Re: [PATCH v8 04/10] mm: thp: Support allocation of anonymous multi-size THP

David Hildenbrand
2023-12-07T10:41:48Z

On 07.12.23 11:37, Ryan Roberts wrote:
> On 06/12/2023 15:44, Ryan Roberts wrote:
>> On 06/12/2023 14:19, Ryan Roberts wrote:
>>> On 05/12/2023 16:32, David Hildenbrand wrote:
>>>> On 04.12.23 11:20, Ryan Roberts wrote:
>>>>> Introduce the logic to allow THP to be configured (through the new sysfs
>>>>> interface we just added) to allocate large folios to back anonymous
>>>>> memory, which are larger than the base page size but smaller than
>>>>> PMD-size. We call this new THP extension "multi-size THP" (mTHP).
>>>>>
>>>>> mTHP continues to be PTE-mapped, but in many cases can still provide
>>>>> similar benefits to traditional PMD-sized THP: Page faults are
>>>>> significantly reduced (by a factor of e.g. 4, 8, 16, etc. depending on
>>>>> the configured order), but latency spikes are much less prominent
>>>>> because the size of each page isn't as huge as the PMD-sized variant and
>>>>> there is less memory to clear in each page fault. The number of per-page
>>>>> operations (e.g. ref counting, rmap management, lru list management) are
>>>>> also significantly reduced since those ops now become per-folio.
>>>>>
>>>>> Some architectures also employ TLB compression mechanisms to squeeze
>>>>> more entries in when a set of PTEs are virtually and physically
>>>>> contiguous and approporiately aligned. In this case, TLB misses will
>>>>> occur less often.
>>>>>
>>>>> The new behaviour is disabled by default, but can be enabled at runtime
>>>>> by writing to /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled
>>>>> (see documentation in previous commit). The long term aim is to change
>>>>> the default to include suitable lower orders, but there are some risks
>>>>> around internal fragmentation that need to be better understood first.
>>>>>
>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>>>
>>>> In general, looks good to me, some comments/nits. And the usual "let's make sure
>>>> we don't degrade order-0 and keep that as fast as possible" comment.
>>>>
>>>>> ---
>>>>>   include/linux/huge_mm.h |   6 ++-
>>>>>   mm/memory.c             | 106 ++++++++++++++++++++++++++++++++++++----
>>>>>   2 files changed, 101 insertions(+), 11 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>>> index bd0eadd3befb..91a53b9835a4 100644
>>>>> --- a/include/linux/huge_mm.h
>>>>> +++ b/include/linux/huge_mm.h
>>>>> @@ -68,9 +68,11 @@ extern struct kobj_attribute shmem_enabled_attr;
>>>>>   #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
>>>>>     /*
>>>>> - * Mask of all large folio orders supported for anonymous THP.
>>>>> + * Mask of all large folio orders supported for anonymous THP; all orders up to
>>>>> + * and including PMD_ORDER, except order-0 (which is not "huge") and order-1
>>>>> + * (which is a limitation of the THP implementation).
>>>>>    */
>>>>> -#define THP_ORDERS_ALL_ANON    BIT(PMD_ORDER)
>>>>> +#define THP_ORDERS_ALL_ANON    ((BIT(PMD_ORDER + 1) - 1) & ~(BIT(0) | BIT(1)))
>>>>>     /*
>>>>>    * Mask of all large folio orders supported for file THP.
>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>> index 3ceeb0f45bf5..bf7e93813018 100644
>>>>> --- a/mm/memory.c
>>>>> +++ b/mm/memory.c
>>>>> @@ -4125,6 +4125,84 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>>>       return ret;
>>>>>   }
>>>>>   +static bool pte_range_none(pte_t *pte, int nr_pages)
>>>>> +{
>>>>> +    int i;
>>>>> +
>>>>> +    for (i = 0; i < nr_pages; i++) {
>>>>> +        if (!pte_none(ptep_get_lockless(pte + i)))
>>>>> +            return false;
>>>>> +    }
>>>>> +
>>>>> +    return true;
>>>>> +}
>>>>> +
>>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>>> +static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>>>>> +{
>>>>> +    gfp_t gfp;
>>>>> +    pte_t *pte;
>>>>> +    unsigned long addr;
>>>>> +    struct folio *folio;
