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

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

On 9/11/24 18:44, David Hildenbrand wrote:
> On 11.09.24 15:05, Dev Jain wrote:
>>
>> 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...
>
> As we are adding a fresh page where there previously wasn't anything 
> mapped (no icache invaldiation required?), and because most anon 
> mappings are not executable, maybe that's why nobody notices so far.

Thanks for the observation!