From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758805Ab0LNBFv (ORCPT ); Mon, 13 Dec 2010 20:05:51 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:45555 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758768Ab0LNBFu (ORCPT ); Mon, 13 Dec 2010 20:05:50 -0500 Date: Mon, 13 Dec 2010 17:05:26 -0800 From: Andrew Morton To: Michel Lespinasse Cc: Linus Torvalds , "linux-mm@kvack.org" , Hugh Dickins , Rik van Riel , Peter Zijlstra , Nick Piggin , KOSAKI Motohiro , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 1/6] mlock: only hold mmap_sem in shared mode when faulting in pages Message-Id: <20101213170526.3b010058.akpm@linux-foundation.org> In-Reply-To: <20101214005140.GA29904@google.com> References: <1291335412-16231-1-git-send-email-walken@google.com> <1291335412-16231-2-git-send-email-walken@google.com> <20101208152740.ac449c3d.akpm@linux-foundation.org> <20101214005140.GA29904@google.com> X-Mailer: Sylpheed 3.0.2 (GTK+ 2.20.1; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 13 Dec 2010 16:51:40 -0800 Michel Lespinasse wrote: > On Thu, Dec 9, 2010 at 10:11 PM, Linus Torvalds wrote: > > On Wednesday, December 8, 2010, Michel Lespinasse wrote: > >> > >> Yes, patch 1/6 changes the long hold time to be in read mode instead > >> of write mode, which is only a band-aid. But, this prepares for patch > >> 5/6, which releases mmap_sem whenever there is contention on it or > >> when blocking on disk reads. > > > > I have to say that I'm not a huge fan of that horribly kludgy > > contention check case. > > > > The "move page-in to read-locked sequence" and the changes to > > get_user_pages look fine, but the contention thing is just disgusting. > > I'd really like to see some other approach if at all possible. > > Andrew, should I amend my patches to remove the rwsem_is_contended() code ? > This would involve: > - remove rwsem-implement-rwsem_is_contended.patch and > x86-rwsem-more-precise-rwsem_is_contended-implementation.patch > - in mlock-do-not-hold-mmap_sem-for-extended-periods-of-time.patch, > drop the one hunk making use of rwsem_is_contended (rest of the patch > would still work without it) I think I fixed all that up. > - optionally, follow up patch to limit batch size to a constant > in do_mlock_pages(): > > diff --git a/mm/mlock.c b/mm/mlock.c > index 569ae6a..a505a7e 100644 > --- a/mm/mlock.c > +++ b/mm/mlock.c > @@ -457,15 +457,23 @@ static int do_mlock_pages(unsigned long start, size_t len) > continue; > if (nstart < vma->vm_start) > nstart = vma->vm_start; > + /* > + * Constrain batch size to limit mmap_sem hold time. > + */ > + if (nend > nstart + 1024 * PAGE_SIZE) > + nend = nstart + 1024 * PAGE_SIZE; > /* > * Now fault in a range of pages. __mlock_vma_pages_range() > * double checks the vma flags, so that it won't mlock pages > * if the vma was already munlocked. > */ > ret = __mlock_vma_pages_range(vma, nstart, nend, &locked); > if (ret < 0) { > ret = __mlock_posix_error_return(ret); > break; > + } else if (locked) { > + locked = 0; > + up_read(&mm->mmap_sem); > } > nend = nstart + ret * PAGE_SIZE; > ret = 0; > > > I don't really prefer using a constant, but I'm not sure how else to make > Linus happy :) rwsem_is_contended() didn't seem so bad to me. Reading 1024 pages can still take a long time. I can't immediately think of a better approach though.