On Sun, Oct 07, 2018 at 04:38:48PM -0700, Mike Kravetz wrote: > The following hugetlbfs truncate/page fault race can be recreated > with programs doing something like the following. > > A huegtlbfs file is mmap(MAP_SHARED) with a size of 4 pages. At > mmap time, 4 huge pages are reserved for the file/mapping. So, > the global reserve count is 4. In addition, since this is a shared > mapping an entry for 4 pages is added to the file's reserve map. > The first 3 of the 4 pages are faulted into the file. As a result, > the global reserve count is now 1. > > Task A starts to fault in the last page (routines hugetlb_fault, > hugetlb_no_page). It allocates a huge page (alloc_huge_page). > The reserve map indicates there is a reserved page, so this is > used and the global reserve count goes to 0. > > Now, task B truncates the file to size 0. It starts by setting > inode size to 0(hugetlb_vmtruncate). It then unmaps all mapping > of the file (hugetlb_vmdelete_list). Since task A's page table > lock is not held at the time, truncation is not blocked. Truncation > removes the 3 pages from the file (remove_inode_hugepages). When > cleaning up the reserved pages (hugetlb_unreserve_pages), it notices > the reserve map was for 4 pages. However, it has only freed 3 pages. > So it assumes there is still (4 - 3) 1 reserved pages. It then > decrements the global reserve count by 1 and it goes negative. > > Task A then continues the page fault process and adds it's newly > acquired page to the page cache. Note that the index of this page > is beyond the size of the truncated file (0). The page fault process > then notices the file has been truncated and exits. However, the > page is left in the cache associated with the file. > > Now, if the file is immediately deleted the truncate code runs again. > It will find and free the one page associated with the file. When > cleaning up reserves, it notices the reserve map is empty. Yet, one > page freed. So, the global reserve count is decremented by (0 - 1) -1. > This returns the global count to 0 as it should be. But, it is > possible for someone else to mmap this file/range before it is deleted. > If this happens, a reserve map entry for the allocated page is created > and the reserved page is forever leaked. > > To avoid all these conditions, let's simply prevent faults to a file > while it is being truncated. Add a new truncation specific rw mutex > to hugetlbfs inode extensions. faults take the mutex in read mode, > truncation takes in write mode. Hm. Don't we have already a lock for this? I mean i_mmap_lock. -- Kirill A. Shutemov
