi1/F28YleiVnGXvp4P0i5drFRgRTJaokmNcB9oYt4PUNvII9hgMsJY3VCxNtGrn6X/Ax3gYsJ3/TlB7LDOFhdm1rU8LFRvh8WC7Y2b4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770472310; c=relaxed/simple; bh=P7o9JGmnVUB5flAkabogxhSURp+zqIVaPeuPujUU5CM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=S4UREhjz3dFkJSHhpVH0sz6YYdGep/CAuENwMuy+RrE8pAOTgsHE/J+6BpwMgvFJTYZxOPGn/6UMje/3AJT15Z9ayiZzmfjwfgUHMwXaQ/XzpLu34dEIlN7POADRgt7uFmhgHDXkHP5BuZxKRKk2kkABR1M8qY/G9MUDtsR8X3c= 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=xHmb1kdD; arc=none smtp.client-ip=95.215.58.187 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="xHmb1kdD" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1770472307; 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=OSB2XM91w6fSVPuXGvQO0MuxeMJ61wrne3hP9UNvlAE=; b=xHmb1kdDVoTY+CPrgUgv92dGrk+LLjmEYijSlvIPVV06VZqPKwOyE9l8A1jKVZdD71Rtq7 ObAcef0sV35J60v1OrfCLMBw7cmKk9n1oY+QHBlCzxz8ZqefBTA+p3MElBX4/FT3bOhNGU WJYb+qvfOgZ9WRsPRzRzGmGiCtXBMHE= Date: Sat, 7 Feb 2026 21:51:27 +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: Barry Song <21cnbao@gmail.com>, Vernon Yang Cc: akpm@linux-foundation.org, david@kernel.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 2026/2/7 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. 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)) > > 2. GUP operation can cause a folio to become dirty. Emm... I don't think we need to do anything special for GUP here :) IIUC, if the range is pinned, MADV_COLLAPSE/khugepaged already fails; We hit the refcount check in hpage_collapse_scan_pmd() (expected vs actual refcount) and return -EAGAIN. ``` /* * Check if the page has any GUP (or other external) pins. * * Here the check may be racy: * it may see folio_mapcount() > folio_ref_count(). * But such case is ephemeral we could always retry collapse * later. However it may report false positive if the page * has excessive GUP pins (i.e. 512). Anyway the same check * will be done again later the risk seems low. */ if (folio_expected_ref_count(folio) != folio_ref_count(folio)) { result = SCAN_PAGE_COUNT; goto out_unmap; } ``` Cheers, Lance > > I see the corner cases from try_to_unmap_one(): > > if (folio_test_dirty(folio) && > !(vma->vm_flags & VM_DROPPABLE)) { > /* > * redirtied either using the > page table or a previously > * obtained GUP reference. > */ > set_ptes(mm, address, > pvmw.pte, pteval, nr_pages); > folio_set_swapbacked(folio); > goto walk_abort; > } > > Should we take these two corner cases into account? > > >> + result = SCAN_PAGE_LAZYFREE; >> + goto out; >> + } >> + >> /* See hpage_collapse_scan_pmd(). */ >> if (folio_maybe_mapped_shared(folio)) { >> ++shared; >> @@ -1335,6 +1342,12 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, >> } >> folio = page_folio(page); >> >> + if (cc->is_khugepaged && !pte_dirty(pteval) && >> + folio_test_lazyfree(folio)) { >> + result = SCAN_PAGE_LAZYFREE; >> + goto out_unmap; >> + } >> + >> if (!folio_test_anon(folio)) { >> result = SCAN_PAGE_ANON; >> goto out_unmap; > > Thanks > Barry