linux-kernel - Re: [PATCH v4 04/10] userfaultfd/shmem: support minor fault registration for shmem

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Message-ID: <alpine.LSU.2.11.2104261920110.2998@eggly.anvils>
Date:   Mon, 26 Apr 2021 19:23:57 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Joe Perches <joe@...ches.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Peter Xu <peterx@...hat.com>, Shaohua Li <shli@...com>,
        Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Wang Qing <wangqing@...o.com>, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
        Brian Geffon <bgeffon@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v4 04/10] userfaultfd/shmem: support minor fault registration
 for shmem

On Tue, 20 Apr 2021, Axel Rasmussen wrote:

> This patch allows shmem-backed VMAs to be registered for minor faults.
> Minor faults are appropriately relayed to userspace in the fault path,
> for VMAs with the relevant flag.
> 
> This commit doesn't hook up the UFFDIO_CONTINUE ioctl for shmem-backed
> minor faults, though, so userspace doesn't yet have a way to resolve
> such faults.
> 
> Acked-by: Peter Xu <peterx@...hat.com>
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>

And if this "04/10" had been numbered 03/10, I would have said
Acked-by: Hugh Dickins <hughd@...gle.com>

Just read the comment above: "so userspace doesn't yet have a way to
resolve such faults" - if it doesn't by this stage, we're in trouble.

> ---
>  fs/userfaultfd.c                 |  6 +++---
>  include/uapi/linux/userfaultfd.h |  7 ++++++-
>  mm/memory.c                      |  8 +++++---
>  mm/shmem.c                       | 12 +++++++++++-
>  4 files changed, 25 insertions(+), 8 deletions(-)