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

Arnd Bergmann
2025-11-07T12:50:28Z

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.

> 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
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).

Arnd