Re: Race between vmtruncate and mapped areas?

William Lee Irwin III
2003-05-13T23:11:39Z

On Tuesday, May 13, 2003 15:49:29 -0700 William Lee Irwin III <wli@holomorphy.com> wrote:
>> That doesn't sound like it's going to help, there isn't a unique
>> mmap_sem to be taken and so we just get caught between acquisitions
>> with the same problem.

On Tue, May 13, 2003 at 06:00:08PM -0500, Dave McCracken wrote:
> Actually it does fix it. I added code in vmtruncate_list() to do a
> down_write(&vma->vm_mm->mmap_sem) around the zap_page_range(), and the
> problem went away. It serializes against any outstanding page faults on a
> particular page table. New faults will see that the page is no longer in
> the file and fail with SIGBUS. Andrew's test case stopped failing.
> I've attached the patch so you can see what I did.
> Can anyone think of any gotchas to this solution?

Okay, what's stopping filemap_nopage() from fetching the page from
pagecache after one of the mm->mmap_sem's is dropped but before
truncate_inode_pages() removes the page? The fault path is only locked
out for one mm during one part of the operation. I can see taking
->i_sem in do_no_page() fixing it, but not ->mmap_sem in vmtruncate()
(but of course that's _far_ too heavy-handed to merge at all).

-- wli