linux-kernel - Re: [PATCH RESEND v6 02/16] mm/gup: Fix __get_user

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Date:   Mon, 2 Mar 2020 20:02:34 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Martin Cracauer <cracauer@...s.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Johannes Weiner <hannes@...xchg.org>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Bobby Powers <bobbypowers@...il.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Jerome Glisse <jglisse@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Marty McFadden <mcfadden8@...l.gov>,
        Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>
Subject: Re: [PATCH RESEND v6 02/16] mm/gup: Fix __get_user_pages() on fault
 retry of hugetlb

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@...hat.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.

Shouldn't we set "ret = i" and assert that i is an error (e.g., EBUSY?).
Or set -EBUSY explicitly?

-- 
Thanks,

David / dhildenb