From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: + hugetlbfs-use-i_mmap_rwsem-to-address-page-fault-truncate-race.patch added to -mm tree Date: Wed, 18 Mar 2020 14:30:07 -0700 Message-ID: <20200318213007.UraObrN7X%akpm@linux-foundation.org> References: <20200305222751.6d781a3f2802d79510941e4e@linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from mail.kernel.org ([198.145.29.99]:55362 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726733AbgCRVaJ (ORCPT ); Wed, 18 Mar 2020 17:30:09 -0400 In-Reply-To: <20200305222751.6d781a3f2802d79510941e4e@linux-foundation.org> Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: aarcange@redhat.com, aneesh.kumar@linux.vnet.ibm.com, dave@stgolabs.net, hughd@google.com, kirill.shutemov@linux.intel.com, mhocko@kernel.org, mike.kravetz@oracle.com, mm-commits@vger.kernel.org, n-horiguchi@ah.jp.nec.com, prakash.sangappa@oracle.com The patch titled Subject: hugetlbfs: Use i_mmap_rwsem to address page fault/truncate race has been added to the -mm tree. Its filename is hugetlbfs-use-i_mmap_rwsem-to-address-page-fault-truncate-race.patch This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/hugetlbfs-use-i_mmap_rwsem-to-address-page-fault-truncate-race.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/hugetlbfs-use-i_mmap_rwsem-to-address-page-fault-truncate-race.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: Mike Kravetz Subject: hugetlbfs: Use i_mmap_rwsem to address page fault/truncate race hugetlbfs page faults can race with truncate and hole punch operations. Current code in the page fault path attempts to handle this by 'backing out' operations if we encounter the race. One obvious omission in the current code is removing a page newly added to the page cache. This is pretty straight forward to address, but there is a more subtle and difficult issue of backing out hugetlb reservations. To handle this correctly, the 'reservation state' before page allocation needs to be noted so that it can be properly backed out. There are four distinct possibilities for reservation state: shared/reserved, shared/no-resv, private/reserved and private/no-resv. Backing out a reservation may require memory allocation which could fail so that needs to be taken into account as well. Instead of writing the required complicated code for this rare occurrence, just eliminate the race. i_mmap_rwsem is now held in read mode for the duration of page fault processing. Hold i_mmap_rwsem in write mode when modifying i_size. In this way, truncation can not proceed when page faults are being processed. In addition, i_size will not change during fault processing so a single check can be made to ensure faults are not beyond (proposed) end of file. Faults can still race with hole punch, but that race is handled by existing code and the use of hugetlb_fault_mutex. With this modification, checks for races with truncation in the page fault path can be simplified and removed. remove_inode_hugepages no longer needs to take hugetlb_fault_mutex in the case of truncation. Comments are expanded to explain reasoning behind locking. Link: http://lkml.kernel.org/r/20200316205756.146666-3-mike.kravetz@oracle.com Signed-off-by: Mike Kravetz Cc: Andrea Arcangeli Cc: "Aneesh Kumar K . V" Cc: Davidlohr Bueso Cc: Hugh Dickins Cc: "Kirill A . Shutemov" Cc: Michal Hocko Cc: Naoya Horiguchi Cc: Prakash Sangappa Signed-off-by: Andrew Morton --- fs/hugetlbfs/inode.c | 28 ++++++++++++++++++++-------- mm/hugetlb.c | 23 +++++++++++------------ 2 files changed, 31 insertions(+), 20 deletions(-) --- a/fs/hugetlbfs/inode.c~hugetlbfs-use-i_mmap_rwsem-to-address-page-fault-truncate-race +++ a/fs/hugetlbfs/inode.c @@ -393,10 +393,9 @@ hugetlb_vmdelete_list(struct rb_root_cac * In this case, we first scan the range and release found pages. * After releasing pages, hugetlb_unreserve_pages cleans up region/reserv * maps and global counts. Page faults can not race with truncation - * in this routine. hugetlb_no_page() prevents page faults in the - * truncated range. It checks i_size before allocation, and again after - * with the page table lock for the page held. The same lock must be - * acquired to unmap a page. + * in this routine. hugetlb_no_page() holds i_mmap_rwsem and prevents + * page faults in the truncated range by checking i_size. i_size is + * modified while holding i_mmap_rwsem. * hole punch is indicated if end is not LLONG_MAX * In the hole punch case we scan the range and release found pages. * Only when releasing a page is the associated region/reserv map @@ -436,7 +435,15 @@ static void remove_inode_hugepages(struc index = page->index; hash = hugetlb_fault_mutex_hash(mapping, index); - mutex_lock(&hugetlb_fault_mutex_table[hash]); + if (!truncate_op) { + /* + * Only need to hold the fault mutex in the + * hole punch case. This prevents races with + * page faults. Races are not possible in the + * case of truncation. + */ + mutex_lock(&hugetlb_fault_mutex_table[hash]); + } /* * If page is mapped, it was faulted in after being @@ -479,7 +486,8 @@ static void remove_inode_hugepages(struc } unlock_page(page); - mutex_unlock(&hugetlb_fault_mutex_table[hash]); + if (!truncate_op) + mutex_unlock(&hugetlb_fault_mutex_table[hash]); } huge_pagevec_release(&pvec); cond_resched(); @@ -517,8 +525,8 @@ static int hugetlb_vmtruncate(struct ino BUG_ON(offset & ~huge_page_mask(h)); pgoff = offset >> PAGE_SHIFT; - i_size_write(inode, offset); i_mmap_lock_write(mapping); + i_size_write(inode, offset); if (!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)) hugetlb_vmdelete_list(&mapping->i_mmap, pgoff, 0); i_mmap_unlock_write(mapping); @@ -640,7 +648,11 @@ static long hugetlbfs_fallocate(struct f /* addr is the offset within the file (zero based) */ addr = index * hpage_size; - /* mutex taken here, fault path and hole punch */ + /* + * fault mutex taken here, protects against fault path + * and hole punch. inode_lock previously taken protects + * against truncation. + */ hash = hugetlb_fault_mutex_hash(mapping, index); mutex_lock(&hugetlb_fault_mutex_table[hash]); --- a/mm/hugetlb.c~hugetlbfs-use-i_mmap_rwsem-to-address-page-fault-truncate-race +++ a/mm/hugetlb.c @@ -3929,16 +3929,17 @@ static vm_fault_t hugetlb_no_page(struct } /* - * Use page lock to guard against racing truncation - * before we get page_table_lock. + * We can not race with truncation due to holding i_mmap_rwsem. + * i_size is modified when holding i_mmap_rwsem, so check here + * once for faults beyond end of file. */ + size = i_size_read(mapping->host) >> huge_page_shift(h); + if (idx >= size) + goto out; + retry: page = find_lock_page(mapping, idx); if (!page) { - size = i_size_read(mapping->host) >> huge_page_shift(h); - if (idx >= size) - goto out; - /* * Check for page in userfault range */ @@ -4044,10 +4045,6 @@ retry: } ptl = huge_pte_lock(h, mm, ptep); - size = i_size_read(mapping->host) >> huge_page_shift(h); - if (idx >= size) - goto backout;