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

[Dev Jain <dev.jain@arm.com>]

On 9/11/24 14:59, David Hildenbrand wrote:
> On 11.09.24 11:27, 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.
>>>
>>
>> Hi,
>>
>>> Signed-off-by: Dev Jain <dev.jain@arm.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)
>>
>> I suggest calling this something like "vma_alloc_anon_folio_pmd()"? Then
>> it's more consistent with vma_alloc_folio().
>>
>> Also, likely we should just only pass in "addr" and calculate "haddr"
>> ourselves, it's cheap and reduces the number of function parameters.
>>
>>>    {
>>> -    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;
>>> +    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;
>>>        }
>>>        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;
>>> +}
>>> +
>>> +static void __pmd_thp_fault_success_stats(struct vm_area_struct *vma)
>>> +{
>>> +    count_vm_event(THP_FAULT_ALLOC);
>>> +    count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC);
>>> +    count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
>>> +}
>>
>> Why isn't that moved into map_pmd_thp()
>>
>> Note that in this patch you do:
>>
>> map_pmd_thp(folio, vmf, vma, haddr);
>> spin_unlock(vmf->ptl);
>> __pmd_thp_fault_success_stats(vma);
>>
>> But in patch #2
>>
>> map_pmd_thp(folio, vmf, vma, haddr);
>> __pmd_thp_fault_success_stats(vma);
>> goto unlock;
>> release:
>>     folio_put(folio);
>> unlock:
>>     spin_unlock(vmf->ptl);
>>
>> Please make that consistent, meaning:
>>
>> 1) Inline __pmd_thp_fault_success_stats() into map_pmd_thp(). No need to
>> have the separated out.
>>
>> 2) Either do the PTL unlocking in __pmd_thp_fault_success_stats() or in
>>      the caller. In the caller is likely easiest. Adjusting the counters
>>      should be cheap, if not we could revisit this later with real data.
>>
>>> +
>>> +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 :)
>>
>>
>> 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.
>>
>>
>> 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)
>
> ... or better "map_anon_folio_pmd" so it better matches 
> vma_alloc_folio_pmd() suggested above.

I'll vote for this.
>
>