Re: [PATCH v2 2/2] hugetlbfs: Use i_mmap_rwsem to fix page fault/truncate race

From: Kirill A. Shutemov
Date: Fri Dec 21 2018 - 15:21:55 EST

On Fri, Dec 21, 2018 at 10:28:25AM -0800, Mike Kravetz wrote:
> On 12/21/18 2:28 AM, Kirill A. Shutemov wrote:
> > On Tue, Dec 18, 2018 at 02:35:57PM -0800, Mike Kravetz wrote:
> >> 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
> >> longer in truncation and hold punch code to cover the call to
> >> remove_inode_hugepages.
> >
> > One of remove_inode_hugepages() callers is noticeably missing --
> > hugetlbfs_evict_inode(). Why?
> >
> > It at least deserves a comment on why the lock rule doesn't apply to it.
> In the case of hugetlbfs_evict_inode, the vfs layer guarantees there are
> no more users of the inode/file.

I'm not convinced that it is true. See documentation for ->evict_inode()
in Documentation/filesystems/porting:

Caller does *not* evict the pagecache or inode-associated
metadata buffers; the method has to use truncate_inode_pages_final() to get rid
of those.

Is hugetlbfs special here?

Kirill A. Shutemov