Re: [PATCH v2 1/2] mm: Abstract THP allocation

[Ryan Roberts <ryan.roberts@arm.com>]

On 06/09/2024 09:45, Dev Jain wrote:
> 
> On 9/6/24 13:58, Ryan Roberts wrote:
>> On 06/09/2024 06:42, Dev Jain wrote:
>>> On 9/5/24 16:38, Ryan Roberts wrote:
>>>> On 04/09/2024 11:09, 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@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..58125fbcc532 100644
>>>>> --- a/mm/huge_memory.c
>>>>> +++ b/mm/huge_memory.c
>>>>> @@ -943,47 +943,89 @@ 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 vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct
>>>>> vm_area_struct *vma,
>>>> Is there a reason for specifying order as a parameter? Previously it was
>>>> hardcoded to HPAGE_PMD_ORDER. But now, thp_fault_alloc() and
>>>> __thp_fault_success_stats() both take order and map_pmd_thp() is still
>>>> implicitly mapping a PMD-sized block. Unless there is a reason you need this
>>>> parameter in the next patch (I don't think there is?) I suggest simplifying.
>>> If I am not wrong, thp_fault_alloc() and __thp_fault_success_stats()
>>> will remain the same in case of mTHP?
>> No, its a bit different for smaller-than-pmd THP - that's handled in
>> alloc_anon_folio() in memory.c. It deals with fallback to smaller orders (inc
>> order-0. Potentially alloc_anon_folio() could be refactored to use your new
>> functions in future but I'm sure there would be a number of subtlties to
>> consider and in any case you would want to make that a separate change. The
>> order param would be added as part of that future change.
> 
> Okay.
> 
>>
>>> I chose to pass order so that these
>>> functions can be used by others in the future.
>> It's not valuable to have an internal interface with no users. My advice is to
>> keep it simple and only extend it when a user pops up.
> 
> Makes sense. In that case, should I call it pmd_thp_fault_alloc() to enforce that
> this is not a non-PMD THP?

Yeah, that works for me.

> 
>>
>>> Therefore, these two functions
>>> can be generically used in the future while map_pmd_thp() (as the name suggests)
>>> maps only a PMD-mappable THP.
>>>
>>>>> +                  unsigned long haddr, struct folio **foliop,
>>>> FWIW, I agree with Kirill's suggestion to just return folio* and drop the
>>>> output
>>>> param.
>>> As I replied to Kirill, I do not think that is a good idea. If you do a git
>>> grep on
>>> the tree for "foliop", you will find several places where that is being used,
>>> for
>>> example, check out alloc_charge_folio() in mm/khugepaged.c. The author
>>> intends to
>>> do the stat computation and setting *foliop in the function itself, so that the
>>> caller is only concerned with the return value.
>> By having 2 return params, you open up the possibility of returning an invalid
>> combination (e.g. ret==FALLBACK, folio!=NULL). You avoid that by having a single
>> return param, which is either NULL (failure) or a valid pointer.
> 
> Okay, I agree with this in a generic sense.
> 
>>
>>> Also, if we return the folio instead of VM_FAULT_FALLBACK from
>>> thp_fault_alloc(),
>>> then you will have two "if (unlikely(!folio))" branches, the first to do the
>>> stat
>>> computation in the function, and the second branch in the caller to set ret =
>>> VM_FAULT_FALLBACK
>>> and then goto out/release.
>> There are already 2 conditionals, that doesn't change.
>>
>>> And, it is already inconsistent to break away the stat computation and the
>>> return value
>>> setting, when the stat computation name is of the form
>>> "count_vm_event(THP_FAULT_FALLBACK)"
>>> and friends, at which point it would just be better to open code the function.
>> I don't understand this.
>>
>>> If I am missing something and your suggestion can be implemented neatly, please
>>> guide.
>> This looks cleaner/simpler to my eye (also removing the order parameter):
>>
>> static folio *thp_fault_alloc(gfp_t gfp, struct vm_area_struct *vma,
>>                   unsigned long haddr, unsigned long addr)
>> {
>>     const int order = HPAGE_PMD_ORDER;
>>     struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
>>
>>     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(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>>         count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>>         goto out;
>>     }
>>     folio_throttle_swaprate(folio, gfp);
>>
>>     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 vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>> {
>>     vm_fault_t ret = VM_FAULT_FALLBACK;
>>     ...
>>
>>     folio = thp_fault_alloc(gfp, vma, haddr, vmf->address);
>>     if (unlikely(!folio))
>>         goto release;
>>     ...
>> }
>>
> 
> Thanks for your help. I will switch to this, just that, ret should still
> be initialized to zero in case we spot the pmd changes after taking the lock.
> So I'll set it in the case of !folio.

ret = thp_fault_alloc() will have already set it back to zero so there isn't a
bug. But if you prefer to explicitly set it in the "if" to make the code clearer
to read, that's fine by me.

