From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-130.freemail.mail.aliyun.com (out30-130.freemail.mail.aliyun.com [115.124.30.130]) (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 AE1DD2AD0C for ; Fri, 16 Oct 2026 01:47:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.130 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792115247; cv=none; b=JKr9IyGaBD9UVmt4J+sA6UPZ9kERE7DtT5fmP2VhgdnJgeKecUBFZOhPtBc+R1+ocOWsSvTNH39Hz3Tq8SttcYkrSEhtLIQwrTBuLdlw2EQtfNE1oXB5jWH4cCZb/q1VLbA8x81gLYIR/GvPvqoLYGjL7Fa9KVhQ+hOdKdGRrvI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792115247; c=relaxed/simple; bh=eJtOKk7P/HGCOr6BwwUa3gt5Ip2aig0MLqeS1fSHCf0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=R8BG0qqjAhCn3aGQVYhqxt/1E4peQorGJ1KUIIC4/XsZQ0cVKW7w+Gte8DnP6QAw5W2vsrti7oCF8gonC1n6QTvYnRafICGoCv+Re1YGI6c5SmfIWVlqxH6jkcSd5WM0vHJLy8GUiPr4MxTjgfoosrvUBuE/FH0zVOfiBwgusSo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=GD/Tpsky; arc=none smtp.client-ip=115.124.30.130 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) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="GD/Tpsky" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1792115241; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=dfUIw3nBCJ2Tb9sCItT56mvQgYidiqFThC9FCi7zIMI=; b=GD/TpskyDMUuwDqy2PreHSaG5knX5fgqORrKK45wunB+3JAWYid7E54ScfvmA6/5NqsvV0gQqo7n8S6fhQRIq1iIJB958yxB9zyr0+i8C7g5J/WTbpBQbmaADYDKpIwCSTuB2SA1wJ+1Fe3Q2q5l8o5UQxCPR9lNo59BLyg0fP8= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R151e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033045098064;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=14;SR=0;TI=SMTPD_---0XCt4VcS_1792115238; Received: from 30.74.144.163(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XCt4VcS_1792115238 cluster:ay36) by smtp.aliyun-inc.com; Fri, 16 Oct 2026 09:47:19 +0800 Message-ID: Date: Fri, 16 Oct 2026 09:47:18 +0800 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 v2] mm: memory: fix truncation removing mapped folios To: "David Hildenbrand (Arm)" , akpm@linux-foundation.org Cc: ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, qi.zheng@linux.dev, jack@suse.cz, pfalcato@suse.de, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <542c97482eae84f86704bcee56482633d1b32321.1791616645.git.baolin.wang@linux.alibaba.com> <8245424a-f068-4be5-9e58-713cbff1f8ae@kernel.org> <405811f5-ee36-4b3c-80d0-7dc6827ab5d4@linux.alibaba.com> <6660d3bb-3485-4c34-8734-3ea99df920f1@kernel.org> <02c9a968-92e2-49e2-bffc-74d264a5c911@linux.alibaba.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10/16/26 3:00 AM, David Hildenbrand (Arm) wrote: > On 10/15/26 08:41, Baolin Wang wrote: >> >> >> On 10/14/26 9:25 PM, David Hildenbrand (Arm) wrote: >>> On 10/13/26 11:36, Baolin Wang wrote: >>>> >>>> >>>> >>>> I had considered this approach before, but it is not feasible. IIUC, the >>>> sync_with_folio_pmd_zap() is used to address the race with zap_huge_pmd(), and >>>> both the PMD clear and the folio mapcount decrement are performed under the PMD >>>> lock in zap_huge_pmd(). >>> >>> Just to be clear, I am thinking about something like the following on the >>> zap_pmd_range() side: >>> >>> diff --git a/mm/internal.h b/mm/internal.h >>> index 38b1165212c94..a8820ad7e2f21 100644 >>> --- a/mm/internal.h >>> +++ b/mm/internal.h >>> @@ -584,14 +584,15 @@ void free_pgtables(struct mmu_gather *tlb, struct >>> unmap_desc *desc); >>>   void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte); >>> >>>   /** >>> - * sync_with_folio_pmd_zap - sync with concurrent zapping of a folio PMD >>> + * sync_with_folio_pmd_zap - sync with concurrent zapping of folios >>>    * @mm: The mm_struct. >>>    * @pmdp: Pointer to the pmd that was found to be pmd_none(). >>>    * >>>    * When we find a pmd_none() while unmapping a folio without holding the PTL, >>> - * zap_huge_pmd() may have cleared the PMD but not yet modified the folio to >>> - * indicate that it's unmapped. Skipping the PMD without synchronization could >>> - * make folio unmapping code assume that unmapping failed. >>> + * zap_huge_pmd() or zap_empty_pte_table() may have cleared the PMD but not >>> + * yet modified the mapped folios to indicate they are unmapped. Skipping the >>> + * PMD without synchronization could make folio unmapping code assume that >>> + * unmapping failed. >>>    * >>>    * Wait for concurrent zapping to complete by grabbing the PTL. >>>    */ >>> diff --git a/mm/memory.c b/mm/memory.c >>> index 8b0c2c735d3de..c585f5eb255c9 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -2139,9 +2139,7 @@ static inline unsigned long zap_pmd_range(struct >>> mmu_gather *tlb, >>>                                  continue; >>>                          } >>>                          /* fall through */ >>> -               } else if (details && details->single_folio && >>> -                          folio_test_pmd_mappable(details->single_folio) && >>> -                          next - addr == HPAGE_PMD_SIZE && pmd_none(*pmd)) { >>> +               } else if (details && details->single_folio && pmd_none(*pmd)) { >>>                          sync_with_folio_pmd_zap(tlb->mm, pmd); >>>                  } >>>                  if (pmd_none(*pmd)) { >>> >> >> Yes, I've considered that. However, implicit memory barriers give me a headache, >> so I ended up going with explicit memory barriers (suggested by Pedro). >> >> If you don't like memory barriers, let me re-check whether the implicit memory >> barriers here can ensure race safety with sync_with_folio_pmd_zap(). A >> simplified logic is as follows: >> >> CPU 0 (truncation)                        CPU 1 (zapping) >> down_read() ->Acquire A1.                