linux-kernel - Re: [PATCH RFC 20/24] userfaultfd: wp: don't wake up when doing write protect

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Date:   Thu, 24 Jan 2019 13:36:47 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Jerome Glisse <jglisse@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Emelyanov <xemul@...allels.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Marty McFadden <mcfadden8@...l.gov>,
        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 RFC 20/24] userfaultfd: wp: don't wake up when doing
 write protect

On Mon, Jan 21, 2019 at 01:10:39PM +0200, Mike Rapoport wrote:
> On Mon, Jan 21, 2019 at 03:57:18PM +0800, Peter Xu wrote:
> > It does not make sense to try to wake up any waiting thread when we're
> > write-protecting a memory region.  Only wake up when resolving a write
> > protected page fault.
> 
> Probably it would be better to make it default to wake up only when
> requested explicitly?

Yes, I think that's what this series does?

Now when we do UFFDIO_WRITEPROTECT with !WP and !DONTWAKE then we'll
first resolve the page fault, then wake up the process properly.  And
we request that explicity using !WP and DONTWAKE.

Or did I misunderstood the question?

> Then we can simply disallow _DONTWAKE for uffd_wp and only use
> UFFDIO_WRITEPROTECT_MODE_WP as possible mode.

I'd admit I don't know the major usage of DONTWAKE (and I'd be glad to
know...), however since we have this flag for both UFFDIO_COPY and
UFFDIO_ZEROCOPY, then it seems sane to have DONTWAKE for WRITEPROTECT
too?  Or is there any other explicit reason to omit it?

Thanks!

-- 
Peter Xu