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

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Message-ID: <30eb3546-9e2c-44a6-9485-604dabc96735@arm.com>
Date: Wed, 11 Sep 2024 18:46:16 +0530
From: Dev Jain <dev.jain@....com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
 willy@...radead.org, kirill.shutemov@...ux.intel.com
Cc: ryan.roberts@....com, anshuman.khandual@....com, catalin.marinas@....com,
 cl@...two.org, vbabka@...e.cz, mhocko@...e.com, apopple@...dia.com,
 dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
 jack@...e.cz, mark.rutland@....com, hughd@...gle.com,
 aneesh.kumar@...nel.org, yang@...amperecomputing.com, peterx@...hat.com,
 ioworker0@...il.com, jglisse@...gle.com, wangkefeng.wang@...wei.com,
 ziy@...dia.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 1/2] mm: Abstract THP allocation


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!