From patchwork Thu Jun 10 22:24:49 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Andrew Morton X-Patchwork-Id: 458474 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER, INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9B61CC48BE5 for ; Thu, 10 Jun 2021 22:25:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7A4DD61410 for ; Thu, 10 Jun 2021 22:25:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230405AbhFJW06 (ORCPT ); Thu, 10 Jun 2021 18:26:58 -0400 Received: from mail.kernel.org ([198.145.29.99]:33134 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230251AbhFJW05 (ORCPT ); Thu, 10 Jun 2021 18:26:57 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 22565613DE; Thu, 10 Jun 2021 22:24:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1623363890; bh=Hp4qZRdRQrlpPL24N/w/4FNL63PeiGKBHt/1pGAfIUA=; h=Date:From:To:Subject:From; b=Jrfnh/i51RkXUCjBlyt+tepHxUIzbxom4EWcdqQ9pdT4PpZo89c2KivtJu241Mgce 17oKnaW8l7OEKHZfIl3Yj9Wya7HtyHTWwMcpvYp7z/Csn7tZ66PlsIcHkvkotCr521 uuqQfxJsukpObp/3gktmOSX0ddOS2CTgNhF9BSlY= Date: Thu, 10 Jun 2021 15:24:49 -0700 From: akpm@linux-foundation.org To: apopple@nvidia.com, hughd@google.com, jack@suse.cz, juew@google.com, kirill.shutemov@linux.intel.com, linmiaohe@huawei.com, minchan@kernel.org, mm-commits@vger.kernel.org, naoya.horiguchi@nec.com, osalvador@suse.de, peterx@redhat.com, rcampbell@nvidia.com, shakeelb@google.com, shy828301@gmail.com, stable@vger.kernel.org, wangyugui@e16-tech.com, willy@infradead.org, ziy@nvidia.com Subject: + mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting.patch added to -mm tree Message-ID: <20210610222449.UzNQjT3GN%akpm@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org The patch titled Subject: mm/thp: try_to_unmap() use TTU_SYNC for safe splitting has been added to the -mm tree. Its filename is mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting.patch This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting.patch and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Hugh Dickins Subject: mm/thp: try_to_unmap() use TTU_SYNC for safe splitting Stressing huge tmpfs often crashed on unmap_page()'s VM_BUG_ON_PAGE (!unmap_success): with dump_page() showing mapcount:1, but then its raw struct page output showing _mapcount ffffffff i.e. mapcount 0. And even if that particular VM_BUG_ON_PAGE(!unmap_success) is removed, it is immediately followed by a VM_BUG_ON_PAGE(compound_mapcount(head)), and further down an IS_ENABLED(CONFIG_DEBUG_VM) total_mapcount BUG(): all indicative of some mapcount difficulty in development here perhaps. But the !CONFIG_DEBUG_VM path handles the failures correctly and silently. I believe the problem is that once a racing unmap has cleared pte or pmd, try_to_unmap_one() may skip taking the page table lock, and emerge from try_to_unmap() before the racing task has reached decrementing mapcount. Instead of abandoning the unsafe VM_BUG_ON_PAGE(), and the ones that follow, use PVMW_SYNC in try_to_unmap_one() in this case: adding TTU_SYNC to the options, and passing that from unmap_page(). When CONFIG_DEBUG_VM, or for non-debug too? Consensus is to do the same for both: the slight overhead added should rarely matter, except perhaps if splitting sparsely-populated multiply-mapped shmem. Once confident that bugs are fixed, TTU_SYNC here can be removed, and the race tolerated. Link: https://lkml.kernel.org/r/c1e95853-8bcd-d8fd-55fa-e7f2488e78f@google.com Fixes: fec89c109f3a ("thp: rewrite freeze_page()/unfreeze_page() with generic rmap walkers") Signed-off-by: Hugh Dickins Cc: Cc: Alistair Popple Cc: Jan Kara Cc: Jue Wang Cc: Kirill A. Shutemov Cc: "Matthew Wilcox (Oracle)" Cc: Miaohe Lin Cc: Minchan Kim Cc: Naoya Horiguchi Cc: Oscar Salvador Cc: Peter Xu Cc: Ralph Campbell Cc: Shakeel Butt Cc: Wang Yugui Cc: Yang Shi Cc: Zi Yan Signed-off-by: Andrew Morton --- include/linux/rmap.h | 1 + mm/huge_memory.c | 2 +- mm/page_vma_mapped.c | 11 +++++++++++ mm/rmap.c | 17 ++++++++++++++++- 4 files changed, 29 insertions(+), 2 deletions(-) --- a/include/linux/rmap.h~mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting +++ a/include/linux/rmap.h @@ -91,6 +91,7 @@ enum ttu_flags { TTU_SPLIT_HUGE_PMD = 0x4, /* split huge PMD if any */ TTU_IGNORE_MLOCK = 0x8, /* ignore mlock */ + TTU_SYNC = 0x10, /* avoid racy checks with PVMW_SYNC */ TTU_IGNORE_HWPOISON = 0x20, /* corrupted page is recoverable */ TTU_BATCH_FLUSH = 0x40, /* Batch TLB flushes where possible * and caller guarantees they will --- a/mm/huge_memory.c~mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting +++ a/mm/huge_memory.c @@ -2350,7 +2350,7 @@ void vma_adjust_trans_huge(struct vm_are static void unmap_page(struct page *page) { - enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | + enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_SYNC | TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD; bool unmap_success; --- a/mm/page_vma_mapped.c~mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting +++ a/mm/page_vma_mapped.c @@ -212,6 +212,17 @@ restart: pvmw->ptl = NULL; } } else if (!pmd_present(pmde)) { + /* + * If PVMW_SYNC, take and drop THP pmd lock so that we + * cannot return prematurely, while zap_huge_pmd() has + * cleared *pmd but not decremented compound_mapcount(). + */ + if ((pvmw->flags & PVMW_SYNC) && + PageTransCompound(pvmw->page)) { + spinlock_t *ptl = pmd_lock(mm, pvmw->pmd); + + spin_unlock(ptl); + } return false; } if (!map_pte(pvmw)) --- a/mm/rmap.c~mm-thp-try_to_unmap-use-ttu_sync-for-safe-splitting +++ a/mm/rmap.c @@ -1405,6 +1405,15 @@ static bool try_to_unmap_one(struct page struct mmu_notifier_range range; enum ttu_flags flags = (enum ttu_flags)(long)arg; + /* + * When racing against e.g. zap_pte_range() on another cpu, + * in between its ptep_get_and_clear_full() and page_remove_rmap(), + * try_to_unmap() may return false when it is about to become true, + * if page table locking is skipped: use TTU_SYNC to wait for that. + */ + if (flags & TTU_SYNC) + pvmw.flags = PVMW_SYNC; + /* munlock has nothing to gain from examining un-locked vmas */ if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED)) return true; @@ -1777,7 +1786,13 @@ bool try_to_unmap(struct page *page, enu else rmap_walk(page, &rwc); - return !page_mapcount(page) ? true : false; + /* + * When racing against e.g. zap_pte_range() on another cpu, + * in between its ptep_get_and_clear_full() and page_remove_rmap(), + * try_to_unmap() may return false when it is about to become true, + * if page table locking is skipped: use TTU_SYNC to wait for that. + */ + return !page_mapcount(page); } /**