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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Aug 07, 2025 at 08:14:09PM +0100, Pedro Falcato wrote:
> 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?

It's within 1 stddev of the original results, so I'd say it's possibly
noise.

Let's see what the bots say. If there's something else we can obviously
take a look, I think Jann's point about cold cache is the key point here,
and the delta here is indicative.

>
> > Reported-by: kernel test robot <oliver.sang@intel.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@oracle.com>
>
> Fix looks great, thanks!
>
> Acked-by: Pedro Falcato <pfalcato@suse.de>

Thanks!

>
> --
> Pedro

Cheers, Lorenzo