[RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[akash.tyagi]

Currently, THP CMA pages share PCP lists with UNMOVABLE and RECLAIMABLE
pages. This may result in CMA THP pages being allocated from the PCP
list for other migratetypes. When this occurs, these pages may fail to
be isolated, leading to CMA allocation failures when drivers request
them.

This patch introduces a dedicated PCP list for the THP CMA migratetype,
ensuring that CMA THP pages are not mixed with other migratetypes and
remain available for CMA allocations as intended.

Signed-off-by: akash.tyagi <akash.tyagi@mediatek.com>
---
 include/linux/mmzone.h | 10 ++++++++--
 mm/page_alloc.c        |  5 +++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 283913d42d7b..dd93088ce851 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -696,11 +696,17 @@ enum zone_watermarks {
 
 /*
  * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER. Two additional lists
- * are added for THP. One PCP list is used by GPF_MOVABLE, and the other PCP list
- * is used by GFP_UNMOVABLE and GFP_RECLAIMABLE.
+ * are added for THP: one for GFP_MOVABLE, and one for GFP_UNMOVABLE and
+ * GFP_RECLAIMABLE. With CMA enabled, an extra THP PCP list is added for
+ * MIGRATE_CMA, allowing further distinction between MIGRATE_MOVABLE and
+ * MIGRATE_CMA for THP allocations.
  */
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#ifdef CONFIG_CMA
+#define NR_PCP_THP 3
+#else
 #define NR_PCP_THP 2
+#endif
 #else
 #define NR_PCP_THP 0
 #endif
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2ef3c07266b3..35f8041afbcc 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -519,6 +519,11 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
 	if (order > PAGE_ALLOC_COSTLY_ORDER) {
 		VM_BUG_ON(order != HPAGE_PMD_ORDER);
 
+#ifdef CONFIG_CMA
+		if (migratetype == MIGRATE_CMA)
+			return NR_LOWORDER_PCP_LISTS + 2;
+#endif
+
 		movable = migratetype == MIGRATE_MOVABLE;
 
 		return NR_LOWORDER_PCP_LISTS + movable;
-- 
2.18.0

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[David Hildenbrand]

On 24.07.25 09:53, akash.tyagi wrote:
> Currently, THP CMA pages share PCP lists with UNMOVABLE and RECLAIMABLE
> pages. This may result in CMA THP pages being allocated from the PCP
> list for other migratetypes. When this occurs, these pages may fail to
> be isolated, leading to CMA allocation failures when drivers request
> them.

Curious, did you run into that in practice?

Having MIGRATE_CMA pages allocated for unmovable allocations would 
indeed be broken.

But, MIGRATE_PCPTYPES does not include MIGRATE_CMA. So there is also no 
dedicated PCP list for VMA?


In free_unref_folios(), we have

"Non-isolated types over MIGRATE_PCPTYPES get added to the 
MIGRATE_MOVABLE pcp list."

	if (unlikely(migratetype >= MIGRATE_PCPTYPES))
		migratetype = MIGRATE_MOVABLE;

So ... shouldn't that safe us here as well for THPs?

> 
> This patch introduces a dedicated PCP list for the THP CMA migratetype,
> ensuring that CMA THP pages are not mixed with other migratetypes and
> remain available for CMA allocations as intended.
> 
> Signed-off-by: akash.tyagi <akash.tyagi@mediatek.com>
> ---
>   include/linux/mmzone.h | 10 ++++++++--
>   mm/page_alloc.c        |  5 +++++
>   2 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 283913d42d7b..dd93088ce851 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -696,11 +696,17 @@ enum zone_watermarks {
>   
>   /*
>    * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER. Two additional lists
> - * are added for THP. One PCP list is used by GPF_MOVABLE, and the other PCP list
> - * is used by GFP_UNMOVABLE and GFP_RECLAIMABLE.
> + * are added for THP: one for GFP_MOVABLE, and one for GFP_UNMOVABLE and
> + * GFP_RECLAIMABLE. With CMA enabled, an extra THP PCP list is added for
> + * MIGRATE_CMA, allowing further distinction between MIGRATE_MOVABLE and
> + * MIGRATE_CMA for THP allocations.
>    */
>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#ifdef CONFIG_CMA
> +#define NR_PCP_THP 3
> +#else
>   #define NR_PCP_THP 2
> +#endif
>   #else
>   #define NR_PCP_THP 0
>   #endif
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2ef3c07266b3..35f8041afbcc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -519,6 +519,11 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
>   	if (order > PAGE_ALLOC_COSTLY_ORDER) {
>   		VM_BUG_ON(order != HPAGE_PMD_ORDER);
>   
> +#ifdef CONFIG_CMA
> +		if (migratetype == MIGRATE_CMA)
> +			return NR_LOWORDER_PCP_LISTS + 2;
> +#endif
> +
>   		movable = migratetype == MIGRATE_MOVABLE;
>   
>   		return NR_LOWORDER_PCP_LISTS + movable;


-- 
Cheers,

David / dhildenb

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[akash.tyagi]

Hi David,

Thank you for your feedback.

