linux-kernel - Re: [PATCH v3 1/3] mm: enable MADV

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Message-ID: <fad42051-6816-08cf-1430-ad4da5dbc951@redhat.com>
Date:   Thu, 17 Feb 2022 09:58:26 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Yin Fengwei <fengwei.yin@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.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>, Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/3] mm: enable MADV_DONTNEED for hugetlb mappings

On 17.02.22 09:32, Yin Fengwei wrote:
> 
> 
> On 2022/2/15 08:23, Mike Kravetz wrote:
>> MADV_DONTNEED is currently disabled for hugetlb mappings.  This
>> certainly makes sense in shared file mappings as the pagecache maintains
>> a reference to the page and it will never be freed.  However, it could
>> be useful to unmap and free pages in private mappings.  In addition,
>> userfaultfd minor fault users may be able to simplify code by using
>> MADV_DONTNEED.
>>
>> The primary thing preventing MADV_DONTNEED from working on hugetlb
>> mappings is a check in can_madv_lru_vma().  To allow support for hugetlb
>> mappings create and use a new routine madvise_dontneed_free_valid_vma()
>> that allows hugetlb mappings in this specific case.
>>
>> For normal mappings, madvise requires the start address be PAGE aligned
>> and rounds up length to the next multiple of PAGE_SIZE.  Do similarly
> 
> In man page of mmap, NOTE for "Huge page (Huge TLB) mappings":
> 
> "For munmap(), addr, and length must both be a multiple of the 
> underlying huge page size."
> 
> Should we apply same rule to MADV_DONTNEED? Thanks.

madvise() already has different rules than mmap() for ordinary pages, so
we'd much rather try staying consistent with madvise() rules.

-- 
Thanks,

David / dhildenb