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

From: David Hildenbrand <david@redhat.com>
To: Mike Kravetz <mike.kravetz@oracle.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: Fri, 11 Feb 2022 09:43:46 +0100	[thread overview]
Message-ID: <356a4b9a-1f56-ae06-b211-bd32fc93ecda@redhat.com> (raw)
In-Reply-To: <55b6e78f-9594-8d5b-6ce2-bb699b9549fd@oracle.com>

>>>
>>> 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?
> 
> Not sure I understand the question.  I was saying that madvise calls which
> operate on hugetlb mappings today only operate on huge pages.  So, this is
> essentially align down starting address and align up end address.

Let me clarify:

If you accidentially
MADV_NORMAL/MADV_RANDOM/MADV_SEQUENTIAL/MADV_WILLNEED a range that's
slightly bigger/smaller than the requested one you don't actually care,
because it will only slightly affect the performance of an application,
if at all.  MADV_COLD/MADV_PAGEOUT should be similar. I assume these
don't apply to hugetlb at all.

The effects of
MADV_MERGEABLE/MADV_UNMERGEABLE/MADV_HUGEPAGE/MADV_NOHUGEPAGE should in
theory be similar, however, there can be some user-space visible effects
when you get it wrong. I assume these don't apply to hugetlb at all.

However, for
MADV_DONTNEED/MADV_REMOVE/MADV_DONTFORK/MADV_DOFORK/MADV_FREE/MADV_WIPEONFORK/MADV_KEEPONFORK/MADV_DONTDUMP/MADV_DODUMP/....
the application could easily detect the difference of the actual range
handling.

> For example consider the MADV_POPULATE calls you recently added.  They will
> only fault in huge pages in a hugetlb vma.

On a related note: I don't see my man page updates upstream yet. And the
last update upstream seems to have happened 5 months ago ... not sure
why the man project seems to have stalled.

https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/

-- 
Thanks,

David / dhildenb