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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id q191sm75708pfq.108.2021.06.03.15.26.59 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Thu, 03 Jun 2021 15:27:00 -0700 (PDT) Date: Thu, 3 Jun 2021 15:26:58 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Andrew Morton cc: Hugh Dickins , "Kirill A. Shutemov" , Yang Shi , Wang Yugui , Matthew Wilcox , Naoya Horiguchi , Alistair Popple , Ralph Campbell , Zi Yan , Miaohe Lin , Minchan Kim , Jue Wang , Peter Xu , Jan Kara , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH 6.1/7] mm: thp: replace DEBUG_VM BUG with VM_WARN when unmap fails for split In-Reply-To: Message-ID: References: User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Authentication-Results: imf30.hostedemail.com; dkim=temperror ("DNS error when getting key") header.d=google.com header.s=20161025 header.b=dddkEQ10; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf30.hostedemail.com: domain of hughd@google.com designates 209.85.216.53 as permitted sender) smtp.mailfrom=hughd@google.com X-Stat-Signature: 3k919dhiw3tc959i51ygd4ob5f9ha8h1 X-Rspamd-Queue-Id: E3BD9E007A53 X-Rspamd-Server: rspam02 X-HE-Tag: 1622759213-517821 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: From: Yang Shi When debugging the bug reported by Wang Yugui [1], try_to_unmap() may fail, but the first VM_BUG_ON_PAGE() just checks page_mapcount() however it may miss the failure when head page is unmapped but other subpage is mapped. Then the second DEBUG_VM BUG() that check total mapcount would catch it. This may incur some confusion. And this is not a fatal issue, so consolidate the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE(). [1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/ Signed-off-by: Yang Shi Reviewed-by: Zi Yan Acked-by: Kirill A. Shutemov Signed-off-by: Hugh Dickins Cc: --- v5: Rediffed by Hugh to fit after 6/7 in his mm/thp series; Cc stable. v4: Updated the subject and commit log per Hugh. Reordered the patches per Hugh. v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag since there is no fundamental change against v2. v2: Removed dead code and updated the comment of try_to_unmap() per Zi Yan. mm/huge_memory.c | 26 ++++++++------------------ 1 file changed, 8 insertions(+), 18 deletions(-) --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2350,19 +2350,19 @@ static void unmap_page(struct page *page { enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD; - bool unmap_success; VM_BUG_ON_PAGE(!PageHead(page), page); if (PageAnon(page)) ttu_flags |= TTU_SPLIT_FREEZE; - /* Make sure that the BUGs will not bite */ + /* If warning below, prevent a race in the mapped accounting */ if (IS_ENABLED(CONFIG_DEBUG_VM)) ttu_flags |= TTU_SYNC; - unmap_success = try_to_unmap(page, ttu_flags); - VM_BUG_ON_PAGE(!unmap_success, page); + try_to_unmap(page, ttu_flags); + + VM_WARN_ON_ONCE_PAGE(page_mapped(page), page); } static void remap_page(struct page *page, unsigned int nr) @@ -2673,7 +2673,7 @@ int split_huge_page_to_list(struct page struct deferred_split *ds_queue = get_deferred_split_queue(head); struct anon_vma *anon_vma = NULL; struct address_space *mapping = NULL; - int count, mapcount, extra_pins, ret; + int extra_pins, ret; pgoff_t end; VM_BUG_ON_PAGE(is_huge_zero_page(head), head); @@ -2732,7 +2732,6 @@ int split_huge_page_to_list(struct page } unmap_page(head); - VM_BUG_ON_PAGE(compound_mapcount(head), head); /* block interrupt reentry in xa_lock and spinlock */ local_irq_disable(); @@ -2750,9 +2749,7 @@ int split_huge_page_to_list(struct page /* Prevent deferred_split_scan() touching ->_refcount */ spin_lock(&ds_queue->split_queue_lock); - count = page_count(head); - mapcount = total_mapcount(head); - if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) { + if (page_ref_freeze(head, 1 + extra_pins)) { if (!list_empty(page_deferred_list(head))) { ds_queue->split_queue_len--; list_del(page_deferred_list(head)); @@ -2772,16 +2769,9 @@ int split_huge_page_to_list(struct page __split_huge_page(page, list, end); ret = 0; } else { - if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) { - pr_alert("total_mapcount: %u, page_count(): %u\n", - mapcount, count); - if (PageTail(page)) - dump_page(head, NULL); - dump_page(page, "total_mapcount(head) > 0"); - BUG(); - } spin_unlock(&ds_queue->split_queue_lock); -fail: if (mapping) +fail: + if (mapping) xa_unlock(&mapping->i_pages); local_irq_enable(); remap_page(head, thp_nr_pages(head));