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Date:   Fri, 21 Oct 2022 17:08:30 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     Randy Dunlap <rdunlap@infradead.org>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
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        "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
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        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        Peter Xu <peterx@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [shmem] 7a7256d5f5: WARNING:possible_recursive_locking_detected
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On Fri, Oct 21, 2022 at 05:02:57PM -0700, Randy Dunlap wrote:
> 
> 
> On 10/21/22 17:00, Ira Weiny wrote:
> 
> > 
> > I was about to get this patch ready to send when I found this:
> > 
> > commit b6ebaedb4cb1a18220ae626c3a9e184ee39dd248
> > Author: Andrea Arcangeli <aarcange@redhat.com>
> > Date:   Fri Sep 4 15:47:08 2015 -0700
> > 
> >     userfaultfd: avoid mmap_sem read recursion in mcopy_atomic
> > 
> >     If the rwsem starves writers it wasn't strictly a bug but lockdep
> >     doesn't like it and this avoids depending on lowlevel implementation  
> >     details of the lock.
> >     
> >     [akpm@linux-foundation.org: delete weird BUILD_BUG_ON()]
> >     Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> >     Acked-by: Pavel Emelyanov <xemul@parallels.com>
> > ...
> > 
> > So I wonder if the true fix is something to lockdep?
> > 
> > Regardless I'll send the below patch because it will restore things to a
> > working order.
> 
> 
> with ; added to each + line.

Indeed!  I will actually compile it prior to sending an actual patch.  I guess
I should have done that below.  :-/  Sorry.

Thanks!
Ira

> 
> > But I'm CC'ing Andrea for comments.
> > 
> > Ira
> > 
> >>
> >> Ira
> >>
> >> diff --git a/mm/shmem.c b/mm/shmem.c
> >> index 8280a5cb48df..6c8e99bf5983 100644
> >> --- a/mm/shmem.c
> >> +++ b/mm/shmem.c
> >> @@ -2424,9 +2424,11 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
> >>
> >>                 if (!zeropage) {        /* COPY */
> >>                         page_kaddr = kmap_local_folio(folio, 0);
> >> +                       pagefault_disable()
> >>                         ret = copy_from_user(page_kaddr,
> >>                                              (const void __user *)src_addr,
> >>                                              PAGE_SIZE);
> >> +                       pagefault_enable()
> >>                         kunmap_local(page_kaddr);
> >>
> >>                         /* fallback to copy_from_user outside mmap_lock */
> >>
> >>
> 
> -- 
> ~Randy
>