We encountered this issue in the Android Common Kernel (version 6.12), which uses PCP lists for CMA pages.

page_owner trace-
Page allocated via order 9, mask 0x52dc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_ZERO), pid 1, tgid 1 (swapper/0), ts 1065952310 ns
PFN 0x23d200 type Unmovable Block 4585 type CMA Flags 0x4000000000000040(head|zone=1|kasantag=0x0)
 post_alloc_hook+0x228/0x230
 prep_new_page+0x28/0x148
 get_page_from_freelist+0x19d0/0x1a38
 __alloc_pages_noprof+0x1b0/0x440
 ___kmalloc_large_node+0xb4/0x1ec
 __kmalloc_large_node_noprof+0x2c/0xec
 __kmalloc_node_noprof+0x39c/0x548
 __kvmalloc_node_noprof+0xd8/0x18c
 nf_ct_alloc_hashtable+0x64/0x108
 nf_nat_init+0x3c/0xf8
 do_one_initcall+0x150/0x3c0
 do_initcall_level+0xa4/0x15c
 do_initcalls+0x70/0xc0
 do_basic_setup+0x1c/0x28
 kernel_init_freeable+0xcc/0x130
 kernel_init+0x20/0x1ac
 
This UNMOVABLE page was allocated from CMA, but it could not be migrated - so CMA alloc failed
At first, we fixed this by adding CMA THP pages to the movable THP PCP list.  
This fixed the issue of CMA pages being put in the wrong list, but now any movable allocation can use these CMA pages.

Later, we saw that a movable allocation used a CMA page and was pinned by __filemap_get_folio(). This page was pinned for too long, and eventually, CMA allocation failed

page_owner trace-
Page allocated via order 0, mask 0x140c48(GFP_NOFS|__GFP_COMP|__GFP_HARDWALL|__GFP_MOVABLE), pid 1198, tgid 1194 (ccci_mdinit), ts 17918751965 ns
PFN 0x207233 type Movable Block 4153 type CMA Flags 0x4020000000008224(referenced|lru|workingset|private|zone=1|kasantag=0x0)
 post_alloc_hook+0x23c/0x254
 prep_new_page+0x28/0x148
 get_page_from_freelist+0x19d8/0x1a40
 __alloc_pages_noprof+0x1a8/0x430
 __folio_alloc_noprof+0x14/0x5c
 __filemap_get_folio+0x1bc/0x430
 bdev_getblk+0xd4/0x294
 __read_extent_tree_block+0x6c/0x260
 ext4_find_extent+0x22c/0x3dc
 ext4_ext_map_blocks+0x88/0x173c
 ext4_map_query_blocks+0x54/0xe0
 ext4_map_blocks+0xf8/0x518
 _ext4_get_block+0x70/0x188
 ext4_get_block+0x18/0x24
 ext4_block_write_begin+0x154/0x62c
 ext4_write_begin+0x20c/0x630
Page has been migrated, last migrate reason: compaction
Charged to memcg /


Currently, free_unref_page treats CMA pages as movable. So, some MOVABLE allocations may use these CMA pages and pinned them. Later, when CMA needs these pages, these pages failed to migrate.
free_unref_page()/free_unref_folios
	migratetype = get_pfnblock_migratetype(page, pfn);
	if (unlikely(migratetype >= MIGRATE_PCPTYPES)) {
		migratetype = MIGRATE_MOVABLE;
	}


Best Regards,
Akash Tyagi

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[David Hildenbrand]

