On Wed, Dec 1, 2010 at 8:35 AM, Rik van Riel <riel@xxxxxxxxxx> wrote:
>
> Surely somebody can have just looked up the page and
> gotten a reference count, right before your ->freepage
> call is invoked?
No.
The removal from the page cache is atomic, even in the presence of the
lockless lookup.
The page cache lookup does a "get_page_unless_zero()" on the count, so
when __remove_mapping() has removed the page using
"page_freeze_refs()", it's really gone, and cannot be looked up.
And if that is broken, then we have much more serious problems (like
aliasing the same page when doing mmap/read etc), so that's more than
just an implementation detail, it's a fundamental requirement of the
whole page-cache design.
And that's the whole point of adding this callback to the
__remove_mapping() stage: that's the _only_ point where we really end
up knowing that "yes, we really removed that page, and there are no
more users".
Linus
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