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

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Date:   Sun, 30 Oct 2022 14:43:51 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.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,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb
 MADV_DONTNEED processing

On Sat, Oct 29, 2022 at 05:54:44PM -0700, Nadav Amit wrote:
> 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]

The loop comes from 7e027b14d53e ("vm: simplify unmap_vmas() calling
convention", 2012-05-06), where zap_page_range() was used to replace a call
to unmap_vmas() because the patch wanted to eliminate the zap details
pointer for unmap_vmas(), which makes sense.

I didn't check the old code, but from what I can tell (and also as Mike
pointed out) I don't think zap_page_range() in the lastest code base is
ever used on multi-vma at all.  Otherwise the mmu notifier is already
broken - see mmu_notifier_range_init() where the vma pointer is also part
of the notification.

Perhaps we should just remove the loop?

> 
> Need to get to it sometime.
> 
> [1] https://lore.kernel.org/lkml/20210926161259.238054-7-namit@vmware.com/
> 

-- 
Peter Xu