>>>>> +    struct vm_area_struct *vma = vmf->vma;
>>>>> +    unsigned long orders;
>>>>> +    int order;
>>>>
>>>> Nit: reverse christmas tree encouraged ;)
>>>
>>> ACK will fix.
>>>
>>>>
>>>>> +
>>>>> +    /*
>>>>> +     * If uffd is active for the vma we need per-page fault fidelity to
>>>>> +     * maintain the uffd semantics.
>>>>> +     */
>>>>> +    if (userfaultfd_armed(vma))
>>>>
>>>> Nit: unlikely()
>>>
>>> ACK will fix.
>>>
>>>>
>>>>> +        goto fallback;
>>>>> +
>>>>> +    /*
>>>>> +     * Get a list of all the (large) orders below PMD_ORDER that are enabled
>>>>> +     * for this vma. Then filter out the orders that can't be allocated over
>>>>> +     * the faulting address and still be fully contained in the vma.
>>>>> +     */
>>>>> +    orders = thp_vma_allowable_orders(vma, vma->vm_flags, false, true, true,
>>>>> +                      BIT(PMD_ORDER) - 1);
>>>>> +    orders = thp_vma_suitable_orders(vma, vmf->address, orders);
>>>>
>>>> Comment: Both will eventually loop over all orders, correct? Could eventually be
>>>> sped up in the future.
>>>
>>> No only thp_vma_suitable_orders() will loop. thp_vma_allowable_orders() only
>>> loops if in_pf=false (it's true here).
>>>
>>>>
>>>> Nit: the orders = ... order = ... looks like this might deserve a helper
>>>> function that makes this easier to read.
>>>
>>> To be honest, the existing function that I've modified is a bit of a mess.
>>> thp_vma_allowable_orders() calls thp_vma_suitable_orders() if we are not in a
>>> page fault, because the page fault handlers already do that check themselves. It
>>> would be nice to refactor the whole thing so that thp_vma_allowable_orders() is
>>> a strict superset of thp_vma_suitable_orders(). Then this can just call
>>> thp_vma_allowable_orders(). But that's going to start touching the PMD and PUD
>>> handlers, so prefer if we leave that for a separate patch set.
>>>
>>>>
>>>> Nit: Why call thp_vma_suitable_orders if the orders are already 0? Again, some
>>>> helper might be reasonable where that is handled internally.
>>>
>>> Because thp_vma_suitable_orders() will handle it safely and is inline, so it
>>> should just as efficient? This would go away with the refactoring described above.
>>>
>>>>
>>>> Comment: For order-0 we'll always perform a function call to both
>>>> thp_vma_allowable_orders() / thp_vma_suitable_orders(). We should perform some
>>>> fast and efficient check if any <PMD THP are even enabled in the system / for
>>>> this VMA, and in that case just fallback before doing more expensive checks.
>>>
>>
>> I just noticed I got these functions round the wrong way in my previous response:
>>
>>> thp_vma_allowable_orders() is inline as you mentioned.
>>
>> ^ Meant thp_vma_suitable_orders() here.
>>
>>>
>>> I was deliberately trying to keep all the decision logic in one place
>>> (thp_vma_suitable_orders) because it's already pretty complicated. But if you
>>
>> ^ Meant thp_vma_allowable_orders() here.
>>
>> Sorry for the confusion.
>>
>>> insist, how about this in the header:
>>>
>>> static inline
>>> unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
>>> unsigned long vm_flags, bool smaps,
>>> bool in_pf, bool enforce_sysfs,
>>> unsigned long orders)
>>> {
>>> /* Optimization to check if required orders are enabled early. */
>>> if (enforce_sysfs && vma_is_anonymous(vma)) {
>>> unsigned long mask = READ_ONCE(huge_anon_orders_always);
>>>
>>> if (vm_flags & VM_HUGEPAGE)
>>> mask |= READ_ONCE(huge_anon_orders_madvise);
>>> if (hugepage_global_always() ||
>>> ((vm_flags & VM_HUGEPAGE) && hugepage_global_enabled()))
>>> mask |= READ_ONCE(huge_anon_orders_inherit);
>>>
>>> orders &= mask;
>>> if (!orders)
>>> return 0;
>>>
>>> enforce_sysfs = false;
>>> }
>>>
>>> return __thp_vma_allowable_orders(vma, vm_flags, smaps, in_pf,
>>> enforce_sysfs, orders);
>>> }
>>>
>>> Then the above check can be removed from __thp_vma_allowable_orders() - it will
