Re: [PATCH v3 1/2] mm: Abstract THP allocation

[Dev Jain <dev.jain@arm.com>]

On 9/11/24 18:30, David Hildenbrand wrote:
> On 11.09.24 14:53, Dev Jain wrote:
>>
>> On 9/11/24 14:57, David Hildenbrand wrote:
>>> On 11.09.24 08:55, Dev Jain wrote:
>>>> In preparation for the second patch, abstract away the THP allocation
>>>> logic present in the create_huge_pmd() path, which corresponds to the
>>>> faulting case when no page is present.
>>>>
>>>> There should be no functional change as a result of applying
>>>> this patch.
>>>>
>>>
>>> [...]
>>>
>>>> +
>>>> +static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>>>> +            struct vm_area_struct *vma, unsigned long haddr)
>>>> +{
>>>> +    pmd_t entry;
>>>> +
>>>> +    entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
>>>> +    entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>>>> +    folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
>>>> +    folio_add_lru_vma(folio, vma);
>>>> +    set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
>>>> +    update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
>>>
>>> It's quite weird to see a mixture of haddr and vmf->address, and
>>> likely this mixture is wrong or not not required.
>>>
>>> Looking at arc's update_mmu_cache_pmd() implementation, I cannot see
>>> how passing in the unaligned address would do the right thing. But
>>> maybe arc also doesn't trigger that code path ... who knows :)
>>
>> If I am reading correctly, arch/arc/mm/tlb.c: update_mmu_cache_pmd()
>> calls update_mmu_cache_range() which is already expecting an unaligned
>> address? But...
>
> So update_mmu_cache_pmd() calls
>
>     update_mmu_cache_range(NULL, vma, addr, &pte, HPAGE_PMD_NR);
>
> But update_mmu_cache_range() only aligns it *to page boundary*:
>
>     unsigned long vaddr = vaddr_unaligned & PAGE_MASK;

Ah, totally missed that it was PAGE_MASK. Thanks.
>
> We obtain the correct hugepage-aligned physical address from the PTE
>
>     phys_addr_t paddr = pte_val(*ptep) & PAGE_MASK_PHYS;
>
> Then, we look at the offset in our folio
>
>     unsigned long offset = offset_in_folio(folio, paddr);
>
> And adjust both vaddr and paddr
>
>     paddr -= offset;
>     vaddr -= offset;
>
> To then use that combination with
>
>     __inv_icache_pages(paddr, vaddr, nr);
>
> If I am not wrong, getting a non-hugepage aligned vaddr messes up 
> things here. But only regarding the icache I think.

Looks like it...
>
>