linux-kernel - Re: [PATCH v2 04/26] mm: allow VM_FAULT

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Date:   Wed, 20 Feb 2019 19:48:49 +0800
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Jerome Glisse <jglisse@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>, Shaohua Li <shli@...com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v2 04/26] mm: allow VM_FAULT_RETRY for multiple times

On Wed, Feb 13, 2019 at 11:34:44AM +0800, Peter Xu wrote:
> On Tue, Feb 12, 2019 at 10:56:10AM +0800, Peter Xu wrote:
> 
> [...]
> 
> > @@ -1351,7 +1351,7 @@ EXPORT_SYMBOL_GPL(__lock_page_killable);
> >  int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
> >  			 unsigned int flags)
> >  {
> > -	if (flags & FAULT_FLAG_ALLOW_RETRY) {
> > +	if (!flags & FAULT_FLAG_TRIED) {
> 
> Sorry, this should be:
> 
>         if (!(flags & FAULT_FLAG_TRIED))

Ok this is problematic too...  Because we for sure allow the page
fault flags to be both !ALLOW_RETRY and !TRIED (e.g., when doing GUP
and when __get_user_pages() is with locked==NULL and !FOLL_NOWAIT).
So current code will fall through the if condition and call
up_read(mmap_sem) even if above condition happened (while we shouldn't
because the GUP caller would assume the mmap_sem should be still
held).  So the correct check should be:

  if ((flags & FAULT_FLAG_ALLOW_RETRY) && !(flags & FAULT_FLAG_TRIED))

To make things easier, I'll just repost this single patch later.
Sorry for the noise.

Regards,

-- 
Peter Xu