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: <d9f8b010-3c80-4d02-99df-30171878c799@lucifer.local>
Date: Mon, 30 Jun 2025 11:41:56 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.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 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?

>
> >
> > >
> > > 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.
> > >
> > > Signed-off-by: David Hildenbrand <david@...hat.com>
> >
> > Other than the query above + nit on name below, this is really nice!
> >
> > > ---
> > >   mm/internal.h  | 11 ++++++++---
> > >   mm/madvise.c   |  4 ++--
> > >   mm/memory.c    |  6 ++----
> > >   mm/mempolicy.c |  3 +--
> > >   mm/mlock.c     |  3 +--
> > >   mm/mremap.c    |  3 +--
> > >   mm/rmap.c      |  3 +--
> > >   mm/util.c      | 29 +++++++++++++++++++++++++++++
> > >   8 files changed, 45 insertions(+), 17 deletions(-)
> > >
> > > diff --git a/mm/internal.h b/mm/internal.h
> > > index ca6590c6d9eab..6000b683f68ee 100644
> > > --- a/mm/internal.h
> > > +++ b/mm/internal.h
> > > @@ -218,9 +218,8 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> > >   }
> > >
> > >   /**
> > > - * folio_pte_batch - detect a PTE batch for a large folio
> > > + * folio_pte_batch_ext - detect a PTE batch for a large folio
> > >    * @folio: The large folio to detect a PTE batch for.
> > > - * @addr: The user virtual address the first page is mapped at.
> > >    * @ptep: Page table pointer for the first entry.
> > >    * @pte: Page table entry for the first page.
> > >    * @max_nr: The maximum number of table entries to consider.
> > > @@ -243,9 +242,12 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> > >    * must be limited by the caller so scanning cannot exceed a single VMA and
> > >    * a single page table.
> > >    *
> > > + * This function will be inlined to optimize based on the input parameters;
> > > + * consider using folio_pte_batch() instead if applicable.
> > > + *
> > >    * Return: the number of table entries in the batch.
> > >    */
> > > -static inline unsigned int folio_pte_batch(struct folio *folio, unsigned long addr,
> > > +static inline unsigned int folio_pte_batch_ext(struct folio *folio,
> > >   		pte_t *ptep, pte_t pte, unsigned int max_nr, fpb_t flags,
> > >   		bool *any_writable, bool *any_young, bool *any_dirty)
> >
> > Sorry this is really really annoying feedback :P but _ext() makes me think of
> > page_ext and ugh :))
> >
> > Wonder if __folio_pte_batch() is better?
> >
> > This is obviously, not a big deal (TM)
>
> 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()?

>
> E.g.,
>
> alloc_frozen_pages() -> alloc_frozen_pages_noprof() ->
> __alloc_frozen_pages_noprof()
>
> --
> Cheers,
>
> David / dhildenb
>