linux-kernel - Re: [PATCH v2 1/3] mm: enable MADV

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Date:   Mon, 7 Feb 2022 15:47:04 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 1/3] mm: enable MADV_DONTNEED for hugetlb mappings

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.

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.
-- 
Mike Kravetz