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

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Message-ID: <a068f1b4-8167-4f1d-ac60-9cb9de19a80b@arm.com>
Date: Wed, 11 Sep 2024 17:52:13 +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 v3 1/2] mm: Abstract THP allocation


On 9/11/24 16:22, Kefeng Wang wrote:
>
>
> On 2024/9/11 14:55, 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.
>>
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>>   mm/huge_memory.c | 110 +++++++++++++++++++++++++++++------------------
>>   1 file changed, 67 insertions(+), 43 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 67c86a5d64a6..b96a1ff2bf40 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -943,47 +943,88 @@ unsigned long thp_get_unmapped_area(struct file 
>> *filp, unsigned long addr,
>>   }
>>   EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
>>   -static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
>> -            struct page *page, gfp_t gfp)
>> +static struct folio *pmd_thp_fault_alloc(gfp_t gfp, struct 
>> vm_area_struct *vma,
>> +                     unsigned long haddr, unsigned long addr)
>>   {
>> -    struct vm_area_struct *vma = vmf->vma;
>> -    struct folio *folio = page_folio(page);
>> -    pgtable_t pgtable;
>> -    unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>> -    vm_fault_t ret = 0;
>> +    const int order = HPAGE_PMD_ORDER;
>
> Maybe move vma_thp_gfp_mask() into this function too.

That's better, thanks.
>
>> +    struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, 
>> true);
>>   -    VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>> +    if (unlikely(!folio)) {
>> +        count_vm_event(THP_FAULT_FALLBACK);
>> +        count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>> +        goto out;
>> +    }
>>   +    VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>>       if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
>>           folio_put(folio);
>>           count_vm_event(THP_FAULT_FALLBACK);
>>           count_vm_event(THP_FAULT_FALLBACK_CHARGE);
>> -        count_mthp_stat(HPAGE_PMD_ORDER, 
>> MTHP_STAT_ANON_FAULT_FALLBACK);
>> -        count_mthp_stat(HPAGE_PMD_ORDER, 
>> MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>> -        return VM_FAULT_FALLBACK;
>> +        count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>> +        count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>> +        goto out;
>
> We need to return NULL here as folio not set to null,

My bad for assuming that folio_put() also sets folio to NULL. I read
through the code path for that and I guess it does not. Thanks.
>
>>       }
>>       folio_throttle_swaprate(folio, gfp);
>>   -    pgtable = pte_alloc_one(vma->vm_mm);
>> -    if (unlikely(!pgtable)) {
>> -        ret = VM_FAULT_OOM;
>> -        goto release;
>> -    }
>> -
>> -    folio_zero_user(folio, vmf->address);
>> +    folio_zero_user(folio, addr);
>>       /*
>>        * The memory barrier inside __folio_mark_uptodate makes sure that
>>        * folio_zero_user writes become visible before the set_pmd_at()
>>        * write.
>>        */
>>       __folio_mark_uptodate(folio);
>> +out:
>> +    return folio;
>> +}
>> +