Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <1f934203-5220-01f6-42d1-65cc984a0201@redhat.com>
Date:   Thu, 5 Jan 2023 09:47:10 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.6.0
Subject: Re: [PATCH 3/3] mm/uffd: Detect pgtable allocation failures
Content-Language: en-US
To:     Peter Xu <peterx@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        James Houghton <jthoughton@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20230104225207.1066932-1-peterx@redhat.com>
 <20230104225207.1066932-4-peterx@redhat.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230104225207.1066932-4-peterx@redhat.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 04.01.23 23:52, Peter Xu wrote:
> Before this patch, when there's any pgtable allocation issues happened
> during change_protection(), the error will be ignored from the syscall.
> For shmem, there will be an error dumped into the host dmesg.  Two issues
> with that:
> 
>    (1) Doing a trace dump when allocation fails is not anything close to
>        grace..

s/..//

> 
>    (2) The user should be notified with any kind of such error, so the user
>        can trap it and decide what to do next, either by retrying, or stop
>        the process properly, or anything else.
> 
> For userfault users, this will change the API of UFFDIO_WRITEPROTECT when
> pgtable allocation failure happened.  It should not normally break anyone,
> though.  If it breaks, then in good ways.
> 
> One man-page update will be on the way to introduce the new -ENOMEM for
> UFFDIO_WRITEPROTECT.  Not marking stable so we keep the old behavior on the
> 5.19-till-now kernels.

We'd now fail after already having modified some state (protected some 
PTEs). I assume that can already happen when protecting across multiple 
VMAs and is expected, right?

-- 
Thanks,

David / dhildenb