From: Peter Xu <peterx@redhat.com>
To: David Hildenbrand <david@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Andrea Arcangeli <aarcange@redhat.com>,
Martin Cracauer <cracauer@cons.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
"Kirill A . Shutemov" <kirill@shutemov.name>,
Johannes Weiner <hannes@cmpxchg.org>,
"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
Bobby Powers <bobbypowers@gmail.com>,
Maya Gokhale <gokhale2@llnl.gov>,
Jerome Glisse <jglisse@redhat.com>,
Mike Kravetz <mike.kravetz@oracle.com>,
Matthew Wilcox <willy@infradead.org>,
Marty McFadden <mcfadden8@llnl.gov>, Mel Gorman <mgorman@suse.de>,
Hugh Dickins <hughd@google.com>,
Brian Geffon <bgeffon@google.com>,
Denis Plotnikov <dplotnikov@virtuozzo.com>,
Pavel Emelyanov <xemul@virtuozzo.com>
Subject: Re: [PATCH RESEND v6 02/16] mm/gup: Fix __get_user_pages() on fault retry of hugetlb
Date: Mon, 2 Mar 2020 15:07:31 -0500 [thread overview]
Message-ID: <20200302200731.GA464129@xz-x1> (raw)
In-Reply-To: <795d9bd0-3f82-d5ce-fe03-4d405d9e6bce@redhat.com>
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 <peterx@redhat.com>
> > ---
> > 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
next prev parent reply other threads:[~2020-03-02 20:07 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-20 15:53 [PATCH RESEND v6 00/16] mm: Page fault enhancements Peter Xu
2020-02-20 15:53 ` [PATCH RESEND v6 01/16] mm/gup: Rename "nonblocking" to "locked" where proper Peter Xu
2020-02-20 15:53 ` [PATCH RESEND v6 02/16] mm/gup: Fix __get_user_pages() on fault retry of hugetlb Peter Xu
2020-03-02 19:02 ` David Hildenbrand
2020-03-02 20:07 ` Peter Xu [this message]
2020-03-02 20:22 ` David Hildenbrand
2020-02-20 15:53 ` [PATCH RESEND v6 03/16] mm: Introduce fault_signal_pending() Peter Xu
2020-03-02 19:04 ` David Hildenbrand
2020-02-20 15:53 ` [PATCH RESEND v6 04/16] x86/mm: Use helper fault_signal_pending() Peter Xu
2020-02-20 15:58 ` [PATCH RESEND v6 05/16] arc/mm: " Peter Xu
2020-02-20 15:59 ` [PATCH RESEND v6 06/16] arm64/mm: " Peter Xu
2020-02-20 16:02 ` [PATCH RESEND v6 07/16] powerpc/mm: " Peter Xu
2020-02-20 16:02 ` [PATCH RESEND v6 08/16] sh/mm: " Peter Xu
2020-02-20 16:02 ` [PATCH RESEND v6 09/16] mm: Return faster for non-fatal signals in user mode faults Peter Xu
2020-02-20 16:02 ` [PATCH RESEND v6 10/16] userfaultfd: Don't retake mmap_sem to emulate NOPAGE Peter Xu
2020-02-20 16:02 ` [PATCH RESEND v6 11/16] mm: Introduce FAULT_FLAG_DEFAULT Peter Xu
2020-02-20 16:02 ` [PATCH RESEND v6 13/16] mm: Allow VM_FAULT_RETRY for multiple times Peter Xu
2020-02-20 16:02 ` [PATCH RESEND v6 15/16] mm/gup: Allow to react to fatal signals Peter Xu
2020-02-20 16:03 ` [PATCH RESEND v6 16/16] mm/userfaultfd: Honor FAULT_FLAG_KILLABLE in fault path Peter Xu
2020-02-20 19:53 ` [PATCH RESEND v6 12/16] mm: Introduce FAULT_FLAG_INTERRUPTIBLE Peter Xu
2020-02-20 19:53 ` [PATCH RESEND v6 14/16] mm/gup: Allow VM_FAULT_RETRY for multiple times Peter Xu
2020-02-21 19:26 ` [PATCH RESEND v6 00/16] mm: Page fault enhancements Brian Geffon
2020-03-02 17:31 ` Peter Xu
2020-02-21 19:32 ` Linus Torvalds
2020-02-21 20:11 ` Peter Xu
2020-03-07 20:33 ` David Hildenbrand
2020-03-07 21:47 ` Peter Xu
2020-03-08 12:12 ` David Hildenbrand
2020-03-09 19:51 ` Peter Xu
2020-03-09 20:06 ` David Hildenbrand
2020-03-08 12:49 ` David Hildenbrand
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