From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx166.postini.com [74.125.245.166]) by kanga.kvack.org (Postfix) with SMTP id AF7876B004F for ; Sat, 14 Jan 2012 00:27:21 -0500 (EST) Received: by wera13 with SMTP id a13so1155885wer.14 for ; Fri, 13 Jan 2012 21:27:19 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20120113153950.7426eee2.akpm@linux-foundation.org> References: <20111222163604.GB14983@tiehlicka.suse.cz> <20120104151632.05e6b3b0.akpm@linux-foundation.org> <20120113153950.7426eee2.akpm@linux-foundation.org> Date: Sat, 14 Jan 2012 13:27:19 +0800 Message-ID: Subject: Re: [PATCH] mm: hugetlb: undo change to page mapcount in fault handler From: Hillf Danton Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Michal Hocko , linux-mm@kvack.org, LKML , KAMEZAWA Hiroyuki On Sat, Jan 14, 2012 at 7:39 AM, Andrew Morton wrote: > On Wed, 11 Jan 2012 20:06:30 +0800 > Hillf Danton wrote: > >> On Thu, Jan 5, 2012 at 7:16 AM, Andrew Morton wrote: >> > On Fri, 23 Dec 2011 21:00:41 +0800 >> > Hillf Danton wrote: >> > >> >> Page mapcount should be updated only if we are sure that the page end= s >> >> up in the page table otherwise we would leak if we couldn't COW due t= o >> >> reservations or if idx is out of bounds. >> > >> > It would be much nicer if we could run vma_needs_reservation() before >> > even looking up or allocating the page. >> > >> > And afaict the interface is set up to do that: you run >> > vma_needs_reservation() before allocating the page and then >> > vma_commit_reservation() afterwards. >> > >> > But hugetlb_no_page() and hugetlb_fault() appear to have forgotten to >> > run vma_commit_reservation() altogether. __Why isn't this as busted as >> > it appears to be? >> >> Hi Andrew >> >> IIUC the two operations, vma_{needs, commit}_reservation, are folded in >> alloc_huge_page(), need to break the pair? > > Looking at it again, it appears that the vma_needs_reservation() calls > are used to predict whether a subsequent COW attempt is going to fail. > > If that's correct then things aren't as bad as I first thought. > However I suspect the code in hugetlb_no_page() is a bit racy: the > vma_needs_reservation() call should happen after we've taken > page_table_lock. =C2=A0As things stand, another thread could sneak in the= re > and steal the reservation which this thread thought was safe. > > What do you think? > Hi Andrew The case of no page, in the fault path, is handled after acquiring hugetlb_instantiation_mutex, and on ohter hand, kmalloc is called if new region required, so no race to check reservation needed but after spinning page_table_lock. Thanks Hillf -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org