Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754312Ab3EGXHr (ORCPT ); Tue, 7 May 2013 19:07:47 -0400 Received: from mail-qe0-f52.google.com ([209.85.128.52]:53766 "EHLO mail-qe0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750881Ab3EGXHq (ORCPT ); Tue, 7 May 2013 19:07:46 -0400 MIME-Version: 1.0 In-Reply-To: <1367967607-vpfj91fc-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> <1367967607-vpfj91fc-mutt-n-horiguchi@ah.jp.nec.com> Date: Tue, 7 May 2013 19:07:45 -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: 3377 Lines: 77 On Tue, May 7, 2013 at 7:00 PM, Naoya Horiguchi 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 >> 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. > > 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 -- 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/