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

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Message-ID: <aUvgjc_KGH_903D3@chandna.localdomain>
Date: Wed, 24 Dec 2025 18:16:05 +0530
From: Sahil Chandna <chandna.sahil@...il.com>
To: akpm@...ux-foundation.org, Lance Yang <ioworker0@...il.com>,
	baolin.wang@...ux.alibaba.com
Cc: Liam.Howlett@...cle.com, akpm@...ux-foundation.org,
	axelrasmussen@...gle.com, chengming.zhou@...ux.dev,
	david.laight.linux@...il.com, 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
Subject: Re: [PATCH v4 1/3] mm: pagewalk: simplify hugepage boundary

On Wed, Dec 24, 2025 at 05:23:32PM +0800, Lance Yang 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 ...
>
>>
>> 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.
Hi Baolin, Lance, Andrew,
Thanks for catching this, I understand why ALIGN() caused an infinite loop.
Please drop this patch, or shall I submit the revert ? 
Apologies for this, I am setting up environment for running the selftest
and will send out corrected patch once it successfully pass mm selftests.

Regards,
Sahil