linux-kernel - Re: [PATCH v5 2/2] mm: Allocate THP on hugezeropage wp-fault

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Message-ID: <903de160-e2cf-42cc-93ee-6b356cdca6de@arm.com>
Date: Wed, 25 Sep 2024 11:01:11 +0530
From: Dev Jain <dev.jain@....com>
To: Kefeng Wang <wangkefeng.wang@...wei.com>, akpm@...ux-foundation.org,
 david@...hat.com, 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, ziy@...dia.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v5 2/2] mm: Allocate THP on hugezeropage wp-fault


On 9/24/24 18:43, Kefeng Wang wrote:
>
>
> On 2024/9/24 18:16, Dev Jain wrote:
>> Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and
>> replace it with a PMD-mapped THP. Remember to flush TLB entry
>> corresponding to the hugezeropage. In case of failure, fallback
>> to splitting the PMD.
>>
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>>   mm/huge_memory.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 43 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index bdbf67c18f6c..fbb195bc2038 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1790,6 +1790,41 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
>>       spin_unlock(vmf->ptl);
>>   }
>>   +static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf)
>> +{
>> +    unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>> +    struct vm_area_struct *vma = vmf->vma;
>> +    struct mmu_notifier_range range;
>> +    struct folio *folio;
>> +    vm_fault_t ret = 0;
>> +
>> +    folio = vma_alloc_anon_folio_pmd(vma, vmf->address);
>> +    if (unlikely(!folio)) {
>> +        ret = VM_FAULT_FALLBACK;
>
> I'd like to return VM_FAULT_FALLBACK if you re-post.

That's cleaner. I need to keep my "goto" obsession in control...
>
> Reviewed-by: Kefeng Wang <wangkefeng.wang@...wei.com>

Thanks!
>
>> +        goto out;
>> +    }
>> +
>> +    mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, 
>> haddr,
>> +                haddr + HPAGE_PMD_SIZE);
>> +    mmu_notifier_invalidate_range_start(&range);
>> +    vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>> +    if (unlikely(!pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
>> +        goto release;
>> +    ret = check_stable_address_space(vma->vm_mm);
>> +    if (ret)
>> +        goto release;
>> +    (void)pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
>> +    map_anon_folio_pmd(folio, vmf->pmd, vma, haddr);
>> +    goto unlock;
>> +release:
>> +    folio_put(folio);
>> +unlock:
>> +    spin_unlock(vmf->ptl);
>> +    mmu_notifier_invalidate_range_end(&range);
>> +out:
>> +    return ret;
>> +}
>> +
>>   vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>>   {
>>       const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
>> @@ -1802,8 +1837,15 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault 
>> *vmf)
>>       vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
>>       VM_BUG_ON_VMA(!vma->anon_vma, vma);
>>   -    if (is_huge_zero_pmd(orig_pmd))
>> +    if (is_huge_zero_pmd(orig_pmd)) {
>> +        vm_fault_t ret = do_huge_zero_wp_pmd(vmf);
>> +
>> +        if (!(ret & VM_FAULT_FALLBACK))
>> +            return ret;
>> +
>> +        /* Fallback to splitting PMD if THP cannot be allocated */
>>           goto fallback;
>> +    }
>>         spin_lock(vmf->ptl);
>
>