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[91.12.100.180]) by smtp.gmail.com with ESMTPSA id j205sm204790wma.42.2020.03.02.12.22.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 02 Mar 2020 12:22:58 -0800 (PST) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable From: David Hildenbrand Mime-Version: 1.0 (1.0) Subject: Re: [PATCH RESEND v6 02/16] mm/gup: Fix __get_user_pages() on fault retry of hugetlb Date: Mon, 2 Mar 2020 21:22:57 +0100 Message-Id: <671E8651-F7BB-48AF-AFA8-AB38C7AE7BFE@redhat.com> References: <20200302200731.GA464129@xz-x1> Cc: David Hildenbrand , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrea Arcangeli , Martin Cracauer , Linus Torvalds , Mike Rapoport , "Kirill A . Shutemov" , Johannes Weiner , "Dr . David Alan Gilbert" , Bobby Powers , Maya Gokhale , Jerome Glisse , Mike Kravetz , Matthew Wilcox , Marty McFadden , Mel Gorman , Hugh Dickins , Brian Geffon , Denis Plotnikov , Pavel Emelyanov In-Reply-To: <20200302200731.GA464129@xz-x1> To: Peter Xu X-Mailer: iPhone Mail (17D50) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Am 02.03.2020 um 21:07 schrieb Peter Xu : >=20 > =EF=BB=BFOn Mon, Mar 02, 2020 at 08:02:34PM +0100, David Hildenbrand wrote= : >>> On 20.02.20 16:53, Peter Xu wrote: >>> When follow_hugetlb_page() returns with *locked=3D=3D0, it means we've g= ot >>> a VM_FAULT_RETRY within the fauling process and we've released the >>> mmap_sem. When that happens, we should stop and bail out. >>>=20 >>> Signed-off-by: Peter Xu >>> --- >>> mm/gup.c | 10 ++++++++++ >>> 1 file changed, 10 insertions(+) >>>=20 >>> diff --git a/mm/gup.c b/mm/gup.c >>> index 1b4411bd0042..76cb420c0fb7 100644 >>> --- a/mm/gup.c >>> +++ b/mm/gup.c >>> @@ -849,6 +849,16 @@ static long __get_user_pages(struct task_struct *ts= k, struct mm_struct *mm, >>> i =3D follow_hugetlb_page(mm, vma, pages, vmas, >>> &start, &nr_pages, i, >>> gup_flags, locked); >>> + if (locked && *locked =3D=3D 0) { >>> + /* >>> + * We've got a VM_FAULT_RETRY >>> + * and we've lost mmap_sem. >>> + * We must stop here. >>> + */ >>> + BUG_ON(gup_flags & FOLL_NOWAIT); >>> + BUG_ON(ret !=3D 0); >>=20 >> Can we be sure ret is really set to !=3D 0 at this point? At least, >> reading the code this is not clear to me. >=20 > Here I wanted to make sure ret is zero (it's BUG_ON, not assert). Sorry, I completely misread that BUG_ON for whatever reason, maybe I was sta= ring for too long into my computer screen :) >=20 > "ret" is the fallback return value only if error happens when i=3D=3D0. > Here we want to make sure even if no page is pinned we'll return zero > gracefully when VM_FAULT_RETRY happened when following the hugetlb > pages. Makes sense! >=20 >>=20 >> Shouldn't we set "ret =3D i" and assert that i is an error (e.g., EBUSY?)= . >> Or set -EBUSY explicitly? >=20 > No. Here "i" could only be either positive (when we've got some pages > pinned no matter where), or zero (when follow_hugetlb_page released > the mmap_sem on the first page that it wants to pin). So imo "i" > should never be negative instead. I briefly scanned the function and spotted some errors being returned, that=E2= =80=98s why I was wondering. Thanks!