From: Mike Kravetz <mike.kravetz@oracle.com>
To: David Hildenbrand <david@redhat.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>,
Axel Rasmussen <axelrasmussen@google.com>,
Mina Almasry <almasrymina@google.com>,
Michal Hocko <mhocko@suse.com>, Peter Xu <peterx@redhat.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Shuah Khan <shuah@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v2 1/3] mm: enable MADV_DONTNEED for hugetlb mappings
Date: Mon, 7 Feb 2022 15:47:04 -0800 [thread overview]
Message-ID: <d7e6ee46-8e0b-2ae3-c159-449a26c17ae9@oracle.com> (raw)
In-Reply-To: <6a82ea68-6e1e-8f5a-ca89-6744fc896a0b@redhat.com>
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
next prev parent reply other threads:[~2022-02-07 23:47 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-02 1:40 [PATCH v2 0/3] Add hugetlb MADV_DONTNEED support Mike Kravetz
2022-02-02 1:40 ` [PATCH v2 1/3] mm: enable MADV_DONTNEED for hugetlb mappings Mike Kravetz
2022-02-02 8:14 ` David Hildenbrand
2022-02-02 19:32 ` Mike Kravetz
2022-02-04 8:35 ` David Hildenbrand
2022-02-07 23:47 ` Mike Kravetz [this message]
2022-02-10 13:09 ` David Hildenbrand
2022-02-10 22:11 ` Mike Kravetz
2022-02-11 8:43 ` David Hildenbrand
2022-02-10 3:21 ` Peter Xu
2022-02-10 21:36 ` Mike Kravetz
2022-02-11 2:28 ` Peter Xu
2022-02-11 19:08 ` Axel Rasmussen
2022-02-11 19:18 ` Mike Kravetz
2022-02-02 1:40 ` [PATCH v2 2/3] selftests/vm: add hugetlb madvise MADV_DONTNEED MADV_REMOVE test Mike Kravetz
2022-02-02 1:40 ` [PATCH v2 3/3] userfaultfd/selftests: enable huegtlb remap and remove event testing Mike Kravetz
2022-02-02 6:11 ` Mike Rapoport
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