Re: [PATCH mm-unstable v2 3/3] mm/hugetlb: use __GFP_COMP for gigantic folios

[Zi Yan <ziy@nvidia.com>]

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On 16 Aug 2024, at 12:02, Yu Zhao wrote:

> On Fri, Aug 16, 2024 at 9:23 AM Zi Yan <ziy@nvidia.com> wrote:
>>
>> On 13 Aug 2024, at 23:54, Yu Zhao wrote:
>>
>>> Use __GFP_COMP for gigantic folios to greatly reduce not only the
>>> amount of code but also the allocation and free time.
>>>
>>> LOC (approximately): +60, -240
>>>
>>> Allocate and free 500 1GB hugeTLB memory without HVO by:
>>>   time echo 500 >/sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>>>   time echo 0 >/sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>>>
>>>        Before  After
>>> Alloc  ~13s    ~10s
>>> Free   ~15s    <1s
>>>
>>> The above magnitude generally holds for multiple x86 and arm64 CPU
>>> models.
>>>
>>> Signed-off-by: Yu Zhao <yuzhao@google.com>
>>> Reported-by: Frank van der Linden <fvdl@google.com>
>>> ---
>>>  include/linux/hugetlb.h |   9 +-
>>>  mm/hugetlb.c            | 293 ++++++++--------------------------------
>>>  2 files changed, 62 insertions(+), 240 deletions(-)
>>>
>>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>>> index 3100a52ceb73..98c47c394b89 100644
>>> --- a/include/linux/hugetlb.h
>>> +++ b/include/linux/hugetlb.h
>>> @@ -896,10 +896,11 @@ static inline bool hugepage_movable_supported(struct hstate *h)
>>>  /* Movability of hugepages depends on migration support. */
>>>  static inline gfp_t htlb_alloc_mask(struct hstate *h)
>>>  {
>>> -     if (hugepage_movable_supported(h))
>>> -             return GFP_HIGHUSER_MOVABLE;
>>> -     else
>>> -             return GFP_HIGHUSER;
>>> +     gfp_t gfp = __GFP_COMP | __GFP_NOWARN;
>>> +
>>> +     gfp |= hugepage_movable_supported(h) ? GFP_HIGHUSER_MOVABLE : GFP_HIGHUSER;
>>> +
>>> +     return gfp;
>>>  }
>>>
>>>  static inline gfp_t htlb_modify_alloc_mask(struct hstate *h, gfp_t gfp_mask)
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 71d469c8e711..efa77ce87dcc 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -56,16 +56,6 @@ struct hstate hstates[HUGE_MAX_HSTATE];
>>>  #ifdef CONFIG_CMA
>>>  static struct cma *hugetlb_cma[MAX_NUMNODES];
>>>  static unsigned long hugetlb_cma_size_in_node[MAX_NUMNODES] __initdata;
>>> -static bool hugetlb_cma_folio(struct folio *folio, unsigned int order)
>>> -{
>>> -     return cma_pages_valid(hugetlb_cma[folio_nid(folio)], &folio->page,
>>> -                             1 << order);
>>> -}
>>> -#else
>>> -static bool hugetlb_cma_folio(struct folio *folio, unsigned int order)
>>> -{
>>> -     return false;
>>> -}
>>>  #endif
>>>  static unsigned long hugetlb_cma_size __initdata;
>>>
>>> @@ -100,6 +90,17 @@ static void hugetlb_unshare_pmds(struct vm_area_struct *vma,
>>>               unsigned long start, unsigned long end);
>>>  static struct resv_map *vma_resv_map(struct vm_area_struct *vma);
>>>
>>> +static void hugetlb_free_folio(struct folio *folio)
>>> +{
>>> +#ifdef CONFIG_CMA
>>> +     int nid = folio_nid(folio);
>>> +
>>> +     if (cma_free_folio(hugetlb_cma[nid], folio))
>>> +             return;
>>> +#endif
>>> +     folio_put(folio);
>>> +}
>>> +
>>
>> It seems that we no longer use free_contig_range() to free gigantic
>> folios from alloc_contig_range().
>
> We switched to two pairs of extern (to the allocator) APIs in this patch:
>   folio_alloc_gigantic()
>   folio_put()
> and
>   cma_alloc_folio()
>   cma_free_folio()
>
>> Will it work? Or did I miss anything?
>
> alloc_contig_range and free_contig_range() also works with __GFP_COMP
> / large folios, but this pair is internal (to the allocator) and
> shouldn't be used directly except to implement external APIs like
> above.

Oh, I missed split_large_buddy() addition to free_one_page() in patch 1.
That handles >MAX_ORDER page freeing. It also makes sure the pageblocks
of a free cross-pageblock gigantic hugetlb page go back to the right
free lists when each pageblock might have different migratetypes,
since the page is freed at pageblock granularity.

The whole series looks good to me.

Acked-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi

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