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Thu, 09 Apr 2020 06:00:18 -0700 (PDT) Received: from xz-x1 ([2607:9880:19c0:32::2]) by smtp.gmail.com with ESMTPSA id l22sm22074735qkj.120.2020.04.09.06.00.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Apr 2020 06:00:17 -0700 (PDT) Date: Thu, 9 Apr 2020 09:00:16 -0400 From: Peter Xu To: Michal Hocko Cc: linux-kernel@vger.kernel.org, Linus Torvalds , linux-mm@kvack.org, Andrew Morton , syzbot+693dc11fcb53120b5559@syzkaller.appspotmail.com Subject: Re: [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal signal Message-ID: <20200409130016.GB362416@xz-x1> References: <20200408014010.80428-1-peterx@redhat.com> <20200408014010.80428-2-peterx@redhat.com> <20200409070253.GB18386@dhcp22.suse.cz> <20200409125258.GA362416@xz-x1> MIME-Version: 1.0 In-Reply-To: <20200409125258.GA362416@xz-x1> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Apr 09, 2020 at 08:52:58AM -0400, Peter Xu wrote: > On Thu, Apr 09, 2020 at 09:02:53AM +0200, Michal Hocko wrote: > > This patch has been merged and it is actually wrong after ae46d2aa6a7f > > has been merged. We can either revert or I suggest just handling >0, > > like the patch below: > >=20 > > From 03fbe30ec61e65b0927d0d41bccc7dff5f7eafd8 Mon Sep 17 00:00:00 2001 > > From: Michal Hocko > > Date: Thu, 9 Apr 2020 08:26:57 +0200 > > Subject: [PATCH] mm, mempolicy: fix up gup usage in lookup_node > >=20 > > ba841078cd05 ("mm/mempolicy: Allow lookup_node() to handle fatal signal= ") has > > added a special casing for 0 return value because that was a possible > > gup return value when interrupted by fatal signal. This has been fixed > > by ae46d2aa6a7f ("mm/gup: Let __get_user_pages_locked() return -EINTR > > for fatal signal") in the mean time so ba841078cd05 can be reverted. > > This patch however doesn't go all the way to revert it because 0 return > > value is impossible. We always get an error or 1 for a single page > > request. > >=20 > > Fixes: ba841078cd05 ("mm/mempolicy: Allow lookup_node() to handle fatal= signal") > > Signed-off-by: Michal Hocko > > --- > > mm/mempolicy.c | 5 +---- > > 1 file changed, 1 insertion(+), 4 deletions(-) > >=20 > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > > index 48ba9729062e..1965e2681877 100644 > > --- a/mm/mempolicy.c > > +++ b/mm/mempolicy.c > > @@ -927,10 +927,7 @@ static int lookup_node(struct mm_struct *mm, unsig= ned long addr) > > =20 > > =09int locked =3D 1; > > =09err =3D get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked); > > -=09if (err =3D=3D 0) { > > -=09=09/* E.g. GUP interrupted by fatal signal */ > > -=09=09err =3D -EFAULT; > > -=09} else if (err > 0) { > > +=09if (err > 0) { > > =09=09err =3D page_to_nid(p); > > =09=09put_page(p); > > =09} >=20 > Hi, Michal, >=20 > I'm totally not against this, but note that get_user_pages_locked() > could still return zero. Although I'm not 100% sure now on whether > npages=3D=3D0 will be the only case, it won't hurt to keep this ret=3D=3D= 0 > check until we consolidate the whole gup code to never return zero. >=20 > Assuming there's another case (even possible for a future gup bug) > that could return a zero, your patch will let err be anything (which > you didn't initialize err with your patch), then the function will > return a random value. So even if you really want this change, I > would suggest you initialize err to some error code. I'm sorry, not a random value, but err=3D0 will be returned as the mem policy by lookup_node(). >=20 > I just don't see much gain we get from removing that check. >=20 > Thanks, >=20 > --=20 > Peter Xu --=20 Peter Xu