linux-kernel - Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb MADV

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Date:   Sat, 29 Oct 2022 17:54:44 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Peter Xu <peterx@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Hildenbrand <david@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Chen <harperchen1110@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb
 MADV_DONTNEED processing

On Oct 29, 2022, at 5:15 PM, Mike Kravetz <mike.kravetz@...cle.com> wrote:

> zap_page_range is a bit confusing.  It appears that the passed range can
> span multiple vmas.  Otherwise, there would be no do while loop.  Yet, there
> is only one mmu_notifier_range_init call specifying the passed vma.
> 
> It appears all callers pass a range entirely within a single vma.
> 
> The modifications above would work for a range within a single vma.  However,
> things would be more complicated if the range can indeed span multiple vmas.
> For multiple vmas, we would need to check the first and last vmas for
> pmd sharing.
> 
> Anyone know more about this seeming confusing behavior?  Perhaps, range
> spanning multiple vmas was left over earlier code?

I don’t have personal knowledge, but I noticed that it does not make much
sense, at least for MADV_DONTNEED. I tried to batch the TLB flushes across
VMAs for madvise’s. [1]

Need to get to it sometime.

[1] https://lore.kernel.org/lkml/20210926161259.238054-7-namit@vmware.com/