From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=115.124.30.97 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793869213; b=r4ZIRF8nPSnRLuQ+r7ifam2vKsGwYUs5gihF3pIBPC13NRPXRxbL6mj4N+lOJl7FBzHb TaRZKwcXARtDaINO6/HqH6BcoEPU2apJePT6G3v95VKtCiMaYRdD2Nfw4zhWy/REG+FVq Qtw43hZISAecGftXXbvoWaBP2c764DNufsUctewvfzq0vFnkrLgbLUforXjjIRyiOi0VX 32dOakg6+yzHHk2CJcc/0C3QtRvP0bGWQhbo1TcibUcYhIBr15xoO4qaEeX8bizlPLML5 MVJEHmqxWuL5tXpMlNNAzfuYiXF6WgY+6DbtZsP5XLRSCP/yownvE9q7qMKbSvEHbIQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793869213; h=DMARC-Filter:DKIM-Signature:X-Alimail-AntiSpam:Received:Message-ID: Date:MIME-Version:User-Agent:Subject:To:Cc:References:From: In-Reply-To:Content-Type:Content-Transfer-Encoding; bh=Qpa3NEcxqfBmlG+KsMNpjxHv3Qlvd6eoPxtMyefULAQ=; b=Hb5/jHBMnyKqrPHuCRaZC86JjZ0bZfbIebkL4aiTiW6hggFV7yjkOrPiiUONoBqsM+fS l7+Ksv5h2SzRS/pRK3XvuT3+Bd2/ehXU4+iwAsVMh3j8BnmZqNgXZxQFl3l54nfeMGtdU D7nVNcBIXwfYuDCK+jHc8BwFM8Wd7IGyLkDDc1aPzMo76EhdzTi/wcZG1jYPQaqYVQxqQ MpCJB8WlpIphlqNyTZmeDv0SjbVs7WmJa9++YUoBjlOHeebSeocBZfrm1vV3i+4Oj64tN voRcIhGPVPnMe5BzTp3Whz3OpwucvMML+MEbH8Hu83j6ZxEoelj6Hmi2ekbSxDD9AUg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass header.d=linux.alibaba.com header.i=@linux.alibaba.com header.a=rsa-sha256 header.s=default header.b="O/0jd9AE"; arc=none smtp.remote-ip=115.124.30.97 Received: from out30-97.freemail.mail.aliyun.com (out30-97.freemail.mail.aliyun.com [115.124.30.97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F08B0606A328 for ; Thu, 5 Nov 2026 09:00:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org F08B0606A328 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.a=rsa-sha256 header.s=default header.b=O/0jd9AE DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1793869207; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=Qpa3NEcxqfBmlG+KsMNpjxHv3Qlvd6eoPxtMyefULAQ=; b=O/0jd9AEo1GMG8+vy3JlMrJYv8Wx4Jbe+VaeeqUG1iwgmFgGTFOx7C9Dqog1EiyUj/ekQfMmnuipgkPEzqcxMNvNyz1lOPa54FCvjLA8QnQOXlBZ6oaZcl0fUAKSwaV8HcPuT1Ijf0406dx17QkwrwN03kX9QV50WGzfhjStC24= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R111e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033037026112;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=15;SR=0;TI=SMTPD_---0XE8DxBe_1793869204; Received: from 30.74.144.136(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XE8DxBe_1793869204 cluster:ay36) by smtp.aliyun-inc.com; Thu, 05 Nov 2026 17:00:05 +0800 Message-ID: Date: Thu, 5 Nov 2026 17:00:03 +0800 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 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes To: Lance Yang , Barry Song Cc: akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, qi.zheng@linux.dev, pfalcato@suse.de, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261105045406.39871-1-lance.yang@linux.dev> <0ddaf336-7cb7-4e83-af2c-f60d23f7aafd@linux.dev> From: Baolin Wang In-Reply-To: <0ddaf336-7cb7-4e83-af2c-f60d23f7aafd@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit CC Barry, who added the TTU_SYNC in shrink_folio_list(). On 11/5/26 1:48 PM, Lance Yang wrote: > > > On 2026/11/5 12:54, Lance Yang wrote: >> >> On Thu, Nov 05, 2026 at 10:56:28AM +0800, Baolin Wang wrote: >>> Similar to commit 243cf1f67b36 ("mm: memory: fix truncation removing >>> mapped >>> folios"), the '!pmd_present()' check in page_vma_mapped_walk() can >>> also race >>> with concurrent page table reclamation via zap_empty_pte_table(). >> >> Good catch! >> >>> page_vma_mapped_walk() is used by many functions including >>> try_to_unmap(), >>> try_to_migrate(), folio_mkclean(), pfn_mkclean_range(), >>> remove_migration_ptes(), >>> page_mapped_in_vma(), and mapping_wrprotect_range(). Except for >>> try_to_unmap() >>> and try_to_migrate(), the other callers do not perform post-walk >>> accounting >>> checks, so the race has no observable impact on them. >>> >>> For migration, the race can cause unnecessary retries. For reclaim via >>> try_to_unmap(), the race can cause failure to reclaim a folio that is in >>> fact already unmapped (as shown below), which may lead to unexpected OOM >>> or increased refaults. >>> >>> CPU 0 (reclaim)                      CPU 1 (MADV_DONTNEED) >>> --------------------------------    -------------------------------- >>> folio_lock(F) >>> try_to_unmap(F, TTU_SYNC) >>>                                     zap_pte_range(full PMD range) >>>                                       spin_lock(ptl) >>>                                       clear PTEs >>>                                       zap_empty_pte_table() >>>                                         spin_trylock(pml) >>>                                         pmd_clear() >>>                                         spin_unlock(pml) >>> >>>   page_vma_mapped_walk() >>>     observe !pmd_present(pmde) >>>     nr_pages < HPAGE_PMD_NR >>>     skip synchronization >>>     return false >>> >>> try_to_unmap() returns >>> folio_mapped(F) == true >>> nr_unmap_fail += folio_nr_pages(F) >>>                                     tlb_flush_mmu_tlbonly() >>>                                     tlb_flush_rmaps() >>>                                       remove F's rmaps >>>                                     spin_unlock(ptl) >>> >>> Commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped >>> folios") >>> has already moved the PMD clear operation (via zap_empty_pte_table()) to >>> after tlb_flush_rmaps(). Similarly, page_vma_mapped_walk() also needs to >>> use sync_with_folio_pmd_zap() to synchronize and avoid memory ordering >>> issues on weakly ordered CPUs. >>> >>> Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in >>> madvise(MADV_DONTNEED)") >>> Cc: stable@vger.kernel.org >>> Suggested-by: David Hildenbrand (Arm) >>> Signed-off-by: Baolin Wang >>> --- >>> mm/page_vma_mapped.c | 5 +---- >>> 1 file changed, 1 insertion(+), 4 deletions(-) >>> >>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >>> index e6b6170c2f4f..8797467b6f0c 100644 >>> --- a/mm/page_vma_mapped.c >>> +++ b/mm/page_vma_mapped.c >>> @@ -293,10 +293,7 @@ bool page_vma_mapped_walk(struct >>> page_vma_mapped_walk *pvmw) >>>             spin_unlock(pvmw->ptl); >>>             pvmw->ptl = NULL; >>>         } else if (!pmd_present(pmde)) { >>> -            if ((pvmw->flags & PVMW_SYNC) && >>> -                thp_vma_suitable_order(vma, pvmw->address, >>> -                           PMD_ORDER) && >>> -                (pvmw->nr_pages >= HPAGE_PMD_NR)) >>> +            if (pvmw->flags & PVMW_SYNC) >> >> Right. Whether the folio can be PMD-mapped doesn't tell us whether its >> PTE table can be reclaimed, so those extra checks can skip >> synchronization >> that the caller explicitly asked for ;) > > Ouch ... one thing I missed ... what about order-0 reclaim? > > shrink_folio_list() only sets TTU_SYNC for large folio, so can't we still > hit the same race there? Thanks Lance. But this matches the purpose of the TTU_SYNC flag, which is used to avoid racy checks with PVMW_SYNC. If no TTU_SYNC flag is set, it means the caller can tolerate some racy checks. For this case, I think we should set TTU_SYNC unconditionally in shrink_folio_list(). Barry, what do you think?