linux-kernel - Re: [PATCH v2 08/10] mm/hugetlb: Make walk_hugetlb

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Date:   Fri, 9 Dec 2022 09:39:30 -0500
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>,
        John Hubbard <jhubbard@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        James Houghton <jthoughton@...gle.com>,
        Jann Horn <jannh@...gle.com>, Rik van Riel <riel@...riel.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH v2 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to
 pmd unshare

On Fri, Dec 09, 2022 at 11:24:55AM +0100, David Hildenbrand wrote:
> For such cases, it would be good to have any evidence that it really helps.

I don't know much on the s390 path, but if a process has a large hugetlb
vma, even MADV_DONTNEED will be blocked for whatever long time if there's
another process or thread scanning pagemap for this vma.

Would this justify a bit?

It's just that the vma lock is taken write far more than mmap_lock taken
write I think, meanwhile what we need here is only release the lock and
retake, nothing else.  It didn't make things over complicated, IMO.

No strong ipinion from my side, as I said to me it's really low hanging
fruit.  If you still think that doesn't justify and if Mike doesn't have a
preference either I can just drop it for later.

-- 
Peter Xu