linux-kernel - Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get

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Message-ID: <0ae22147-e1a1-4bcb-8a4c-f900f3f8c39e@redhat.com>
Date: Tue, 26 Mar 2024 17:17:38 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Mark Rutland <mark.rutland@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64

On 15.02.24 13:17, Ryan Roberts wrote:
> This is an RFC for a series that aims to reduce the cost and complexity of
> ptep_get_lockless() for arm64 when supporting transparent contpte mappings [1].
> The approach came from discussion with Mark and David [2].
> 
> It introduces a new helper, ptep_get_lockless_norecency(), which allows the
> access and dirty bits in the returned pte to be incorrect. This relaxation
> permits arm64's implementation to just read the single target pte, and avoids
> having to iterate over the full contpte block to gather the access and dirty
> bits, for the contpte case.
> 
> It turns out that none of the call sites using ptep_get_lockless() require
> accurate access and dirty bit information, so we can also convert those sites.
> Although a couple of places need care (see patches 2 and 3).
> 
> Arguably patch 3 is a bit fragile, given the wide accessibility of
> vmf->orig_pte. So it might make sense to drop this patch and stick to using
> ptep_get_lockless() in the page fault path. I'm keen to hear opinions.

Yes. Especially as we have these pte_same() checks that might just fail 
now because of wrong accessed/dirty bits?

Likely, we just want to read "the real deal" on both sides of the 
pte_same() handling.

> 
> I've chosen the name "recency" because it's shortish and somewhat descriptive,
> and is alredy used in a couple of places to mean similar things (see mglru and
> damon). I'm open to other names if anyone has better ideas.

Not a native speaker; works for me.

> 
> If concensus is that this approach is generally acceptable, I intend to create a
> series in future to do a similar thing with ptep_get() -> ptep_get_norecency().

Yes, sounds good to me.

-- 
Cheers,

David / dhildenb