On 25.07.25 07:08, akash.tyagi wrote:
> Hi David,
> 
> Thank you for your feedback.
> 
> We encountered this issue in the Android Common Kernel (version 6.12), which uses PCP lists for CMA pages.
> 
> page_owner trace-
> Page allocated via order 9, mask 0x52dc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_ZERO), pid 1, tgid 1 (swapper/0), ts 1065952310 ns
> PFN 0x23d200 type Unmovable Block 4585 type CMA Flags 0x4000000000000040(head|zone=1|kasantag=0x0)
>   post_alloc_hook+0x228/0x230
>   prep_new_page+0x28/0x148
>   get_page_from_freelist+0x19d0/0x1a38
>   __alloc_pages_noprof+0x1b0/0x440
>   ___kmalloc_large_node+0xb4/0x1ec
>   __kmalloc_large_node_noprof+0x2c/0xec
>   __kmalloc_node_noprof+0x39c/0x548
>   __kvmalloc_node_noprof+0xd8/0x18c
>   nf_ct_alloc_hashtable+0x64/0x108
>   nf_nat_init+0x3c/0xf8
>   do_one_initcall+0x150/0x3c0
>   do_initcall_level+0xa4/0x15c
>   do_initcalls+0x70/0xc0
>   do_basic_setup+0x1c/0x28
>   kernel_init_freeable+0xcc/0x130
>   kernel_init+0x20/0x1ac
>   
> This UNMOVABLE page was allocated from CMA, but it could not be migrated - so CMA alloc failed
> At first, we fixed this by adding CMA THP pages to the movable THP PCP list.
> This fixed the issue of CMA pages being put in the wrong list, but now any movable allocation can use these CMA pages.
> 
> Later, we saw that a movable allocation used a CMA page and was pinned by __filemap_get_folio(). This page was pinned for too long, and eventually, CMA allocation failed
> 
> page_owner trace-
> Page allocated via order 0, mask 0x140c48(GFP_NOFS|__GFP_COMP|__GFP_HARDWALL|__GFP_MOVABLE), pid 1198, tgid 1194 (ccci_mdinit), ts 17918751965 ns
> PFN 0x207233 type Movable Block 4153 type CMA Flags 0x4020000000008224(referenced|lru|workingset|private|zone=1|kasantag=0x0)
>   post_alloc_hook+0x23c/0x254
>   prep_new_page+0x28/0x148
>   get_page_from_freelist+0x19d8/0x1a40
>   __alloc_pages_noprof+0x1a8/0x430
>   __folio_alloc_noprof+0x14/0x5c
>   __filemap_get_folio+0x1bc/0x430
>   bdev_getblk+0xd4/0x294
>   __read_extent_tree_block+0x6c/0x260
>   ext4_find_extent+0x22c/0x3dc
>   ext4_ext_map_blocks+0x88/0x173c
>   ext4_map_query_blocks+0x54/0xe0
>   ext4_map_blocks+0xf8/0x518
>   _ext4_get_block+0x70/0x188
>   ext4_get_block+0x18/0x24
>   ext4_block_write_begin+0x154/0x62c
>   ext4_write_begin+0x20c/0x630
> Page has been migrated, last migrate reason: compaction
> Charged to memcg /
> 
> 
> Currently, free_unref_page treats CMA pages as movable. So, some MOVABLE allocations may use these CMA pages and pinned them. Later, when CMA needs these pages, these pages failed to migrate.


MOVABLE allocations commonly fallback to CMA allocations, independent of pcp.

Long-term pinning is forbidden on MIGRATE_CMA pages. We had a bug recently fixed,
maybe you ran into that?

See

commit 517f496e1e61bd169d585dab4dd77e7147506322
Author: David Hildenbrand <david@redhat.com>
Date:   Wed Jun 11 15:13:14 2025 +0200

     mm/gup: revert "mm: gup: fix infinite loop within __get_longterm_locked"
     
     After commit 1aaf8c122918 ("mm: gup: fix infinite loop within
     __get_longterm_locked") we are able to longterm pin folios that are not
     supposed to get longterm pinned, simply because they temporarily have the
     LRU flag cleared (esp.  temporarily isolated).
     
     For example, two __get_longterm_locked() callers can race, or
     __get_longterm_locked() can race with anything else that temporarily
     isolates folios.

But there is this known problem that CMA can fail temporarily due to
short-term pinnings. See the "reliable CMA" work (don't remember the exact name).

So what you proposed in the patch at least does not apply I think.

