linux-kernel - Re: [PATCH v4 1/3] mm: pagewalk: simplify hugepage boundary

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Message-ID: <20251224140829.2ec7bbae@pumpkin>
Date: Wed, 24 Dec 2025 14:08:29 +0000
From: David Laight <david.laight.linux@...il.com>
To: Lance Yang <ioworker0@...il.com>
Cc: baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 akpm@...ux-foundation.org, axelrasmussen@...gle.com,
 chandna.sahil@...il.com, chengming.zhou@...ux.dev, david@...nel.org,
 david@...hat.com, hannes@...xchg.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, lorenzo.stoakes@...cle.com, mhocko@...e.com,
 nphamcs@...il.com, rppt@...nel.org, shakeel.butt@...ux.dev,
 surenb@...gle.com, vbabka@...e.cz, weixugc@...gle.com, willy@...radead.org,
 yosry.ahmed@...ux.dev, yuanchu@...gle.com, zhengqi.arch@...edance.com,
 Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH v4 1/3] mm: pagewalk: simplify hugepage boundary

On Wed, 24 Dec 2025 17:23:32 +0800
Lance Yang <ioworker0@...il.com> wrote:

> From: Lance Yang <lance.yang@...ux.dev>
> 
> 
> On Wed, 24 Dec 2025 15:50:34 +0800, Baolin Wang wrote:
> > Hi Andrew,
> > 
> > On 2025/11/28 15:01, Sahil Chandna wrote:  
> > > Replace hugepage boundary computation with ALIGN() helper instead of
> > > an open coded expression. This helps to improves code readability.
> > > 
> > > This was flagged by Coccinelle (misc/minmax.cocci) as an opportunity
> > > to use min(), after which the boundary computation was updated following
> > > review suggestions.
> > > 
> > > Found by: make coccicheck MODE=report M=mm/
> > > No functional change intended.
> > > 
> > > Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>
> > > Suggested-by: David Hildenbrand (Red Hat) <david@...nel.org>
> > > Suggested-by: Matthew Wilcox <willy@...radead.org>
> > > Suggested-by: David Laight <david.laight.linux@...il.com>
> > > Signed-off-by: Sahil Chandna <chandna.sahil@...il.com>
> > > ---
> > >   mm/pagewalk.c | 3 +--
> > >   1 file changed, 1 insertion(+), 2 deletions(-)
> > > 
> > > diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> > > index 9f91cf85a5be..9fd59d517f37 100644
> > > --- a/mm/pagewalk.c
> > > +++ b/mm/pagewalk.c
> > > @@ -312,8 +312,7 @@ static int walk_pgd_range(unsigned long addr, unsigned long end,
> > >   static unsigned long hugetlb_entry_end(struct hstate *h, unsigned long addr,
> > >   				       unsigned long end)
> > >   {
> > > -	unsigned long boundary = (addr & huge_page_mask(h)) + huge_page_size(h);
> > > -	return boundary < end ? boundary : end;
> > > +	return min(ALIGN(addr, huge_page_size(h)), end);
> > >   }  
> > 
> > Please drop this patch from the mm-new branch, as it causes 
> > 'run_vmtests.sh' to hang. Specifically, it leads to the system hanging 
> > when executing hugepage-vmemmap test, because the program falls into an 
> > infinite loop in walk_hugetlb_range() and cannot break out.  
> 
> Good catch! The problem is that ALIGN() returns addr itself when already
> aligned, causing the infinite loop ...

Using ALIGN(addr + 1, huge_page_size(h)) would work.
Although it could be (addr + 1) & ~huge_page_mask(h) which is probably
the easiest to understand.
Some of the 'helper' macros don't really make the code easier to read.
(And that includes a lot of uses of min().)

	David

> 
> > 
> > This patch does introduce functional changes and makes an incorrect 
> > assumption that the 'end' must be aligned to the hugepage size. However,   
> 
> Yep. This patch is not equivalent to the original code when addr is
> already aligned :)
> 
> > this is not necessarily the case. For example, see how pagemap_read() 
> > calculates the 'end':
> > 
> > "
> > end = start_vaddr + ((count / PM_ENTRY_BYTES) << PAGE_SHIFT);
> > "
> > 
> > Revert this patch, mm selftests work well.