Re: [PATCH Resend] mm: Refine __{pgd,p4d,pud,pmd,pte}_alloc_one_*() about HIGHMEM

Lance Yang
2025-11-07T14:18:08Z

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@linux.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