-- 
Cheers,

David / dhildenb

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Zi Yan]

On 25 Jul 2025, at 3:04, David Hildenbrand wrote:

> On 25.07.25 07:08, akash.tyagi wrote:
>> Hi David,
>>
>> Thank you for your feedback.
>>
>> We encountered this issue in the Android Common Kernel (version 6.12), which uses PCP lists for CMA pages.
>>
>> page_owner trace-
>> Page allocated via order 9, mask 0x52dc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_ZERO), pid 1, tgid 1 (swapper/0), ts 1065952310 ns
>> PFN 0x23d200 type Unmovable Block 4585 type CMA Flags 0x4000000000000040(head|zone=1|kasantag=0x0)
>>   post_alloc_hook+0x228/0x230
>>   prep_new_page+0x28/0x148
>>   get_page_from_freelist+0x19d0/0x1a38
>>   __alloc_pages_noprof+0x1b0/0x440
>>   ___kmalloc_large_node+0xb4/0x1ec
>>   __kmalloc_large_node_noprof+0x2c/0xec
>>   __kmalloc_node_noprof+0x39c/0x548
>>   __kvmalloc_node_noprof+0xd8/0x18c
>>   nf_ct_alloc_hashtable+0x64/0x108
>>   nf_nat_init+0x3c/0xf8
>>   do_one_initcall+0x150/0x3c0
>>   do_initcall_level+0xa4/0x15c
>>   do_initcalls+0x70/0xc0
>>   do_basic_setup+0x1c/0x28
>>   kernel_init_freeable+0xcc/0x130
>>   kernel_init+0x20/0x1ac
>>  This UNMOVABLE page was allocated from CMA, but it could not be migrated - so CMA alloc failed
>> At first, we fixed this by adding CMA THP pages to the movable THP PCP list.
>> This fixed the issue of CMA pages being put in the wrong list, but now any movable allocation can use these CMA pages.
>>
>> Later, we saw that a movable allocation used a CMA page and was pinned by __filemap_get_folio(). This page was pinned for too long, and eventually, CMA allocation failed
>>
>> page_owner trace-
>> Page allocated via order 0, mask 0x140c48(GFP_NOFS|__GFP_COMP|__GFP_HARDWALL|__GFP_MOVABLE), pid 1198, tgid 1194 (ccci_mdinit), ts 17918751965 ns
>> PFN 0x207233 type Movable Block 4153 type CMA Flags 0x4020000000008224(referenced|lru|workingset|private|zone=1|kasantag=0x0)
>>   post_alloc_hook+0x23c/0x254
>>   prep_new_page+0x28/0x148
>>   get_page_from_freelist+0x19d8/0x1a40
>>   __alloc_pages_noprof+0x1a8/0x430
>>   __folio_alloc_noprof+0x14/0x5c
>>   __filemap_get_folio+0x1bc/0x430
>>   bdev_getblk+0xd4/0x294
>>   __read_extent_tree_block+0x6c/0x260
>>   ext4_find_extent+0x22c/0x3dc
>>   ext4_ext_map_blocks+0x88/0x173c
>>   ext4_map_query_blocks+0x54/0xe0
>>   ext4_map_blocks+0xf8/0x518
>>   _ext4_get_block+0x70/0x188
>>   ext4_get_block+0x18/0x24
>>   ext4_block_write_begin+0x154/0x62c
>>   ext4_write_begin+0x20c/0x630
>> Page has been migrated, last migrate reason: compaction
>> Charged to memcg /
>>
>>
>> Currently, free_unref_page treats CMA pages as movable. So, some MOVABLE allocations may use these CMA pages and pinned them. Later, when CMA needs these pages, these pages failed to migrate.
>
>
> MOVABLE allocations commonly fallback to CMA allocations, independent of pcp.
>
> Long-term pinning is forbidden on MIGRATE_CMA pages. We had a bug recently fixed,
> maybe you ran into that?
>
> See
>
> commit 517f496e1e61bd169d585dab4dd77e7147506322
> Author: David Hildenbrand <david@redhat.com>
> Date:   Wed Jun 11 15:13:14 2025 +0200
>
>     mm/gup: revert "mm: gup: fix infinite loop within __get_longterm_locked"
>         After commit 1aaf8c122918 ("mm: gup: fix infinite loop within
>     __get_longterm_locked") we are able to longterm pin folios that are not
>     supposed to get longterm pinned, simply because they temporarily have the
>     LRU flag cleared (esp.  temporarily isolated).
>         For example, two __get_longterm_locked() callers can race, or
>     __get_longterm_locked() can race with anything else that temporarily
>     isolates folios.
>
> But there is this known problem that CMA can fail temporarily due to
> short-term pinnings. See the "reliable CMA" work (don't remember the exact name).
I think you mean Guaranteed CMA[1].

