Re: [RESEND PATCH v7 04/10] mm: thp: Support allocation of anonymous small-sized THP

[Ryan Roberts]

On 27/11/2023 03:41, Barry Song wrote:
>> +	if ((nr_pages == 1 && vmf_pte_changed(vmf)) ||
>> +	    (nr_pages  > 1 && !pte_range_none(vmf->pte, nr_pages))) {
>> +		for (i = 0; i < nr_pages; i++)
>> +			update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
>>  		goto release;
>>  	}
> 
> Hi Ryan,
> what has stopped nr_pages == 1 from using !pte_range_none(vmf->pte, 1)
> directly, then the code can become,
> +	if (!pte_range_none(vmf->pte, nr_pages)) {
> +		for (i = 0; i < nr_pages; i++)
> +			update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
>  		goto release;
>  	}
> 
> for both 1 and > 1 cases?

We can get to do_anonymous_page() from 2 routes:

 - page fault on unallocated memory (pte_none())
 - page fault on uffd_wp pte marker

In the latter case, we guarrantee that we are only operating on nr_pages == 1
because when uffd is in the picture we need to preserve any uffd state per-pte.
It also means we can't just check the pte is none because in this case it is not
none, it has a pte marker so we need to check it hasn't changed.

I was previously abstracting this in vmf_pte_range_changed() but there were
complaints [1] about the semantic being different based on the number of pages,
so this was my attempt to make it more understandable.

[1] https://lore.kernel.org/linux-mm/a6fa0847-a950-4044-972c-e5dc8cbc7922@xxxxxxx/

Thanks,
Ryan


> 
> Thanks
> Barry
>