Re: [RFC, PATCHv2 0/2] mm: map few pages around fault address if they are in page cache

[Rik van Riel]

On 02/17/2014 02:01 PM, Linus Torvalds wrote:

>  - increment the page _mapcount (iow, do "page_add_file_rmap()"
> early). This guarantees that any *subsequent* unmap activity on this
> page will walk the file mapping lists, and become serialized by the
> page table lock we hold.
> 
>  - mb_after_atomic_inc() (this is generally free)
> 
>  - test that the page is still unlocked and uptodate, and the page
> mapping still points to our page.
> 
>  - if that is true, we're all good, we can use the page, otherwise we
> decrement the mapcount (page_remove_rmap()) and skip the page.
> 
> Hmm? Doing something like this means that we would never lock the
> pages we prefault, and you can go back to your gang lookup rather than
> that "one page at a time". And the race case is basically never going
> to trigger.
> 
> Comments?

What would the direct io code do when it runs into a page with
elevated mapcount, but for which a mapping cannot be found yet?

Looking at the code, it looks like the above scheme could cause
some trouble with invalidate_inode_pages2_range(), which has
the following sequence:

                        if (page_mapped(page)) {
				... unmap page
			}
                        BUG_ON(page_mapped(page));

In other words, it looks like incrementing _mapcount first could
lead to an oops in the truncate and direct IO code.

The page lock is used to prevent such races.

*sigh*

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