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

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Message-ID: <20200214231954.GA29849@redhat.com>
Date:   Fri, 14 Feb 2020 18:19:54 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Brian Geffon <bgeffon@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [RFC PATCH] userfaultfd: Address race after fault.

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