From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with SMTP id 458788D0039 for ; Wed, 2 Feb 2011 08:32:04 -0500 (EST) Date: Wed, 2 Feb 2011 15:31:57 +0200 From: Gleb Natapov Subject: Re: [PATCH 1/2] Allow GUP to fail instead of waiting on a page. Message-ID: <20110202133157.GI14984@redhat.com> References: <1296559307-14637-1-git-send-email-gleb@redhat.com> <1296559307-14637-2-git-send-email-gleb@redhat.com> <20110201164240.9a5c06e9.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110201164240.9a5c06e9.akpm@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: avi@redhat.com, mtosatti@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Linus Torvalds , Rik van Riel , Hugh Dickins On Tue, Feb 01, 2011 at 04:42:40PM -0800, Andrew Morton wrote: > On Tue, 1 Feb 2011 13:21:46 +0200 > Gleb Natapov wrote: > > > GUP user may want to try to acquire a reference to a page if it is already > > in memory, but not if IO, to bring it in, is needed. For example KVM may > > tell vcpu to schedule another guest process if current one is trying to > > access swapped out page. Meanwhile, the page will be swapped in and the > > guest process, that depends on it, will be able to run again. > > > > This patch adds FAULT_FLAG_RETRY_NOWAIT (suggested by Linus) and > > FOLL_NOWAIT follow_page flags. FAULT_FLAG_RETRY_NOWAIT, when used in > > conjunction with VM_FAULT_ALLOW_RETRY, indicates to handle_mm_fault that > > it shouldn't drop mmap_sem and wait on a page, but return VM_FAULT_RETRY > > instead. > > > > ... > > > > +#define FOLL_NOWAIT 0x20 /* return if disk transfer is needed */ > > The comment is a little misleading. Or incomplete. > > For both swap-backed and file-backed pages, the code will initiate the > disk transfer and will then return without waiting for it to complete. > This (important!) information isn't really presented in either the > changelog or the code itself. > > This? > Yes, this is better. Thanks you. I see that the patch below is in your queue already. Should I re-spin my patch with improved comment anyway? > --- a/include/linux/mm.h~mm-allow-gup-to-fail-instead-of-waiting-on-a-page-fix > +++ a/include/linux/mm.h > @@ -1537,7 +1537,8 @@ struct page *follow_page(struct vm_area_ > #define FOLL_GET 0x04 /* do get_page on page */ > #define FOLL_DUMP 0x08 /* give error on hole if it would be zero */ > #define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */ > -#define FOLL_NOWAIT 0x20 /* return if disk transfer is needed */ > +#define FOLL_NOWAIT 0x20 /* if a disk transfer is needed, start the IO > + * and return without waiting upon it */ > #define FOLL_MLOCK 0x40 /* mark page as mlocked */ > #define FOLL_SPLIT 0x80 /* don't return transhuge pages, split them */ > > _ -- Gleb. -- 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 policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: email@kvack.org