netdev - Re: [RFC PATCH] mm: remove zap_page_range and change callers to use zap_vma_page

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Date:   Thu, 05 Jan 2023 12:14:36 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rik van Riel <riel@...riel.com>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Palmer Dabbelt <palmer@...belt.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Christian Brauner <brauner@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] mm: remove zap_page_range and change callers to use
 zap_vma_page_range


Mike Kravetz <mike.kravetz@...cle.com> writes:

> On 12/19/22 13:06, Michal Hocko wrote:
>> On Fri 16-12-22 11:20:12, Mike Kravetz wrote:
>> > zap_page_range was originally designed to unmap pages within an address
>> > range that could span multiple vmas.  While working on [1], it was
>> > discovered that all callers of zap_page_range pass a range entirely within
>> > a single vma.  In addition, the mmu notification call within zap_page
>> > range does not correctly handle ranges that span multiple vmas as calls
>> > should be vma specific.
>> 
>> Could you spend a sentence or two explaining what is wrong here?
>
> Hmmmm?  My assumption was that the range passed to mmu_notifier_range_init()
> was supposed to be within the specified vma.  When looking into the notifier
> routines, I could not find any documentation about the usage of the vma within
> the mmu_notifier_range structure.  It was introduced with commit bf198b2b34bf
> "mm/mmu_notifier: pass down vma and reasons why mmu notifier is happening".
> However, I do not see this being used today.
>
> Of course, I could be missing something, so adding Jérôme.

The only use for mmu_notifier_range->vma I can find is in
mmu_notifier_range_update_to_read_only() which was introduced in
c6d23413f81b ("mm/mmu_notifier: mmu_notifier_range_update_to_read_only()
helper"). However there are no users of that symbol so I think we can
remove it along with the mmu_notifier_range->vma field.

I will put togeather a patch to do that.

>> 
>> > Instead of fixing zap_page_range, change all callers to use the new
>> > routine zap_vma_page_range.  zap_vma_page_range is just a wrapper around
>> > zap_page_range_single passing in NULL zap details.  The name is also
>> > more in line with other exported routines that operate within a vma.
>> > We can then remove zap_page_range.
>> 
>> I would stick with zap_page_range_single rather than adding a new
>> wrapper but nothing really critical.
>
> I am fine with doing that as well.  My only reason for the wrapper is that all 
> callers outside mm/memory.c would pass in NULL zap details.
>
>> 
>> > Also, change madvise_dontneed_single_vma to use this new routine.
>> > 
>> > [1] https://lore.kernel.org/linux-mm/20221114235507.294320-2-mike.kravetz@oracle.com/
>> > Suggested-by: Peter Xu <peterx@...hat.com>
>> > Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
>> 
>> Other than that LGTM
>> Acked-by: Michal Hocko <mhocko@...e.com>
>> 
>> Thanks!
>
> Thanks for taking a look.