Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754243Ab3EGW2W (ORCPT ); Tue, 7 May 2013 18:28:22 -0400 Received: from mail-qc0-f178.google.com ([209.85.216.178]:48475 "EHLO mail-qc0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751297Ab3EGW2T (ORCPT ); Tue, 7 May 2013 18:28:19 -0400 MIME-Version: 1.0 In-Reply-To: <1367963581-fsskl9vz-mutt-n-horiguchi@ah.jp.nec.com> References: <1367959554-3218-1-git-send-email-j.glisse@gmail.com> <1367963581-fsskl9vz-mutt-n-horiguchi@ah.jp.nec.com> Date: Tue, 7 May 2013 18:28:18 -0400 Message-ID: Subject: Re: [PATCH 1/3] mm: honor FOLL_GET flag in follow_hugetlb_page From: Jerome Glisse To: Naoya Horiguchi Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jerome Glisse Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2540 Lines: 63 On Tue, May 7, 2013 at 5:53 PM, Naoya Horiguchi wrote: > 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(). 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. > 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). Cheers, Jerome -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/