linux-kernel - Re: [PATCH v2 3/4] mm: split folio_pte_batch() into folio_pte_batch() and folio_pte_batch

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Message-ID: <b91a6670-0d85-49eb-90fb-804fcf6fa2a6@arm.com>
Date: Thu, 3 Jul 2025 16:15:16 +0530
From: Dev Jain <dev.jain@....com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...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>,
 Lance Yang <ioworker0@...il.com>, Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v2 3/4] mm: split folio_pte_batch() into folio_pte_batch()
 and folio_pte_batch_flags()

On 02/07/25 4:19 pm, David Hildenbrand wrote:
> Many users (including upcoming ones) don't really need the flags etc,
> and can live with the possible overhead of a function call.
>
> So let's provide a basic, non-inlined folio_pte_batch(), to avoid code
> bloat while still providing a variant that optimizes out all flag
> checks at runtime. folio_pte_batch_flags() will get inlined into
> folio_pte_batch(), optimizing out any conditionals that depend on input
> flags.
>
> folio_pte_batch() will behave like folio_pte_batch_flags() when no
> flags are specified. It's okay to add new users of
> folio_pte_batch_flags(), but using folio_pte_batch() if applicable is
> preferred.
>
> So, before this change, folio_pte_batch() was inlined into the C file
> optimized by propagating constants within the resulting object file.
>
> With this change, we now also have a folio_pte_batch() that is
> optimized by propagating all constants. But instead of having one instance
> per object file, we have a single shared one.
>
> In zap_present_ptes(), where we care about performance, the compiler
> already seem to generate a call to a common inlined folio_pte_batch()
> variant, shared with fork() code. So calling the new non-inlined variant
> should not make a difference.
>
> While at it, drop the "addr" parameter that is unused.
>
> Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
> Link: https://lore.kernel.org/linux-mm/20250503182858.5a02729fcffd6d4723afcfc2@linux-foundation.org/
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>   

Having one place to share the inlined copy is amazing! In the near
future I will be doing some GNU toolchain work so hopefully I gain
knowledge on compiler stuff; it was fun reading the inlined
vs non-inlined discussion.

Reviewed-by: Dev Jain <dev.jain@....com>