linux-kernel - Re: [PATCH] userfaultfd: replace ENOSPC with ESRCH in case mm has gone during copy/zeropage

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Message-ID: <20170810093349.GK23863@dhcp22.suse.cz>
Date:   Thu, 10 Aug 2017 11:33:49 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH] userfaultfd: replace ENOSPC with ESRCH in case mm has
 gone during copy/zeropage

On Tue 08-08-17 09:08:17, Mike Rapoport wrote:
> (adding Michal)

Thanks

> On Mon, Aug 07, 2017 at 04:12:25PM +0300, Mike Rapoport wrote:
> > When the process exit races with outstanding mcopy_atomic, it would be
> > better to return ESRCH error. When such race occurs the process and it's mm
> > are going away and returning "no such process" to the uffd monitor seems
> > better fit than ENOSPC.

Not only the error message would be less confusing I also think that
error handling should be more straightforward. Although I cannot find
any guidelines for ENOSPC handling I've considered this errno as
potentially temporary and retry might be feasible while ESRCH is a
terminal error. I do not expect any userfaultfd users would retry on
error but who knows how the interface will be used in future so better
be prepared.

> > Suggested-by: Michal Hocko <mhocko@...e.com>
> > Cc: Andrea Arcangeli <aarcange@...hat.com>
> > Cc: "Dr. David Alan Gilbert" <dgilbert@...hat.com>
> > Cc: Pavel Emelyanov <xemul@...tuozzo.com>
> > Cc: Mike Kravetz <mike.kravetz@...cle.com>
> > Signed-off-by: Mike Rapoport <rppt@...ux.vnet.ibm.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> > ---
> > The man-pages update is ready and I'll send it out once the patch is
> > merged.
> > 
> >  fs/userfaultfd.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 06ea26b8c996..b0d5897bc4e6 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -1600,7 +1600,7 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
> >  				   uffdio_copy.len);
> >  		mmput(ctx->mm);
> >  	} else {
> > -		return -ENOSPC;
> > +		return -ESRCH;
> >  	}
> >  	if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
> >  		return -EFAULT;
> > @@ -1647,7 +1647,7 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
> >  				     uffdio_zeropage.range.len);
> >  		mmput(ctx->mm);
> >  	} else {
> > -		return -ENOSPC;
> > +		return -ESRCH;
> >  	}
> >  	if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
> >  		return -EFAULT;
> > -- 
> > 2.7.4
> > 
> 
> -- 
> Sincerely yours,
> Mike.
> 

-- 
Michal Hocko
SUSE Labs