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

[David Hildenbrand]

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