Re: Fix bogus get_page() calls in hugepage code

[Andrew Morton <akpm@osdl.org>]

David Gibson <david@gibson.dropbear.id.au> wrote:
>
> On some archs the functions used to implement follow_page() for
>  hugepages do a get_page().  This is unlike the normal-page path for
>  follow_page(), so presumably a bug.  This patch fixes it.

get_user_pages() is supposed to pin the pages which it placed into the
callers pages[] array.

And the caller of get_user_pages() is supposed to unpin those pages when
they are finished with.

So follow_hugetlb_page() is currently doing the right thing.  The asymmetry
with follow_page() is awkward, but the overall intent was to minimise the
amount of impact which the hugepage code has on core MM.


Aside: note that get_user_pages() doesn't hold page_table_lock while
walking the pagetables, whereas it does hold that lock while walking the
regular pages' pagetables.  This is because the caller of get_user_pages()
holds down_read(mmap_sem), whereas hugetlb pagetable setup and teardown
always happens under down_write(mmap_sem).