Re: [Lse-tech] Re: Hugetlbpages in very large memory machines.......

[Andrew Morton <akpm@osdl.org>]

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.