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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory
Message-ID: <740cdd51-137b-2b08-8b7f-9757d8d847cb@redhat.com>
Date:   Thu, 18 Feb 2021 12:38:27 +0100
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>>>>      If we hit
>>>>      hardware errors on pages, ignore them - nothing we really can or
>>>>      should do.
>>>> 3. On errors during MADV_POPULATED, some memory might have been
>>>>      populated. Callers have to clean up if they care.
>>>
>>> How does caller find out? madvise reports 0 on success so how do you
>>> find out how much has been populated?
>>
>> If there is an error, something might have been populated. In my QEMU
>> implementation, I simply discard the range again, good enough. I don't think
>> we need to really indicate "error and populated" or "error and not
>> populated".
> 
> Agreed. The wording just suggests that the syscall actually provides any
> means for an effective way to handle those errors. Maybe you should just
> stick with the first sentence and drop the second.

Makes sense. "On errors during MADV_POPULATE, some memory might have 
been populated."

>   
>>>> 4. Concurrent changes to the virtual memory layour are tolerated - we
>>>>      process each and every PFN only once, though.
>>>
>>> I do not understand this. madvise is about virtual address space not a
>>> physical address space.
>>
>> What I wanted to express: if we detect a change in the mapping we don't
>> restart at the beginning, we always make forward progress. We process each
>> virtual address once (on a per-page basis, thus I accidentally used "PFN").
> 
> This is an implicit assumption. Your range can have the same page mapped
> several times in the given address range and all you care about is that
> you fault those which are not present during the virtual address space
> walk. Your syscall can return and large part of the address space might
> be unpopulated because memory reclaim just dropped those pages and that
> would be fine. This shouldn't really imply memory presence - mlock does
> that.

"Concurrent changes to the virtual memory layout are tolerated. The 
range is processed exactly once."

> 
>>>> 5. If MADV_POPULATE succeeds, all memory in the range can be accessed
>>>>      without SIGBUS. (of course, not if user space changed mappings in the
>>>>      meantime or KSM kicked in on anonymous memory).
>>>
>>> I do not see how KSM would change anything here and maybe it is not
>>> really important to mention it. KSM should be really transparent from
>>> the users space POV. Parallel and destructive virtual address space
>>> operations are also expected to change the outcome and there is nothing
>>> kernel do about at and provide any meaningful guarantees. I guess we
>>> want to assume a reasonable userspace behavior here.
>>
>> It's just a note that we cannot protect from someone interfering
>> (discard/ksm/whatever). I'm making that clearer in the cover letter.
> 
> Again that is implicit expectation. madvise will not work for anybody
> shooting an own foot.

Okay, I'll drop that part, thanks!

-- 
Thanks,

David / dhildenb