Re: [PATCH v2 1/3] mm: enable MADV_DONTNEED for hugetlb mappings

[David Hildenbrand]

On 08.02.22 00:47, Mike Kravetz wrote:
> On 2/4/22 00:35, David Hildenbrand wrote:
>>> I thought this was simple. :)
>>
>> It really bugs me that it's under-specified what's supposed to happen
>> when the length is not aligned.
>>
>> BUT: in the posix world, "calling posix_madvise() shall not affect the
>> semantics of access to memory in the specified range". So we don't care
>> too much about if we align up/down, because it wouldn't affect the
>> semantics. Especially for MADV_DONTNEED/MADV_REMOVE as implemented by
>> Linux this is certainly different and the alignment handling matters.
>>
>> So I guess especially for MADV_DONTNEED/MADV_REMOVE we need a clear
>> specification what's supposed to happen if the length falls into the
>> middle of a huge page. We should document alignment handling for
>> madvise() in general I assume.
>>
>> IMHO we should have bailed out right from the start whenever something
>> is not properly aligned, but that ship has sailed. So I agree, maybe we
>> can make at least hugetlb MADV_DONTNEED obey the same (weird) rules as
>> ordinary pages.
>>
>> So b) would mean, requiring start to be hugepage aligned and aligning-up
>> the end. Still feels wrong but at least matches existing semantics.
>>
>> Hugetlb MADV_REMOVE semantics are unfortunate and we should document the
>> exception.
> 
> Thank you for all your comments David!
> 
> So, my plan was to make MADV_DONTNEED behave as described above:
> - EINVAL if start address not huge page size aligned
> - Align end/length up to huge page size.
> 
> The code I had for this was very specific to MADV_DONTNEED.  I then thought,
> why not do the same for MADV_REMOVE as well?  Or even more general, add this
> check and alignment to the vma parsing code in madvise.
> 
> It was then that I realized there are several madvise behaviors that take
> non-huge page size aligned addresses for hugetlb mappings today.  Making
> huge page size alignment a requirement for all madvise behaviors could break
> existing code.  So, it seems like it could only be added to MADV_DONTNEED as
> this functionality does not exist today.  We then end up with MADV_DONTNEED
> as the only behavior requiring huge page size alignment for hugetlb mappings.
> Sigh!!!

:/

> 
> I am now rethinking the decision to proceed with b) as described above.
> 
> With the exception of MADV_REMOVE (which we may be able to change for
> hugetlb),  madvise operations operate on huge page size pages for hugetlb
> mappings.  If start address is in the middle of a hugetlb page, we essentially
> align down to the beginning of the hugetlb page.  If length lands in the
> middle of a hugetlb page, we essentially round up.

Which MADV calls would be affected?

The "bad" thing about MADV_DONTNEED and MADV_REMOVE are that they
destroy data, which is why they heavily diverge from the original posix
madvise odea.

> 
> When adding MADV_REMOVE perhaps we should go with this align down start and
> align up end strategy that is used everywhere else?  I really wish we could
> go back and change things, but as you know it is too late for that.

I assume whatever we do, we should document it at least cleanly in the
man page. Unfortunately, hugetlb is a gift that keeps on giving. Making
it at least somehow consistent, even if it's "hugtlb being consistent in
its own mess", that would be preferable I guess.

-- 
Thanks,

David / dhildenb