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

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Message-ID: <20251225100106.53e03519@pumpkin>
Date: Thu, 25 Dec 2025 10:01:06 +0000
From: David Laight <david.laight.linux@...il.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Matthew Wilcox <willy@...radead.org>, Lance Yang <ioworker0@...il.com>,
 baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 akpm@...ux-foundation.org, axelrasmussen@...gle.com,
 chandna.sahil@...il.com, chengming.zhou@...ux.dev, 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, 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 Thu, 25 Dec 2025 10:32:46 +0100
"David Hildenbrand (Red Hat)" <david@...nel.org> wrote:

> On 12/24/25 19:06, Matthew Wilcox wrote:
> > On Wed, Dec 24, 2025 at 02:08:29PM +0000, David Laight wrote:  
> >>>>> +++ 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().)  
> > 
> > Or we could go back to my original suggestion.
> > 
> > https://lore.kernel.org/linux-mm/aRyOWrARRlUCeEz6@casper.infradead.org/
> > 
> > which was in v2:
> > 
> > https://lore.kernel.org/linux-mm/f802959f58865371ba1b10081bced98e3784c5e4.1763796152.git.chandna.sahil@gmail.com/  
> 
> I'm starting to wonder whether we should just leave that code alone :)
> 

Maybe 'we' should stop checkpatch (etc) suggesting min() in trivial
cases. It doesn't really make the code better.

	David