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Message-ID: <84B216E2-238F-4BB3-8015-3BADC48F5F43@nvidia.com>
Date: Wed, 23 Oct 2024 12:55:12 -0400
From: Zi Yan <ziy@...dia.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: yuzhao@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
hannes@...xchg.org, riel@...riel.com, shakeel.butt@...ux.dev,
roman.gushchin@...ux.dev, david@...hat.com, npache@...hat.com,
baohua@...nel.org, ryan.roberts@....com, rppt@...nel.org,
willy@...radead.org, cerasuolodomenico@...il.com, ryncsn@...il.com,
corbet@....net, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
kernel-team@...a.com, Shuang Zhai <zhais@...gle.com>
Subject: Re: [PATCH v5 2/6] mm: remap unused subpages to shared zeropage when
splitting isolated thp
On 23 Oct 2024, at 12:50, Usama Arif wrote:
> On 23/10/2024 17:21, Zi Yan wrote:
>> On 30 Aug 2024, at 6:03, Usama Arif wrote:
>>
>>> From: Yu Zhao <yuzhao@...gle.com>
>>>
>>> Here being unused means containing only zeros and inaccessible to
>>> userspace. When splitting an isolated thp under reclaim or migration,
>>> the unused subpages can be mapped to the shared zeropage, hence saving
>>> memory. This is particularly helpful when the internal
>>> fragmentation of a thp is high, i.e. it has many untouched subpages.
>>>
>>> This is also a prerequisite for THP low utilization shrinker which will
>>> be introduced in later patches, where underutilized THPs are split, and
>>> the zero-filled pages are freed saving memory.
>>>
>>> Signed-off-by: Yu Zhao <yuzhao@...gle.com>
>>> Tested-by: Shuang Zhai <zhais@...gle.com>
>>> Signed-off-by: Usama Arif <usamaarif642@...il.com>
>>> ---
>>> include/linux/rmap.h | 7 ++++-
>>> mm/huge_memory.c | 8 ++---
>>> mm/migrate.c | 72 ++++++++++++++++++++++++++++++++++++++------
>>> mm/migrate_device.c | 4 +--
>>> 4 files changed, 75 insertions(+), 16 deletions(-)
>>>
>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>>> index 91b5935e8485..d5e93e44322e 100644
>>> --- a/include/linux/rmap.h
>>> +++ b/include/linux/rmap.h
>>> @@ -745,7 +745,12 @@ int folio_mkclean(struct folio *);
>>> int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,
>>> struct vm_area_struct *vma);
>>>
>>> -void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked);
>>> +enum rmp_flags {
>>> + RMP_LOCKED = 1 << 0,
>>> + RMP_USE_SHARED_ZEROPAGE = 1 << 1,
>>> +};
>>> +
>>> +void remove_migration_ptes(struct folio *src, struct folio *dst, int flags);
>>>
>>> /*
>>> * rmap_walk_control: To control rmap traversing for specific needs
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 0c48806ccb9a..af60684e7c70 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3020,7 +3020,7 @@ bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
>>> return false;
>>> }
>>>
>>> -static void remap_page(struct folio *folio, unsigned long nr)
>>> +static void remap_page(struct folio *folio, unsigned long nr, int flags)
>>> {
>>> int i = 0;
>>>
>>> @@ -3028,7 +3028,7 @@ static void remap_page(struct folio *folio, unsigned long nr)
>>> if (!folio_test_anon(folio))
>>> return;
>>> for (;;) {
>>> - remove_migration_ptes(folio, folio, true);
>>> + remove_migration_ptes(folio, folio, RMP_LOCKED | flags);
>>> i += folio_nr_pages(folio);
>>> if (i >= nr)
>>> break;
>>> @@ -3240,7 +3240,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>>
>>> if (nr_dropped)
>>> shmem_uncharge(folio->mapping->host, nr_dropped);
>>> - remap_page(folio, nr);
>>> + remap_page(folio, nr, PageAnon(head) ? RMP_USE_SHARED_ZEROPAGE : 0);
>>>
>>> /*
>>> * set page to its compound_head when split to non order-0 pages, so
>>> @@ -3542,7 +3542,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>> if (mapping)
>>> xas_unlock(&xas);
>>> local_irq_enable();
>>> - remap_page(folio, folio_nr_pages(folio));
>>> + remap_page(folio, folio_nr_pages(folio), 0);
>>> ret = -EAGAIN;
>>> }
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 6f9c62c746be..d039863e014b 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -204,13 +204,57 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
>>> return true;
>>> }
>>>
>>> +static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>>> + struct folio *folio,
>>> + unsigned long idx)
>>> +{
>>> + struct page *page = folio_page(folio, idx);
>>> + bool contains_data;
>>> + pte_t newpte;
>>> + void *addr;
>>> +
>>> + VM_BUG_ON_PAGE(PageCompound(page), page);
>>
>> This should be:
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index e950fd62607f..7ffdbe078aa7 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -206,7 +206,8 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>> pte_t newpte;
>> void *addr;
>>
>> - VM_BUG_ON_PAGE(PageCompound(page), page);
>> + if (PageCompound(page))
>> + return false;
>> VM_BUG_ON_PAGE(!PageAnon(page), page);
>> VM_BUG_ON_PAGE(!PageLocked(page), page);
>> VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);
>>
>> Otherwise, splitting anonymous large folios to non order-0 ones just
>> triggers this BUG_ON.
>>
>
> That makes sense, would you like to send the fix?
>
> Adding Yu Zhao to "To" incase he has any objections.
>
Sure, will do.
Best Regards,
Yan, Zi
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