linux-kernel - Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on mremap folio pte batch

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Message-ID: <47ce3db2-38ad-4a6b-917a-c6300fc39543@lucifer.local>
Date: Mon, 11 Aug 2025 05:57:09 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Barry Song <21cnbao@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Dev Jain <dev.jain@....com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on
 mremap folio pte batch

On Mon, Aug 11, 2025 at 10:40:50AM +0800, Barry Song wrote:
> On Fri, Aug 8, 2025 at 2:59 AM Lorenzo Stoakes
> > The expectation by those discussing this from the start was that
> > vm_normal_folio() (invoked by mremap_folio_pte_batch()) would likely be the
> > culprit due to having to retrieve memory from the vmemmap (which mremap()
> > page table moves does not otherwise do, meaning this is inevitably cold
> > memory).
>
> If vm_normal_folio() is so expensive, does that mean it negates the
> benefits that commit f822a9a81a31 (“mm: optimize mremap() by PTE
> batching”) was originally intended to achieve through PTE batching?

Not for arm64 apparently. And the hint check introduces here should avoid
regressions even there when small folios are in place.

In similar series to these in other areas, it appears we need the folio
anyway so there is no additional overhead to deal with, in mremap() you'd
otherwise just be looking at page tables which makes so this egregious
here.

> > Reported-by: kernel test robot <oliver.sang@...el.com>
> > Closes: https://lore.kernel.org/oe-lkp/202508071609.4e743d7c-lkp@intel.com
> > Fixes: f822a9a81a31 ("mm: optimize mremap() by PTE batching")
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Barry Song <baohua@...nel.org>

Thanks!