Re: Race between vmtruncate and mapped areas?

William Lee Irwin III
2003-05-13T23:20:38Z

On Tuesday, May 13, 2003 16:11:39 -0700 William Lee Irwin III <wli@holomorphy.com> wrote:
>> 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).

On Tue, May 13, 2003 at 06:16:16PM -0500, Dave McCracken wrote:
> mmap_sem is held for read across the entire fault, so by the time
> vmtruncate_list() can call zap_page_range() the page has been instantiated
> in the page table and will get removed.

That's not quite the answer, inode->i_size is.

The mmap_sem works because then ->i_size can't be sampled by
filemap_nopage() before the pagetable wiping operation starts.

-- wli