>>> still retain the `if (enforce_sysfs && !vma_is_anonymous(vma))` part.
>>>
>>>
>>>>
>>>>> +
>>>>> +    if (!orders)
>>>>> +        goto fallback;
>>>>> +
>>>>> +    pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
>>>>> +    if (!pte)
>>>>> +        return ERR_PTR(-EAGAIN);
>>>>> +
>>>>> +    order = first_order(orders);
>>>>> +    while (orders) {
>>>>> +        addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
>>>>> +        vmf->pte = pte + pte_index(addr);
>>>>> +        if (pte_range_none(vmf->pte, 1 << order))
>>>>> +            break;
>>>>
>>>> Comment: Likely it would make sense to scan only once and determine the "largest
>>>> none range" around that address, having the largest suitable order in mind.
>>>
>>> Yes, that's how I used to do it, but Yu Zhou requested simplifying to this,
>>> IIRC. Perhaps this an optimization opportunity for later?
>>>
>>>>
>>>>> +        order = next_order(&orders, order);
>>>>> +    }
>>>>> +
>>>>> +    vmf->pte = NULL;
>>>>
>>>> Nit: Can you elaborate why you are messing with vmf->pte here? A simple helper
>>>> variable will make this code look less magical. Unless I am missing something
>>>> important :)
>>>
>>> Gahh, I used to pass the vmf to what pte_range_none() was refactored into (an
>>> approach that was suggested by Yu Zhou IIRC). But since I did some refactoring
>>> based on some comments from JohnH, I see I don't need that anymore. Agreed; it
>>> will be much clearer just to use a local variable. Will fix.
>>>
>>>>
>>>>> +    pte_unmap(pte);
>>>>> +
>>>>> +    gfp = vma_thp_gfp_mask(vma);
>>>>> +
>>>>> +    while (orders) {
>>>>> +        addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
>>>>> +        folio = vma_alloc_folio(gfp, order, vma, addr, true);
>>>>> +        if (folio) {
>>>>> +            clear_huge_page(&folio->page, addr, 1 << order);
>>>>> +            return folio;
>>>>> +        }
>>>>> +        order = next_order(&orders, order);
>>>>> +    }
>>>>> +
>>>>
>>>> Queestion: would it make sense to combine both loops? I suspect memory
>>>> allocations with pte_offset_map()/kmao are problematic.
>>>
>>> They are both operating on separate orders; next_order() is "consuming" an order
>>> by removing the current one from the orders bitfield and returning the next one.
>>>
>>> So the first loop starts at the highest order and keeps checking lower orders
>>> until one fully fits in the VMA. And the second loop starts at the first order
>>> that was found to fully fits and loops to lower orders until an allocation is
>>> successful.
>>>
>>> So I don't see a need to combine the loops.
>>>
>>>>
>>>>> +fallback:
>>>>> +    return vma_alloc_zeroed_movable_folio(vma, vmf->address);
>>>>> +}
>>>>> +#else
>>>>> +#define alloc_anon_folio(vmf) \
>>>>> +        vma_alloc_zeroed_movable_folio((vmf)->vma, (vmf)->address)
>>>>> +#endif
>>>>> +
>>>>>   /*
>>>>>    * We enter with non-exclusive mmap_lock (to exclude vma changes,
>>>>>    * but allow concurrent faults), and pte mapped but not yet locked.
>>>>> @@ -4132,6 +4210,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>>>    */
>>>>>   static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>>>>>   {
>>>>> +    int i;
>>>>> +    int nr_pages = 1;
>>>>> +    unsigned long addr = vmf->address;
>>>>>       bool uffd_wp = vmf_orig_pte_uffd_wp(vmf);
>>>>>       struct vm_area_struct *vma = vmf->vma;
>>>>>       struct folio *folio;
>>>>
>>>> Nit: reverse christmas tree :)
>>>
>>> ACK
>>>
>>>>
>>>>> @@ -4176,10 +4257,15 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>>>>>       /* Allocate our own private page. */
>>>>>       if (unlikely(anon_vma_prepare(vma)))
>>>>>           goto oom;
>>>>> -    folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
>>>>> +    folio = alloc_anon_folio(vmf);
>>>>> +    if (IS_ERR(folio))
>>>>> +        return 0;
>>>>>       if (!folio)
>>>>>           goto oom;
>>>>>   +    nr_pages = folio_nr_pages(folio);
