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Thu, 09 Apr 2020 05:53:00 -0700 (PDT) Received: from xz-x1 ([2607:9880:19c0:32::2]) by smtp.gmail.com with ESMTPSA id p10sm2274167qtu.14.2020.04.09.05.52.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Apr 2020 05:52:59 -0700 (PDT) Date: Thu, 9 Apr 2020 08:52:58 -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: <20200409125258.GA362416@xz-x1> References: <20200408014010.80428-1-peterx@redhat.com> <20200408014010.80428-2-peterx@redhat.com> <20200409070253.GB18386@dhcp22.suse.cz> MIME-Version: 1.0 In-Reply-To: <20200409070253.GB18386@dhcp22.suse.cz> 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 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 s= ignal") > 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, unsigne= d 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} Hi, Michal, 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=3D0 check until we consolidate the whole gup code to never return zero. 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 just don't see much gain we get from removing that check. Thanks, --=20 Peter Xu