Re: Race between vmtruncate and mapped areas?

[William Lee Irwin III]

On Tue, May 13, 2003 at 03:44:45PM -0500, Dave McCracken wrote:
> As part of chasing the BUG() we've been seeing in objrmap I took a good
> look at vmtruncate().  I believe I've identified a race condition that no
> only  triggers that BUG(), but also could cause some strange behavior
> without the objrmap patch.
> Basically vmtruncate() does the following steps:  first, it unmaps the
> truncated pages from all page tables using zap_page_range().  Then it
> removes those pages from the page cache using truncate_inode_pages().
> These steps are done without any lock that I can find, so it's possible for
> another task to get in between the unmap and the remove, and remap one or
> more pages back into its page tables.
> The result of this is a page that has been disconnected from the file but
> is mapped in a task's address space as if it were still part of that file.
> Any further modifications to this page will be lost.
> I can easily detect this condition by adding a bugcheck for page_mapped()
> in truncate_complete_page(), then running Andrew's bash-shared-mapping test
> case.
> Please feel free to poke holes in my analysis.  I'm not at all sure I
> haven't missed some subtlety here.

There are various flavors of pain here. I think this was the new page
state hugh was talking about in an earlier post on the subject.

-- wli