>>>>> +    addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
>>>>> +
>>>>>       if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL))
>>>>>           goto oom_free_page;
>>>>>       folio_throttle_swaprate(folio, GFP_KERNEL);
>>>>> @@ -4196,12 +4282,13 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>>>>>       if (vma->vm_flags & VM_WRITE)
>>>>>           entry = pte_mkwrite(pte_mkdirty(entry), vma);
>>>>>   -    vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
>>>>> -            &vmf->ptl);
>>>>> +    vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
>>>>>       if (!vmf->pte)
>>>>>           goto release;
>>>>> -    if (vmf_pte_changed(vmf)) {
>>>>> -        update_mmu_tlb(vma, vmf->address, vmf->pte);
>>>>> +    if ((nr_pages == 1 && vmf_pte_changed(vmf)) ||
>>>>> +        (nr_pages  > 1 && !pte_range_none(vmf->pte, nr_pages))) {
>>>>> +        for (i = 0; i < nr_pages; i++)
>>>>> +            update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
>>>>
>>>> Comment: separating the order-0 case from the other case might make this easier
>>>> to read.
>>>
>>> Yeah fair enough. Will fix.
>>>
>>>>
>>>>>           goto release;
>>>>>       }
>>>>>   @@ -4216,16 +4303,17 @@ static vm_fault_t do_anonymous_page(struct vm_fault
>>>>> *vmf)
>>>>>           return handle_userfault(vmf, VM_UFFD_MISSING);
>>>>>       }
>>>>>   -    inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
>>>>> -    folio_add_new_anon_rmap(folio, vma, vmf->address);
>>>>> +    folio_ref_add(folio, nr_pages - 1);
>>>>> +    add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
>>>>> +    folio_add_new_anon_rmap(folio, vma, addr);
>>>>>       folio_add_lru_vma(folio, vma);
>>>>>   setpte:
>>>>>       if (uffd_wp)
>>>>>           entry = pte_mkuffd_wp(entry);
>>>>> -    set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
>>>>> +    set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr_pages);
>>>>>         /* No need to invalidate - it was non-present before */
>>>>> -    update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
>>>>> +    update_mmu_cache_range(vmf, vma, addr, vmf->pte, nr_pages);
>>>>>   unlock:
>>>>>       if (vmf->pte)
>>>>>           pte_unmap_unlock(vmf->pte, vmf->ptl);
>>>>
>>>> Benchmarking order-0 allocations might be interesting. There will be some added
>>>> checks + multiple loops/conditionals for order-0 that could be avoided by having
>>>> two separate code paths. If we can't measure a difference, all good.
>>>
>>> Yep will do - will post numbers once I have them. I've been assuming that the
>>> major cost is clearing the page, but perhaps I'm wrong.
>>>
>
> I added a "write-fault-byte" benchmark to the microbenchmark tool you gave me.
> This elides the normal memset page population routine, and instead writes the
> first byte of every page while the timer is running.
>
> I ran with 100 iterations per run, then ran the whole thing 16 times. I ran it
> for a baseline kernel, as well as v8 (this series) and v9 (with changes from
> your review). I repeated on Ampere Altra (bare metal) and Apple M2 (VM):
>
> | | m2 vm | altra |
> |--------------|---------------------|--------------------:|
> | kernel | mean | std_rel | mean | std_rel |
> |--------------|----------|----------|----------|---------:|
> | baseline | 0.000% | 0.341% | 0.000% | 3.581% |
> | anonfolio-v8 | 0.005% | 0.272% | 5.068% | 1.128% |
> | anonfolio-v9 | -0.013% | 0.442% | 0.107% | 1.788% |
>
> No measurable difference on M2, but altra has a slow down in v8 which is fixed
> in v9; Looking at the changes, this is either down to the new unlikely() for the
> uffd or due to moving the THP order check to be inline within
> thp_vma_allowable_orders().

I suspect the last one.

>
> So I have all the changes done and perf numbers to show no regression for
> order-0. I'm gonna do a final check and post v9 later today.

Good!

Let me catch up on your comments real quick.

--
Cheers,

David / dhildenb