pte spin_lock() ->Acquire A4 >>                                          clear mapcount >> >>                                          pmd spin_lock() ->Acquire A5 >>                                          clear pmd >>                                          pmd spin_unlock() ->Release R5 >> >> if (pmd_none()) { //read pmd >>                                          pte spin_unlock() ->Release R4 >>     pmd spin_lock() ->Acquire A2 >>     pmd spin_unlock() ->Release R2 >> } >> up_read() -> Release R1 >> >> spin_lock(y) ->Acquire A3 >> folio_mapped() // read mapcount >> spin_unlock(y) ->Release R3 >> >> CPU 0 constraints: >> 1. R2 is a release, so "read pmd" completes before R2. >> 2. A2 completes before A3 (ACQUIRE vs ACQUIRE). >> 3. "read mapcount" completes after A3, hence after A2. >> >> CPU 1 constraints: >> 1. R5 is a release. Both "clear mapcount" and "clear pmd" complete before R5. >> >> According to above constraints, if CPU0 sees pmd_none() caused by CPU1's clear >> pmd, and CPU0 then acquires the same pmd lock released by CPU1 at R5, then >> CPU1's clear mapcount happens before CPU0's later folio_mapped(). >> >> Therefore folio_mapped() is guaranteed not to read the old mapcount in this >> case. So I think sync_with_folio_pmd_zap() can work, but needs a small change to >> avoid the double pmd_none() check. > > > I'd be okay with some (follow-up?) memory barrier handling inside > sync_with_folio_pmd_zap(), for example, based on the folio size and with proper > documentation why we need the PT lock in one case and why the memory barrier is > ok in the other case. I'm afraid that we can't fit all the memory barriers into sync_with_folio_pmd_zap(). As the current patch does, we need memory barriers on both the writer and reader sides. >> David, does the following diff look good to you? > > I think we should extend the sync_with_folio_pmd_zap() doc as suggested. Ack. >> diff --git a/mm/memory.c b/mm/memory.c >> index 6a8e7772b8d6..c0aadab1b59e 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -2036,6 +2036,15 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, >>                 } >>         } while (pte += nr, addr += PAGE_SIZE * nr, addr != end); >> >> +       add_mm_rss_vec(mm, rss); >> +       lazy_mmu_mode_disable(); >> + >> +       /* Do the actual TLB flush before dropping ptl */ >> +       if (force_flush) { >> +               tlb_flush_mmu_tlbonly(tlb); >> +               tlb_flush_rmaps(tlb, vma); >> +       } >> + >>         /* >>          * Fast path: try to hold the pmd lock and unmap the PTE page. >>          * >> @@ -2044,16 +2053,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, >>          * to ensure they are still none, thereby preventing the pte entries >>          * from being repopulated by another thread. >>          */ >> -       if (can_reclaim_pt && direct_reclaim && addr == end) >> +       if (can_reclaim_pt && direct_reclaim && addr == end) { >>                 direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, &pmdval); >> - >> -       add_mm_rss_vec(mm, rss); >> -       lazy_mmu_mode_disable(); >> - >> -       /* Do the actual TLB flush before dropping ptl */ >> -       if (force_flush) { >> -               tlb_flush_mmu_tlbonly(tlb); >> -               tlb_flush_rmaps(tlb, vma); >>         } >>         pte_unmap_unlock(start_pte, ptl); >> >> @@ -2102,12 +2103,11 @@ static inline unsigned long zap_pmd_range(struct >> mmu_gather *tlb, >>                                 continue; >>                         } >>                         /* fall through */ >> -               } else if (details && details->single_folio && >> -                          folio_test_pmd_mappable(details->single_folio) && >> -                          next - addr == HPAGE_PMD_SIZE && pmd_none(*pmd)) { >> -                       sync_with_folio_pmd_zap(tlb->mm, pmd); >>                 } >> + >>                 if (pmd_none(*pmd)) { >> +                       if (details && details->single_folio) >> +                               sync_with_folio_pmd_zap(tlb->mm, pmd); >>                         addr = next; >>                         continue; >>                 } > > We do have another sync_with_folio_pmd_zap() user in page_vma_mapped_walk(), > does that one need care as well? Good point. But I don't want to add page_vma_mapped_walk() changes into this fix patch. I need to analyze all callers of page_vma_mapped_walk() and the potential races. I'll look into whether it should be changed in a follow-up patch. Thanks for your comments.