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

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Message-ID: <62df853c-98fb-4900-9cc6-04208e9611c6@arm.com>
Date: Wed, 11 Sep 2024 18:35:35 +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 18:30, 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...
>
>