[1] https://lore.kernel.org/linux-mm/CAJuCfpEWVEqsivd7oTvp4foEho_HaD1XNP8KTeKWzG_X2skfGQ@mail.gmail.com/

Best Regards,
Yan, Zi

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[akash.tyagi]

On Fri, 25 Jul 2025 at 10:27, Zi Yan <ziy@nvidia.com> wrote:
> But there is this known problem that CMA can fail temporarily due to
> short-term pinnings. See the "reliable CMA" work (don't remember the exact name).
> I think you mean Guaranteed CMA[1].
>
> [1] https://lore.kernel.org/linux-mm/CAJuCfpEWVEqsivd7oTvp4foEho_HaD1XNP8KTeKWzG_X2skfGQ@mail.gmail.com/
>
> Best Regards,
> Yan, Zi


Hi,

Yes, the issue I described is actually related to Guaranteed CMA[1].

I have rewritten our problem statement to address concerns more specifically related to the Android common kernels.

Problem statement:
Android Common kernels usually have an out-of-tree patch to prevent file-backed page allocated from CMA.
It allows some allocations which have lower chance of being pinned to use CMA to improve CMA utilization controlled by a flag __GFP_CMA.
https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/


Additionally, android kernels create cma pcp list for pages less than PAGE_ALLOC_COSTLY_ORDER, but not for THP pages.

This way we noticed some UNMOVABLE allocation also occured from CMA via pcplist as for THP there is pcp either for movable or UNMOVABLE/RECLAIMABLE but not for CMA.
so we moved CMA pages to movable for THP.

		movable = migratetype == MIGRATE_MOVABLE;
#ifdef CONFIG_CMA
		movable |= migratetype == MIGRATE_CMA;
#endif

		return NR_LOWORDER_PCP_LISTS + movable;
		
		

Now, this way we fixes the issue where CMA pages wrongly placed in UNMOVABLE PCP. 
But now if there is a GFP_MOVABLE allocation (even without __GFP_CMA) (which android kernel maintains out-of-tree patch as share above), might pull that CMA page from the PCP.
This breaks the intended use case of the above patch, which is to allow only allocations that use the __GFP_CMA flag.

To address this, we have proposed introducing a CMA PCP for THP pages as well.

I would appreciate your review and feedback on whether this is a feasible approach for adding a new PCP in Android common kernel perspective becuase Because having many MIGRATE_CMA pages 
in the THP lists could cause several performance issues.


Best Regards,
Akash Tyagi

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[David Hildenbrand]

On 29.07.25 14:30, akash.tyagi wrote:
> On Fri, 25 Jul 2025 at 10:27, Zi Yan <ziy@nvidia.com> wrote:
>> But there is this known problem that CMA can fail temporarily due to
>> short-term pinnings. See the "reliable CMA" work (don't remember the exact name).
>> I think you mean Guaranteed CMA[1].
>>
>> [1] https://lore.kernel.org/linux-mm/CAJuCfpEWVEqsivd7oTvp4foEho_HaD1XNP8KTeKWzG_X2skfGQ@mail.gmail.com/
>>
>> Best Regards,
>> Yan, Zi
> 
> 
> Hi,
> 
> Yes, the issue I described is actually related to Guaranteed CMA[1].
> 
> I have rewritten our problem statement to address concerns more specifically related to the Android common kernels.
> 
> Problem statement:
> Android Common kernels usually have an out-of-tree patch to prevent file-backed page allocated from CMA.
> It allows some allocations which have lower chance of being pinned to use CMA to improve CMA utilization controlled by a flag __GFP_CMA.
> https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/
> 
> 
> Additionally, android kernels create cma pcp list for pages less than PAGE_ALLOC_COSTLY_ORDER, but not for THP pages.

I'm afraid you have to fix this in the android kernels.

-- 
Cheers,

David / dhildenb

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Matthew Wilcox]

On Tue, Jul 29, 2025 at 06:00:28PM +0530, akash.tyagi wrote:
> Additionally, android kernels create cma pcp list for pages less than PAGE_ALLOC_COSTLY_ORDER, but not for THP pages.

Why bother?  If it's a CMA allocaation, just free it back to CMA
straight away.

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Juan Yescas]

Hi David/Zi,

Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?

There are many devices that need fast allocation of MIGRATE_CMA pages,
and they have to get them from the buddy allocator, which is a bit
slower in comparison to the PCP lists.

We also have cases where the MIGRATE_CMA memory requirements are big.
For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
These cases would benefit if we have THPs for CMAs.

Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?

Thanks

[RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Juan Yescas]

Hi David/Zi,

Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?

