From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx204.postini.com [74.125.245.204]) by kanga.kvack.org (Postfix) with SMTP id 84F156B00E3 for ; Tue, 7 May 2013 17:53:06 -0400 (EDT) Date: Tue, 07 May 2013 17:53:01 -0400 From: Naoya Horiguchi Message-ID: <1367963581-fsskl9vz-mutt-n-horiguchi@ah.jp.nec.com> In-Reply-To: <1367959554-3218-1-git-send-email-j.glisse@gmail.com> References: <1367959554-3218-1-git-send-email-j.glisse@gmail.com> Subject: Re: [PATCH 1/3] mm: honor FOLL_GET flag in follow_hugetlb_page Mime-Version: 1.0 Content-Type: text/plain; charset=iso-2022-jp Content-Transfer-Encoding: 7bit Content-Disposition: inline Sender: owner-linux-mm@kvack.org List-ID: To: j.glisse@gmail.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jerome Glisse On Tue, May 07, 2013 at 04:45:54PM -0400, j.glisse@gmail.com wrote: > From: Jerome Glisse > > Do not increase page count if FOLL_GET is not set. > > Signed-off-by: Jerome Glisse > --- > 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(). 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). Thanks, Naoya Horiguchi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org