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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 Subject: Re: [PATCH RESEND v6 02/16] mm/gup: Fix __get_user_pages() on fault retry of hugetlb Message-ID: <20200302200731.GA464129@xz-x1> References: <20200220155353.8676-1-peterx@redhat.com> <20200220155353.8676-3-peterx@redhat.com> <795d9bd0-3f82-d5ce-fe03-4d405d9e6bce@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <795d9bd0-3f82-d5ce-fe03-4d405d9e6bce@redhat.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 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==0, it means we've got > > a VM_FAULT_RETRY within the fauling process and we've released the > > mmap_sem. When that happens, we should stop and bail out. > > > > Signed-off-by: Peter Xu > > --- > > mm/gup.c | 10 ++++++++++ > > 1 file changed, 10 insertions(+) > > > > 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 *tsk, struct mm_struct *mm, > > i = follow_hugetlb_page(mm, vma, pages, vmas, > > &start, &nr_pages, i, > > gup_flags, locked); > > + if (locked && *locked == 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 != 0); > > Can we be sure ret is really set to != 0 at this point? At least, > reading the code this is not clear to me. Here I wanted to make sure ret is zero (it's BUG_ON, not assert). "ret" is the fallback return value only if error happens when i==0. 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. > > Shouldn't we set "ret = i" and assert that i is an error (e.g., EBUSY?). > Or set -EBUSY explicitly? 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. Thanks, -- Peter Xu