linux-kernel - Re: [RFC PATCH] userfaultfd: Address race after fault.

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Date:   Mon, 17 Feb 2020 11:07:39 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Brian Geffon <bgeffon@...gle.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [RFC PATCH] userfaultfd: Address race after fault.

On Sat, Feb 15, 2020 at 09:29:46AM -0500, Brian Geffon wrote:
> Hi Andrea,
> Thanks for the quick reply. That's great to hear that Peter has been
> working on those improvements. I didn't try the entire patchset but I
> did confirm that patch 13, not surprisingly, also resolves that issue
> on at least on x86:
>   https://lkml.org/lkml/2019/9/26/179
> 
> Given that seems pretty low risk and it definitely resolves a pretty
> big issue for the non-cooperative userfaultfd case, any chance it
> could be landed ahead of the rest of the series?

Thanks Andrea & Brian!  Yes it would be great if the series (or some
of the patches) could be moved forward.  Please just let me know if
there's still anything I can do from my side.

Thanks,

> 
> Thanks,
> Brian
> 
> On Fri, Feb 14, 2020 at 6:20 PM Andrea Arcangeli <aarcange@...hat.com> wrote:
> >
> > Hello,
> >
> > this and other enhancements have already implemented by Peter (CC'ed)
> > and in the right way, by altering the retry logic in the page fault
> > code. This is a requirement for other kind of usages too, notably the
> > UFFD_WRITEPROTECT ioctl after which multiple consecutive faults can
> > happen and must be handled.
> >
> > IIRC Kirill asked at last LSF-MM uffd-wp talk if there's any
> > particular reason the fault couldn't be retried currently. I had no
> > sure answer other than there's apparently no strong reason why
> > VM_FAULT_RETRY is only allowed 1 time currently, so there should be no
> > issue in lifting that artificial restriction.
> >
> > I'm running with this patchset applied in my systems since Nov with no
> > regression at all. I got sidetracked by various other issues, so
> > unfortunately I didn' post a proper reviewed-by on the last submit yet
> > (pending), but I did at least test it and it was rock solid so far.
> >
> > https://lore.kernel.org/lkml/20190926093904.5090-1-peterx@redhat.com/
> >
> > Can you test and verify it too if it solves your use case?
> >
> > Also note the complete uffd-WP support submit also from Peter:
> >
> > https://lore.kernel.org/lkml/20190620022008.19172-1-peterx@redhat.com/
> >
> > https://github.com/xzpeter/linux/tree/uffd-wp-merged
> >
> > Thanks,
> > Andrea
> >
> 

-- 
Peter Xu