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SubjectRe: [Lse-tech] Re: Hugetlbpages in very large memory machines.......
Andrew Morton <akpm@osdl.org> wrote:
>
> Well that's just a dumb implementation. hugetlb_prefault() doesn't need
> page_table_lock while it is zeroing the page: just drop it, test for
> -EEXIST returned from add_to_page_cache().
>
> In fact we need to do that anyway: the current code is buggy if some other
> process with a different mm gets in there and instantiates the page in the
> pagecache before this process does: hugetlb_prefault() will return -EEXIST
> instead of simply accepting the race and using the page which someone else
> put there.
>
> After we have the page in pagecache we need to retake page_table_lock and
> check that the target pte is still pte_none(). If it is not, you know that
> some other thread has already instantiated a pte there so the new ref to
> the pagecache page can simply be dropped. See how do_no_page() handles it.
> Of course, this only applies if mmap_sem is no longer held in there.

But before implementing any of this we should move hugetlb_prefault() and
any other generic-looking functions into mm/hugetlbpage.c. We're getting
too much duplication in there.

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