Thanks,
Ryan

> 
>>
>>>> Thanks,
>>>> Ryan
>>>>
>>>>> +                  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;
>>>>> +    struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
>>>>>    -    VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>>>>> +    *foliop = folio;
>>>>> +    if (unlikely(!folio)) {
>>>>> +        count_vm_event(THP_FAULT_FALLBACK);
>>>>> +        count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>>>>> +        return VM_FAULT_FALLBACK;
>>>>> +    }
>>>>>    +    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);
>>>>> +        count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>>>>> +        count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>>>>>            return VM_FAULT_FALLBACK;
>>>>>        }
>>>>>        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);
>>>>> +    return 0;
>>>>> +}
>>>>> +
>>>>> +static void __thp_fault_success_stats(struct vm_area_struct *vma, int order)
>>>>> +{
>>>>> +    count_vm_event(THP_FAULT_ALLOC);
>>>>> +    count_mthp_stat(order, MTHP_STAT_ANON_FAULT_ALLOC);
>>>>> +    count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
>>>>> +}
>>>>> +
>>>>> +static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>>>>> +            struct vm_area_struct *vma, unsigned long haddr,
>>>>> +            pgtable_t pgtable)
>>>>> +{
>>>>> +    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);
>>>>> +    pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
>>>>> +    set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
>>>>> +    update_mmu_cache_pmd(vma, vmf->address, vmf->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 = NULL;
>>>>> +    pgtable_t pgtable;
>>>>> +    unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>>>>> +    vm_fault_t ret = 0;
>>>>> +    gfp_t gfp = vma_thp_gfp_mask(vma);
>>>>> +
>>>>> +    pgtable = pte_alloc_one(vma->vm_mm);
>>>>> +    if (unlikely(!pgtable)) {
>>>>> +        ret = VM_FAULT_OOM;
>>>>> +        goto release;
>>>>> +    }
>>>>> +
>>>>> +    ret = thp_fault_alloc(gfp, HPAGE_PMD_ORDER, vma, haddr, &folio,
>>>>> +                  vmf->address);
>>>>> +    if (ret)
>>>>> +        goto release;
>>>>>          vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>>>>> +
>>>>>        if (unlikely(!pmd_none(*vmf->pmd))) {
>>>>>            goto unlock_release;
>>>>>        } else {
>>>>> -        pmd_t entry;
>>>>> -
>>>>>            ret = check_stable_address_space(vma->vm_mm);
>>>>>            if (ret)
>>>>>                goto unlock_release;
>>>>> @@ -997,20 +1039,9 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct
>>>>> vm_fault *vmf,
>>>>>                VM_BUG_ON(ret & VM_FAULT_FALLBACK);
>>>>>                return ret;
>>>>>            }
>>>>> -
>>>>> -        entry = mk_huge_pmd(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);
>>>>> -        pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
>>>>> -        set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
>>>>> -        update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
>>>>> -        add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>>>>> -        mm_inc_nr_ptes(vma->vm_mm);
>>>>> +        map_pmd_thp(folio, vmf, vma, haddr, pgtable);
>>>>>            spin_unlock(vmf->ptl);
>>>>> -        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);
>>>>> +        __thp_fault_success_stats(vma, HPAGE_PMD_ORDER);
>>>>>        }
>>>>>          return 0;
>>>>> @@ -1019,7 +1050,8 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct
>>>>> vm_fault *vmf,
>>>>>    release:
>>>>>        if (pgtable)
>>>>>            pte_free(vma->vm_mm, pgtable);
>>>>> -    folio_put(folio);
>>>>> +    if (folio)
>>>>> +        folio_put(folio);
>>>>>        return ret;
>>>>>      }
>>>>> @@ -1077,8 +1109,6 @@ static void set_huge_zero_folio(pgtable_t pgtable,
>>>>> struct mm_struct *mm,
>>>>>    vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>>>>>    {
>>>>>        struct vm_area_struct *vma = vmf->vma;
>>>>> -    gfp_t gfp;
>>>>> -    struct folio *folio;
>>>>>        unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>>>>>        vm_fault_t ret;
>>>>>    @@ -1129,14 +1159,8 @@ vm_fault_t do_huge_pmd_anonymous_page(struct
>>>>> vm_fault *vmf)
>>>>>            }
>>>>>            return ret;
>>>>>        }
>>>>> -    gfp = vma_thp_gfp_mask(vma);
>>>>> -    folio = vma_alloc_folio(gfp, HPAGE_PMD_ORDER, vma, haddr, true);
>>>>> -    if (unlikely(!folio)) {
>>>>> -        count_vm_event(THP_FAULT_FALLBACK);
>>>>> -        count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
>>>>> -        return VM_FAULT_FALLBACK;
>>>>> -    }
>>>>> -    return __do_huge_pmd_anonymous_page(vmf, &folio->page, gfp);
>>>>> +
>>>>> +    return __do_huge_pmd_anonymous_page(vmf);
>>>>>    }
>>>>>      static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long
>>>>> addr,