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

[Nadav Amit <nadav.amit@gmail.com>]

> On Jan 4, 2023, at 2:52 PM, Peter Xu <peterx@redhat.com> 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..
> 
>  (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.

I understand that the current assumption is that change_protection() should
fully succeed or fail, and I guess this is the current behavior.

However, to be more “future-proof” perhaps this needs to be revisited.

For instance, UFFDIO_WRITEPROTECT can benefit from the ability to (based on
userspace request) prevent write-protection of pages that are pinned. This is
necessary to allow userspace uffd monitor to avoid write-protection of
O_DIRECT’d memory, for instance, that might change even if a uffd monitor
considers it write-protected.

In such a case, a “partial failure” is possible, since only part of the memory
was write-protected. The uffd monitor should be allowed to continue
execution, but it has to know the part of the memory that was successfully
write-protected. 

To support “partial failure”, the kernel should return to
UFFDIO_WRITEPROTECT-users the number of pages/bytes that were not
successfully write-protected, unless no memory was successfully
write-protected. (Unlike NUMA, pages that were skipped should be accounted
as “successfully write-protected"). 

I am only raising this subject to avoid multiple API changes.