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

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Message-ID: <8bc0e51b-0e9b-4eeb-997f-e2a0b1a0c0f8@arm.com>
Date: Wed, 11 Sep 2024 18:23:47 +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 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...
>
>
> Staring at some other update_mmu_cache_pmd() users, it's quite 
> inconsistent. Primarily only do_huge_pmd_numa_page() and 
> __do_huge_pmd_anonymous_page() use the unaligned address. The others 
> seem to use the aligned address ... as one would expect when modifying 
> a PMD.

Looking at riscv: 
update_mmu_cache_pmd()->update_mmu_cache()->update_mmu_cache_range(). 
The argument getting passed to local_flush_tlb_page() seems like, should 
expect an aligned address.
>
>
> I suggest to change this function to *not* pass in the vmf, and rename 
> it to something like:
>
> static void folio_map_anon_pmd(struct folio *folio, struct 
> vm_area_struct *vma, pmd_t *pmd, unsigned long haddr)
>
> Then use haddr also to do the update_mmu_cache_pmd().
>
>> +    add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>> +    mm_inc_nr_ptes(vma->vm_mm);
>> +}
>> +
>> +static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>> +{
>> +    struct vm_area_struct *vma = vmf->vma;
>> +    struct folio *folio;
>> +    pgtable_t pgtable;
>> +    unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>> +    vm_fault_t ret = 0;
>> +    gfp_t gfp = vma_thp_gfp_mask(vma);
>
> Nit: While at it, try to use reverse christmas-tree where possible, 
> makes things more reasible. You could make haddr const.
>
> struct vm_area_struct *vma = vmf->vma;
> unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> gfp_t gfp = vma_thp_gfp_mask(vma);
> struct folio *folio;
> vm_fault_t ret = 0;
> ...
>
>> +
>> +    folio = pmd_thp_fault_alloc(gfp, vma, haddr, vmf->address);
>> +    if (unlikely(!folio)) {
>> +        ret = VM_FAULT_FALLBACK;
>> +        goto release;
>> +    }
>> +
>> +    pgtable = pte_alloc_one(vma->vm_mm);
>> +    if (unlikely(!pgtable)) {
>> +        ret = VM_FAULT_OOM;
>> +        goto release;
>> +    }
>>         vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>> +
>
> Nit Unrelated change.
>
>>       if (unlikely(!pmd_none(*vmf->pmd))) {
>>           goto unlock_release;
>
>