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

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Date: Tue, 26 Mar 2024 16:31:07 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.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 26/03/2024 16:17, David Hildenbrand wrote:
> 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?

Which pte_same() checks are you referring to? I've changed them all to
pte_same_norecency() which ignores the access/dirty bits when doing the comparison.

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

Sorry I'm not sure I understand? You mean read the full pte including
access/dirty? That's the same as dropping the patch, right? Of course if we do
that, we still have to keep pte_get_lockless() around for this case. In an ideal
world we would convert everything over to ptep_get_lockless_norecency() and
delete ptep_get_lockless() to remove the ugliness from arm64.

> 
>>
>> 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.
>