From 87bf91d39bb52b688fb411d668fbe7df278b29ae Mon Sep 17 00:00:00 2001 From: Mike Kravetz Date: Wed, 1 Apr 2020 21:11:08 -0700 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. Signed-off-by: Mike Kravetz Signed-off-by: Andrew Morton 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 Link: http://lkml.kernel.org/r/20200316205756.146666-3-mike.kravetz@oracle.com Signed-off-by: Linus Torvalds --- fs/hugetlbfs/inode.c | 28 ++++++++++++++++++++-------- 1 file changed, 20 insertions(+), 8 deletions(-) (limited to 'fs/hugetlbfs') diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c index ce9d354ea5c2..991c60c7ffe0 100644 --- a/fs/hugetlbfs/inode.c +++ b/fs/hugetlbfs/inode.c @@ -393,10 +393,9 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end) * 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(struct inode *inode, loff_t lstart, 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(struct inode *inode, loff_t lstart, } 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 inode *inode, loff_t offset) 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 file *file, int mode, loff_t offset, /* 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]); -- cgit v1.2.3