Message-ID: <68604797-f3a9-4555-aaf2-228f6e7f820a@arm.com>
Date: Wed, 28 May 2025 11:45:36 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: akpm@...ux-foundation.org, Liam.Howlett@...cle.com, vbabka@...e.cz,
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Subject: Re: [PATCH v3 2/2] mm: Optimize mremap() by PTE batching


On 28/05/25 10:19 am, Lorenzo Stoakes wrote:
> On Wed, May 28, 2025 at 09:02:26AM +0530, Dev Jain wrote:
>> On 27/05/25 10:16 pm, Lorenzo Stoakes wrote:
>>> On Tue, May 27, 2025 at 10:08:59PM +0530, Dev Jain wrote:
>>>> On 27/05/25 9:59 pm, Lorenzo Stoakes wrote:
>>> [snip]
>>>>> If I invoke split_huge_pmd(), I end up with a bunch of PTEs mapping the same
>>>>> large folio. The folio itself is not split, so nr_ptes surely will be equal to
>>>>> something >1 here right?
>>>> Thanks for elaborating.
>>>>
>>>> So,
>>>>
>>>> Case 1: folio splitting => nr_ptes = 1 => the question of a/d bit smearing
>>>> disappears.
>>>>
>>>> Case 2: page table splitting => consec PTEs point to the same large folio
>>>> => nr_ptes > 1 => get_and_clear_full_ptes() will smear a/d bits on the
>>>> new ptes, which is correct because we are still pointing to the same large
>>>> folio.
>>>>
>>> OK awesome, I thought as much, just wanted to make sure :) we are good then.
>>>
>>> The accessed/dirty bits really matter at a folio granularity (and especially
>>> with respect to reclaim/writeback which both operate at folio level) so the
>>> smearing as you say is fine.
>>>
>>> This patch therefore looks fine, only the trivial comment fixup.
>>>
>>> I ran the series on my x86-64 setup (fwiw) with no build/mm selftest errors.
>> Thanks!
>>
>>
>>> Sorry to be a pain but could you respin with the commit message for this patch
>>> updated to explicitly mention that the logic applies for the non-contPTE split
>>> PTE case (and therefore also helps performance there)? That and the trivial
>>> thing of dropping that comment.
>> What do you mean by the non-contpte case? In that case the PTEs do not point
>> to the same folio or are misaligned, and there will be no optimization. This
> Split page table large folio.
>
>> patch is optimizing two things: 1) ptep_get() READ_ONCE accesses 2) reduction
>> in number of TLBIs for contig blocks, both of which happen in the contpte case.
>>
> But it impacts split huge pages. Your code changes this behaviour. We need to
> make this clear :)
>
>> In general, the patch should have a minor improvement on other arches because
>> we are detecting a batch and processing it together, thus saving on a few
>> function calls, but the main benefit is for arm64.
> Ack, but you are changing this behaviour. The commit message doesn't make this
> clear and seems to imply this only impacts contPTE cases. Or at least isn't
> clear enough
>
> A simple additional paragraph like:
>
> 'Transparent huge pages which have been split into PTEs will also be impacted,
> however the performance gain in this case is expected to be modest'

Ah okay, sounds good.

>
> Will sort this out.
>
> Thanks!
>
>>> Then we should be good for a tag unless somebody else spots something
>>> egregious :)
>>>
>>> Thanks for this! Good improvement.
>>>
>>> [snip]
>>>
>>> Cheers, Lorenzo

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