linux-kernel - Re: [PATCH v1 3/4] mm: split folio_pte_batch() into folio_pte_batch() and folio_pte_batch

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Message-ID: <6466a90a-6af0-4dc5-bbb6-19ec6d2f5c8c@redhat.com>
Date: Mon, 30 Jun 2025 12:54:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Zi Yan <ziy@...dia.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>, Pedro Falcato <pfalcato@...e.de>,
 Rik van Riel <riel@...riel.com>, Harry Yoo <harry.yoo@...cle.com>
Subject: Re: [PATCH v1 3/4] mm: split folio_pte_batch() into folio_pte_batch()
 and folio_pte_batch_ext()

On 30.06.25 12:41, Lorenzo Stoakes wrote:
> On Mon, Jun 30, 2025 at 11:19:13AM +0200, David Hildenbrand wrote:
>> On 27.06.25 20:48, Lorenzo Stoakes wrote:
>>> On Fri, Jun 27, 2025 at 01:55:09PM +0200, David Hildenbrand wrote:
>>>> Many users (including upcoming ones) don't really need the flags etc,
>>>> and can live with a function call.
>>>>
>>>> So let's provide a basic, non-inlined folio_pte_batch().
>>>
>>> Hm, but why non-inlined, when it invokes an inlined function? Seems odd no?
>>
>> We want to always generate a function that uses as little runtime checks as
>> possible. Essentially, optimize out the "flags" as much as possible.
>>
>> In case of folio_pte_batch(), where we won't use any flags, any checks will
>> be optimized out by the compiler.
>>
>> So we get a single, specialized, non-inlined function.
> 
> I mean I suppose code bloat is a thing too. Would the compiler not also optimise
> out checks if it were inlined though?

The compiler will optimize all (most) inlined variants, yes.

But we will end up creating the same optimized variant for each 
folio_pte_batch() user before this change.

And as Andrew put it

"And why the heck is folio_pte_batch() inlined?  It's larger then my 
first hard disk" [1]

I should probably add a suggested-by + link to that discussion.

[1] 
https://lore.kernel.org/linux-mm/20250503182858.5a02729fcffd6d4723afcfc2@linux-foundation.org/

[...]

>>
>> Obviously, I had that as part of the development, and decided against it at
>> some point. :)
>>
>> Yeah, _ext() is not common in MM yet, in contrast to other subsystems. The
>> only user is indeed page_ext. On arm we seem to have set_pte_ext(). But it's
>> really "page_ext", that's the problematic part, not "_ext" :P
>>
>> No strong opinion, but I tend to dislike here "__", because often it means
>> "internal helper you're not supposed to used", which isn't really the case
>> here.
> 
> Yeah, and of course we break this convention all over the place :)
> 
> Maybe folio_pte_batch_flags()?

Works for me as well.

-- 
Cheers,

David / dhildenb