[PATCH] mm: page_alloc: use the correct THP order for THP PCP

[PATCH] mm: page_alloc: use the correct THP order for THP PCP

[Baolin Wang]

Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
on the per-cpu lists") extends the PCP allocator to store THP pages, and
it determines whether to cache THP pags in PCP by comparing with pageblock_order.
But the pageblock_order is not always equal to THP order, it might also
be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.

Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
THP for PCP can fix this issue

Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 mm/page_alloc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1beb56f75319..915f4ef070da 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -506,7 +506,7 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
 {
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	if (order > PAGE_ALLOC_COSTLY_ORDER) {
-		VM_BUG_ON(order != pageblock_order);
+		VM_BUG_ON(order != HPAGE_PMD_ORDER);
 		return NR_LOWORDER_PCP_LISTS;
 	}
 #else
@@ -522,7 +522,7 @@ static inline int pindex_to_order(unsigned int pindex)
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	if (pindex == NR_LOWORDER_PCP_LISTS)
-		order = pageblock_order;
+		order = HPAGE_PMD_ORDER;
 #else
 	VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
 #endif
@@ -535,7 +535,7 @@ static inline bool pcp_allowed_order(unsigned int order)
 	if (order <= PAGE_ALLOC_COSTLY_ORDER)
 		return true;
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	if (order == pageblock_order)
+	if (order == HPAGE_PMD_ORDER)
 		return true;
 #endif
 	return false;
-- 
2.39.3

Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

[Vlastimil Babka]

On 4/3/24 3:47 PM, Baolin Wang wrote:
> Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
> on the per-cpu lists") extends the PCP allocator to store THP pages, and
> it determines whether to cache THP pags in PCP by comparing with pageblock_order.
> But the pageblock_order is not always equal to THP order, it might also
> be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.
> 
> Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
> THP for PCP can fix this issue
> 
> Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

IIUC this happens with CONFIG_HUGETLB_PAGE disabled because HUGETLBFS is
disabled? But THPs are still enabled? I think there might be more of THP
working suboptimally in that case with pageblock_order being larger
(MAX_PAGE_ORDER).

In other words, should be rather make pageblock_order itself defined as

 min_t(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)

in case with !CONFIG_HUGETLB_PAGE but THP enabled.

> ---
>  mm/page_alloc.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1beb56f75319..915f4ef070da 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -506,7 +506,7 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
>  {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	if (order > PAGE_ALLOC_COSTLY_ORDER) {
> -		VM_BUG_ON(order != pageblock_order);
> +		VM_BUG_ON(order != HPAGE_PMD_ORDER);
>  		return NR_LOWORDER_PCP_LISTS;
>  	}
>  #else
> @@ -522,7 +522,7 @@ static inline int pindex_to_order(unsigned int pindex)
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	if (pindex == NR_LOWORDER_PCP_LISTS)
> -		order = pageblock_order;
> +		order = HPAGE_PMD_ORDER;
>  #else
>  	VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
>  #endif
> @@ -535,7 +535,7 @@ static inline bool pcp_allowed_order(unsigned int order)
>  	if (order <= PAGE_ALLOC_COSTLY_ORDER)
>  		return true;
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	if (order == pageblock_order)
> +	if (order == HPAGE_PMD_ORDER)
>  		return true;
>  #endif
>  	return false;

Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

[Baolin Wang]

On 2024/4/4 18:03, Vlastimil Babka wrote:
> On 4/3/24 3:47 PM, Baolin Wang wrote:
>> Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
>> on the per-cpu lists") extends the PCP allocator to store THP pages, and
>> it determines whether to cache THP pags in PCP by comparing with pageblock_order.
>> But the pageblock_order is not always equal to THP order, it might also
>> be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.
>>
>> Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
>> THP for PCP can fix this issue
>>
>> Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> IIUC this happens with CONFIG_HUGETLB_PAGE disabled because HUGETLBFS is
> disabled? But THPs are still enabled? I think there might be more of THP

Right, and seems the Powerpc arch will set pageblock_order via 
set_pageblock_order() when the huge page sizes are variable (not sure if 
this is always equal to THP order).

Moreover, it still does not make sense to use pageblock_order to 
indicate a THP page, especially when we already have HPAGE_PMD_ORDER to 
represent THP.

> working suboptimally in that case with pageblock_order being larger
> (MAX_PAGE_ORDER).
> 
> In other words, should be rather make pageblock_order itself defined as
> 
>   min_t(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)
> 
> in case with !CONFIG_HUGETLB_PAGE but THP enabled.

Yes, this makes sense to me (I wonder why this wasn't done before?). I 
can create a seperate patch to do this, what do you think? Thanks.

Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

[Vlastimil Babka]

On 4/3/24 3:47 PM, Baolin Wang wrote:
> Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
> on the per-cpu lists") extends the PCP allocator to store THP pages, and
> it determines whether to cache THP pags in PCP by comparing with pageblock_order.
> But the pageblock_order is not always equal to THP order, it might also
> be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.
> 
> Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
> THP for PCP can fix this issue
> 
> Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_alloc.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1beb56f75319..915f4ef070da 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -506,7 +506,7 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
>  {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	if (order > PAGE_ALLOC_COSTLY_ORDER) {
> -		VM_BUG_ON(order != pageblock_order);
> +		VM_BUG_ON(order != HPAGE_PMD_ORDER);
>  		return NR_LOWORDER_PCP_LISTS;
>  	}
>  #else
> @@ -522,7 +522,7 @@ static inline int pindex_to_order(unsigned int pindex)
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	if (pindex == NR_LOWORDER_PCP_LISTS)
> -		order = pageblock_order;
> +		order = HPAGE_PMD_ORDER;
>  #else
>  	VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
>  #endif
> @@ -535,7 +535,7 @@ static inline bool pcp_allowed_order(unsigned int order)
>  	if (order <= PAGE_ALLOC_COSTLY_ORDER)
>  		return true;
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	if (order == pageblock_order)
> +	if (order == HPAGE_PMD_ORDER)
>  		return true;
>  #endif
>  	return false;

Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

[Vlastimil Babka]

On 4/4/24 2:19 PM, Baolin Wang wrote:
> 
> 
> On 2024/4/4 18:03, Vlastimil Babka wrote:
>> On 4/3/24 3:47 PM, Baolin Wang wrote:
>>> Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
>>> on the per-cpu lists") extends the PCP allocator to store THP pages, and
>>> it determines whether to cache THP pags in PCP by comparing with pageblock_order.
>>> But the pageblock_order is not always equal to THP order, it might also
>>> be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.
>>>
>>> Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
>>> THP for PCP can fix this issue
>>>
>>> Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> 
>> IIUC this happens with CONFIG_HUGETLB_PAGE disabled because HUGETLBFS is
>> disabled? But THPs are still enabled? I think there might be more of THP
> 
> Right, and seems the Powerpc arch will set pageblock_order via 
> set_pageblock_order() when the huge page sizes are variable (not sure if 
> this is always equal to THP order).
> 
> Moreover, it still does not make sense to use pageblock_order to 
> indicate a THP page, especially when we already have HPAGE_PMD_ORDER to 
> represent THP.
> 
>> working suboptimally in that case with pageblock_order being larger
>> (MAX_PAGE_ORDER).
>> 
>> In other words, should be rather make pageblock_order itself defined as
>> 
>>   min_t(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)
>> 
>> in case with !CONFIG_HUGETLB_PAGE but THP enabled.
> 
> Yes, this makes sense to me (I wonder why this wasn't done before?). I 
> can create a seperate patch to do this, what do you think? Thanks.

It probably wasn't anticipated that hugetlbfs would be disabled and THP
enabled. If you can create such patch, great!

Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

[Barry Song]

On Thu, Apr 4, 2024 at 2:47 AM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
> Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
> on the per-cpu lists") extends the PCP allocator to store THP pages, and
> it determines whether to cache THP pags in PCP by comparing with pageblock_order.
> But the pageblock_order is not always equal to THP order, it might also
> be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.
>
> Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
> THP for PCP can fix this issue
>
> Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

LGTM,
Reviewed-by: Barry Song <baohua@kernel.org>

In the context of using mTHP, perhaps there arises a need for PCP
allocation for frequently
requested mTHP orders. These orders typically exceed PAGE_ALLOC_COSTLY_ORDER
but are smaller than HPAGE_PMD_ORDER.

> ---
>  mm/page_alloc.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1beb56f75319..915f4ef070da 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -506,7 +506,7 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
>  {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>         if (order > PAGE_ALLOC_COSTLY_ORDER) {
> -               VM_BUG_ON(order != pageblock_order);
> +               VM_BUG_ON(order != HPAGE_PMD_ORDER);
>                 return NR_LOWORDER_PCP_LISTS;
>         }
>  #else
> @@ -522,7 +522,7 @@ static inline int pindex_to_order(unsigned int pindex)
>
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>         if (pindex == NR_LOWORDER_PCP_LISTS)
> -               order = pageblock_order;
> +               order = HPAGE_PMD_ORDER;
>  #else
>         VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
>  #endif
> @@ -535,7 +535,7 @@ static inline bool pcp_allowed_order(unsigned int order)
>         if (order <= PAGE_ALLOC_COSTLY_ORDER)
>                 return true;
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -       if (order == pageblock_order)
> +       if (order == HPAGE_PMD_ORDER)
>                 return true;
>  #endif
>         return false;
> --
> 2.39.3
>

Thanks
Barry

Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

[Baolin Wang]

On 2024/4/5 12:53, Barry Song wrote:
> On Thu, Apr 4, 2024 at 2:47 AM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>>
>> Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
>> on the per-cpu lists") extends the PCP allocator to store THP pages, and
>> it determines whether to cache THP pags in PCP by comparing with pageblock_order.
>> But the pageblock_order is not always equal to THP order, it might also
>> be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.
>>
>> Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
>> THP for PCP can fix this issue
>>
>> Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> LGTM,
> Reviewed-by: Barry Song <baohua@kernel.org>
> 
> In the context of using mTHP, perhaps there arises a need for PCP
> allocation for frequently
> requested mTHP orders. These orders typically exceed PAGE_ALLOC_COSTLY_ORDER
> but are smaller than HPAGE_PMD_ORDER.

Yes, I have also considered this, but haven't found some time to do more 
investigation and run some benchmarks.:) May be we can create a new 
thread to talk about this first.