linux-kernel - Re: [PATCH 1/2] mm: Constify folio_order()/folio_test_pmd

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Date: Wed, 26 Jun 2024 04:30:10 +0000
From: ran xiaokai <ranxiaokai627@....com>
To: ziy@...dia.com
Cc: yang.yang29@....com.cn,
	si.hao@....com.cn,
	akpm@...ux-foundation.org,
	baohua@...nel.org,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	peterx@...hat.com,
	ran.xiaokai@....com.cn,
	ranxiaokai627@....com,
	ryan.roberts@....com,
	svetly.todorov@...verge.com,
	vbabka@...e.cz,
	willy@...radead.org
Subject: Re: [PATCH 1/2] mm: Constify folio_order()/folio_test_pmd_mappable()

> On Tue Jun 25, 2024 at 10:49 PM EDT, ran xiaokai wrote:
> > From: Ran Xiaokai <ran.xiaokai@....com.cn>
> >
> > Constify folio_order()/folio_test_pmd_mappable().
> > No functional changes, just a preparation for the next patch.
> 
> What warning/error are you seeing when you just apply patch 2? I wonder why it
> did not show up in other places. Thanks.

fs/proc/page.c: In function 'stable_page_flags':
fs/proc/page.c:152:35: warning: passing argument 1 of 'folio_test_pmd_mappable' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  152 |  else if (folio_test_pmd_mappable(folio)) {
      |                                   ^~~~~
In file included from include/linux/mm.h:1115,
                 from include/linux/memblock.h:12,
                 from fs/proc/page.c:2:
include/linux/huge_mm.h:380:58: note: expected 'struct folio *' but argument is of type 'const struct folio *'
  380 | static inline bool folio_test_pmd_mappable(struct folio *folio)

u64 stable_page_flags(const struct page *page)
{
	const struct folio *folio; // the const definition causes the warning
	...
}

As almost all the folio_test_XXX(flags) have converted to received
a const parameter, it is Ok to also do this for folio_order()?

> >
> > Signed-off-by: Ran Xiaokai <ran.xiaokai@....com.cn>
> > ---
> >  include/linux/huge_mm.h | 2 +-
> >  include/linux/mm.h      | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 2aa986a5cd1b..8d66e4eaa1bc 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -377,7 +377,7 @@ static inline spinlock_t *pud_trans_huge_lock(pud_t *pud,
> >   * folio_test_pmd_mappable - Can we map this folio with a PMD?
> >   * @folio: The folio to test
> >   */
> > -static inline bool folio_test_pmd_mappable(struct folio *folio)
> > +static inline bool folio_test_pmd_mappable(const struct folio *folio)
> >  {
> >  	return folio_order(folio) >= HPAGE_PMD_ORDER;
> >  }
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 9a5652c5fadd..b1c11371a2a3 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1105,7 +1105,7 @@ static inline unsigned int compound_order(struct page *page)
> >   *
> >   * Return: The order of the folio.
> >   */
> > -static inline unsigned int folio_order(struct folio *folio)
> > +static inline unsigned int folio_order(const struct folio *folio)
> >  {
> >  	if (!folio_test_large(folio))
> >  		return 0;