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

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Message-ID: <ujfeyn2awquoc4pc7qxcnavgqcu57kvmdb66x2hyaeoiasq3w4@u5qlyyklfrld>
Date: Thu, 7 Aug 2025 20:14:09 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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>, 
	Barry Song <baohua@...nel.org>, 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 Thu, Aug 07, 2025 at 07:58:19PM +0100, Lorenzo Stoakes wrote:
> It was discovered in the attached report that commit f822a9a81a31 ("mm:
> optimize mremap() by PTE batching") introduced a significant performance
> regression on a number of metrics on x86-64, most notably
> stress-ng.bigheap.realloc_calls_per_sec - indicating a 37.3% regression in
> number of mremap() calls per second.
> 
> I was able to reproduce this locally on an intel x86-64 raptor lake system,
> noting an average of 143,857 realloc calls/sec (with a stddev of 4,531 or
> 3.1%) prior to this patch being applied, and 81,503 afterwards (stddev of
> 2,131 or 2.6%) - a 43.3% regression.
> 
> During testing I was able to determine that there was no meaningful
> difference in efforts to optimise the folio_pte_batch() operation, nor
> checking folio_test_large().
> 
> This is within expectation, as a regression this large is likely to
> indicate we are accessing memory that is not yet in a cache line (and
> perhaps may even cause a main memory fetch).
> 
> 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).
> 
> I was able to definitively determine that this theory is indeed correct and
> the cause of the issue.
> 
> The solution is to restore part of an approach previously discarded on
> review, that is to invoke pte_batch_hint() which explicitly determines,
> through reference to the PTE alone (thus no vmemmap lookup), what the PTE
> batch size may be.
> 
> On platforms other than arm64 this is currently hardcoded to return 1, so
> this naturally resolves the issue for x86-64, and for arm64 introduces
> little to no overhead as the pte cache line will be hot.
> 
> With this patch applied, we move from 81,503 realloc calls/sec to
> 138,701 (stddev of 496.1 or 0.4%), which is a -3.6% regression, however
> accounting for the variance in the original result, this is broadly
> restoring performance to its prior state.
>

So, do we still have a regression then? If so, do we have any idea why?

> 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>

Fix looks great, thanks!

Acked-by: Pedro Falcato <pfalcato@...e.de>

-- 
Pedro