linux-kernel - Re: [PATCH 3/3] mm/uffd: Detect pgtable allocation failures

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Date:   Thu, 5 Jan 2023 09:47:10 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        James Houghton <jthoughton@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] mm/uffd: Detect pgtable allocation failures

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