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

[David Hildenbrand <david@redhat.com>]

On 05.01.23 19:01, Nadav Amit wrote:
> 
> 
>> On Jan 5, 2023, at 12:59 AM, David Hildenbrand <david@redhat.com> wrote:
>>
>> On 05.01.23 04:10, Nadav Amit wrote:
>>>> 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.
>>
>> Just a note that this is pretty tricky IMHO, because:
>>
>> a) We cannot distinguished "pinned readable" from "pinned writable"
>> b) We can have false positives ("pinned") even for compound pages due to
>>    concurrent GUP-fast.
>> c) Synchronizing against GUP-fast is pretty tricky ... as we learned.
>>    Concurrent pinning is usually problematic.
>> d) O_DIRECT still uses FOLL_GET and we cannot identify that. (at least
>>    that should be figured out at one point)
> 
> My prototype used the page-count IIRC, so it had false-positives (but

I suspect GUP-fast is still problematic, I might be wrong.

> addressed O_DIRECT). And yes, precise refinement is complicated. However,
> if you need to uffd-wp memory, then without such a mechanism you need to
> ensure no kerenl/DMA write to these pages is possible. The only other
> option I can think of is interposing/seccomp on a variety of syscalls,
> to prevent uffd-wp of such memory.

The whole thing reminds me of MADV_DONTNEED+pinning: an application 
shouldn't do it, because you can only get it wrong :) I know, that's a 
bad answer.

-- 
Thanks,

David / dhildenb