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SubjectRe: [PATCH RFC 1/1] hugetlbfs: introduce truncation/fault mutex to avoid races
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

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