linux-kernel - Re: [PATCH v2 00/24] userfaultfd-wp: Support shmem and hugetlbfs

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Message-ID: <alpine.LSU.2.11.2105132312350.3969@eggly.anvils>
Date:   Fri, 14 May 2021 00:07:38 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Peter Xu <peterx@...hat.com>
cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v2 00/24] userfaultfd-wp: Support shmem and hugetlbfs

On Wed, 12 May 2021, Peter Xu wrote:
> On Tue, Apr 27, 2021 at 12:12:53PM -0400, Peter Xu wrote:
> > This is v2 of uffd-wp shmem & hugetlbfs support, which completes uffd-wp as a
> > kernel full feature, as it only supports anonymous before this series.
> 
> Ping..
> 
> Thinking about a repost, as this series shouldn't be able to apply after we've
> got more relevant patches into -mm.  E.g., the full minor fault, and also some
> small stuff like pagemap, as we need one more patch to support shmem/hugetlbfs
> too.
> 
> Hugh, haven't received any further comment from you on shmem side (or on the
> general idea).  It would be great to still have some of your input.
> 
> Let me know if you prefer to read a fresh new version otherwise.

I am very sorry to let you down, Peter, repeatedly; but it is now very
clear that I shall *never* have time to review your patchset - I am too
slow, have too much else to attend to, and take too long each time to
sink myself deep enough into userfaultfd.

I realize that you're being considerate, and expecting no more than
a few comments from me, rather than asking for formal review; but it's
still too much for me to get into.

The only reason I was involved at all, was when you were wondering how
to handle the pagetable entries for shmem.  I suggested one encoding,
Andrea suggested slightly differently: Andrea's was more elegant (no
"swap type" required), and it looked like you went with his - good.

I wonder whether you noticed
https://lore.kernel.org/linux-mm/20210407084238.20443-2-apopple@nvidia.com/
which might interfere.  I've had no more time to look at that than yours,
so no opinion on it (and I don't know what happened to it after that).

Please keep uppermost in mind when modifying mm/shmem.c for userfaultfd,
the difference between shared and private; and be on guard against the
ways in which CONFIG_USERFAULTFD=y might open a door to abuse.

Thanks,
Hugh