From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx104.postini.com [74.125.245.104]) by kanga.kvack.org (Postfix) with SMTP id 645EF6B004D for ; Wed, 2 May 2012 05:09:55 -0400 (EDT) Received: by obbwd18 with SMTP id wd18so932914obb.14 for ; Wed, 02 May 2012 02:09:54 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20120502081705.GB16976@quack.suse.cz> References: <1335778207-6511-1-git-send-email-jack@suse.cz> <20120502081705.GB16976@quack.suse.cz> Date: Wed, 2 May 2012 19:09:54 +1000 Message-ID: Subject: Re: [PATCH] Describe race of direct read and fork for unaligned buffers From: Nick Piggin Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: Jeff Moyer , KOSAKI Motohiro , Michael Kerrisk , LKML , linux-man@vger.kernel.org, linux-mm@kvack.org, mgorman@suse.de, Andrea Arcangeli , Woodman On 2 May 2012 18:17, Jan Kara wrote: > On Wed 02-05-12 01:50:46, Nick Piggin wrote: >> KOSAKI-san is correct, I think. >> >> The race is something like this: >> >> DIO-read >> =C2=A0 =C2=A0 page =3D get_user_pages() >> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 fork() >> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 COW(page= ) >> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0touch(page) >> =C2=A0 =C2=A0 DMA(page) >> =C2=A0 =C2=A0 page_cache_release(page); >> >> So whether parent or child touches the page, determines who gets the >> actual DMA target, and who gets the copy. > =C2=A0OK, this is roughly what I understood from original threads as well= . So > if our buffer is page aligned and its size is page aligned, you would hit > the corruption only if you do modify the buffer while IO to / from that b= uffer > is in progress. And that would seem like a really bad programming practic= e > anyway. So I still believe that having everything page size aligned will > effectively remove the problem although I agree it does not aim at the co= re > of it. I see what you mean. I'm not sure, though. For most apps it's bad practice I think. If you get i= nto realm of sophisticated, performance critical IO/storage managers, it would not surprise me if such concurrent buffer modifications could be allowed. We allow exactly such a thing in our pagecache layer. Although probably those would be using shared mmaps for their buffer cache. I think it is safest to make a default policy of asking for IOs against pri= vate cow-able mappings to be quiesced before fork, so there are no surprises or reliance on COW details in the mm. Do you think? -- 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