Re: [PATCH 1/3] mm: honor FOLL_GET flag in follow_hugetlb_page

[Jerome Glisse]

On Tue, May 7, 2013 at 7:00 PM, Naoya Horiguchi
<n-horiguchi@xxxxxxxxxxxxx> wrote:
> On Tue, May 07, 2013 at 06:28:18PM -0400, Jerome Glisse wrote:
>> On Tue, May 7, 2013 at 5:53 PM, Naoya Horiguchi
>> <n-horiguchi@xxxxxxxxxxxxx> wrote:
>> > On Tue, May 07, 2013 at 04:45:54PM -0400, j.glisse@xxxxxxxxx wrote:
>> >> From: Jerome Glisse <jglisse@xxxxxxxxxx>
>> >>
>> >> Do not increase page count if FOLL_GET is not set.
>> >>
>> >> Signed-off-by: Jerome Glisse <jglisse@xxxxxxxxxx>
>> >> ---
>> >>  mm/hugetlb.c | 4 +++-
>> >>  1 file changed, 3 insertions(+), 1 deletion(-)
>> >>
>> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> >> index 1a12f5b..5d1e46b 100644
>> >> --- a/mm/hugetlb.c
>> >> +++ b/mm/hugetlb.c
>> >> @@ -2991,7 +2991,9 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>> >>  same_page:
>> >>               if (pages) {
>> >>                       pages[i] = mem_map_offset(page, pfn_offset);
>> >> -                     get_page(pages[i]);
>> >> +                     if (flags & FOLL_GET) {
>> >> +                             get_page_foll(pages[i]);
>> >> +                     }
>> >>               }
>> >>
>> >>               if (vmas)
>> >> --
>> >
>> > Hi Jerome,
>> >
>> > I think that we need to be careful in handling tail pages, because
>> > __get_page_tail_foll() uses page->_mapcount as refcount.
>> > When you get refcount on a tail page and free the hugepage without
>> > putting the *mapcount*, you will hit BUG_ON() in free_huge_page().
>> > Yes, this is a very tricky workaround for thp, so to avoid making
>> > things too complicated, I think either of the following is better:
>> >  - to get refcount only for head pages, or
>> >  - to introduce a hugetlbfs variant of get_page_foll().
>>
>> Maybe a simpler variant is to just not take any refcount, ie like
>> current code if FOLL_GET is set then take refcount on all page wether
>> they are head/tail or not. I will resend with that.
>
> Hmm, I think that FOLL_GET flag means "do get_page on page", so
> the "not take any refcount" variant seems to make no sense for me.
> Would it be better just call without FOLL_GET?
>
>> > BTW, who do you expect is the caller of follow_hugetlb_page()
>> > with FOLL_GET (I can't find your subsequent patches 2/3 or 3/3)?
>> > I'm interested in this change because in my project it's necessary
>> > to implement this for hugepage migration
>> > (see https://lkml.org/lkml/2013/3/22/553).
>>
>> I can not talk about the patchset yet (and it's not fully cook) but i
>> need to be able to get the page without taking reference so without
>> the FOLL_GET flag set but i need splitting, well no real splitting, i
>> need pfn for each fake sub page of huge page (interested in physical
>> address not in the page struct).
>
> If the caller knows the vma is backed by hugetlbfs, we can get pfn of
> the tail page by adding page offset (which can be calcurated by the virtual
> address with proper huge page mask) to the head page's pfn.

Yes, i am just lazy in my code on that front. But i probably should do that
anyway, it will save me some temporary allocation.

Cheers,
Jerome
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