linux-kernel - Re: [PATCH Resend] mm: Refine __{pgd,p4d,pud,pmd,pte}_alloc_one

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Message-ID: <46a03f86-ab38-4a6c-b1fb-6f77122eff0d@linux.dev>
Date: Fri, 7 Nov 2025 22:17:44 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Arnd Bergmann <arnd@...db.de>, Huacai Chen <chenhuacai@...ngson.cn>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Huacai Chen <chenhuacai@...nel.org>, Jan Kara <jack@...e.cz>,
 Kevin Brodsky <kevin.brodsky@....com>,
 Linux-Arch <linux-arch@...r.kernel.org>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, david@...nel.org, Lance Yang <ioworker0@...il.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, vishal.moola@...il.com
Subject: Re: [PATCH Resend] mm: Refine __{pgd,p4d,pud,pmd,pte}_alloc_one_*()
 about HIGHMEM



On 2025/11/7 20:50, Arnd Bergmann wrote:
> On Fri, Nov 7, 2025, at 12:44, Lance Yang wrote:
>> From: Lance Yang <lance.yang@...ux.dev>
>> On Fri,  7 Nov 2025 17:59:22 +0800, Huacai Chen wrote:
>>>
>>>    */
>>>   static inline pte_t *__pte_alloc_one_kernel_noprof(struct mm_struct *mm)
>>>   {
>>> -	struct ptdesc *ptdesc = pagetable_alloc_noprof(GFP_PGTABLE_KERNEL &
>>> -			~__GFP_HIGHMEM, 0);
>>> +	struct ptdesc *ptdesc = pagetable_alloc_noprof(GFP_PGTABLE_KERNEL, 0);
>>
>> I looked into the history and it seems you are right. This defensive pattern
>> was likely introduced by Vishal Moola in commit c787ae5[1].
> 
> Right, so not even so long ago, so we need to make sure we agree
> on a direction and don't send opposite patches in the name of
> cleanups.

Yes, better to get on the same page now than to have conflicting
cleanups down the line ;)

> 
>> After this cleanup, would it make sense to add a BUILD_BUG_ON() somewhere
>> to check that __GFP_HIGHMEM is not present in GFP_PGTABLE_KERNEL and
>> GFP_PGTABLE_USER? This would prevent any future regression ;)
>>
>> Just a thought ...
> 
> I think we can go either way here, but I'd tend towards not
> adding more checks but instead removing any mention of __GFP_HIGHMEM
> that we can show is either pointless or can be avoided, with

Makes sense to me :)

> the goal of having only a small number of actual highmem
> allocations remaining in places we do care about (normal
> page cache, zram, possibly huge pages).

Right! That's the ideal end state. Making the code cleaner and
the intention clearer ;p

Cheers,
Lance