There are many devices that need fast allocation of MIGRATE_CMA pages,
and they have to get them from the buddy allocator, which is a bit
slower in comparison to the PCP lists.

We also have cases where the MIGRATE_CMA memory requirements are big.
For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
These cases would benefit if we have THPs for CMAs.

Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?

Thanks

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[David Hildenbrand]

On 04.08.25 20:20, Juan Yescas wrote:
> Hi David/Zi,
> 
> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
> 
> There are many devices that need fast allocation of MIGRATE_CMA pages,
> and they have to get them from the buddy allocator, which is a bit
> slower in comparison to the PCP lists.
> 
> We also have cases where the MIGRATE_CMA memory requirements are big.
> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
> These cases would benefit if we have THPs for CMAs.
> 
> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?

Remember how CMA memory is used:

The owner allocates it through cma_alloc() and friends, where the CMA 
allocator will try allocating *specific physical memory regions* using 
alloc_contig_range(). It doesn't just go ahead and pick a random CMA 
page from the buddy (or PCP) lists. Doesn't work (just imagine having 
different CMA areas etc).

Anybody else is free to use CMA pages for MOVABLE allocations. So we 
treat them as being MOVABLE on the PCP.

Having a separate CMA PCP list doesn't solve or speedup anything, really.

I still have no clue what this patch here tried to solve: it doesn't 
make any sense.

-- 
Cheers,

David / dhildenb

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Zi Yan]

On 4 Aug 2025, at 14:49, David Hildenbrand wrote:

> On 04.08.25 20:20, Juan Yescas wrote:
>> Hi David/Zi,
>>
>> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
>>
>> There are many devices that need fast allocation of MIGRATE_CMA pages,
>> and they have to get them from the buddy allocator, which is a bit
>> slower in comparison to the PCP lists.
>>
>> We also have cases where the MIGRATE_CMA memory requirements are big.
>> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
>> These cases would benefit if we have THPs for CMAs.
>>
>> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
>
> Remember how CMA memory is used:
>
> The owner allocates it through cma_alloc() and friends, where the CMA allocator will try allocating *specific physical memory regions* using alloc_contig_range(). It doesn't just go ahead and pick a random CMA page from the buddy (or PCP) lists. Doesn't work (just imagine having different CMA areas etc).

Yeah, unless some code is relying on gfp_to_alloc_flags_cma() to get ALLOC_CMA
to try to get CMA pages from buddy.

>
> Anybody else is free to use CMA pages for MOVABLE allocations. So we treat them as being MOVABLE on the PCP.
>
> Having a separate CMA PCP list doesn't solve or speedup anything, really.

It can be slower when small CMA pages are on PCP lists and large CMA pages
cannot be allocated, one needs to drain PCP lists. This assumes the code is
trying to get CMA pages from buddy, which is not how CMA memory is designed
to be used like David mentioned above.

>
> I still have no clue what this patch here tried to solve: it doesn't make any sense.


Best Regards,
Yan, Zi

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[David Hildenbrand]

On 04.08.25 21:00, Zi Yan wrote:
> On 4 Aug 2025, at 14:49, David Hildenbrand wrote:
> 
>> On 04.08.25 20:20, Juan Yescas wrote:
>>> Hi David/Zi,
>>>
>>> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
>>>
>>> There are many devices that need fast allocation of MIGRATE_CMA pages,
>>> and they have to get them from the buddy allocator, which is a bit
>>> slower in comparison to the PCP lists.
>>>
>>> We also have cases where the MIGRATE_CMA memory requirements are big.
>>> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
>>> These cases would benefit if we have THPs for CMAs.
>>>
>>> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
>>
>> Remember how CMA memory is used:
>>
>> The owner allocates it through cma_alloc() and friends, where the CMA allocator will try allocating *specific physical memory regions* using alloc_contig_range(). It doesn't just go ahead and pick a random CMA page from the buddy (or PCP) lists. Doesn't work (just imagine having different CMA areas etc).
> 
> Yeah, unless some code is relying on gfp_to_alloc_flags_cma() to get ALLOC_CMA
> to try to get CMA pages from buddy.

Right, but that's just for internal purposes IIUC, to grab pages from 
the CMA lists when serving movable allocations.

> 
>>
>> Anybody else is free to use CMA pages for MOVABLE allocations. So we treat them as being MOVABLE on the PCP.
>>
>> Having a separate CMA PCP list doesn't solve or speedup anything, really.
> 
> It can be slower when small CMA pages are on PCP lists and large CMA pages
> cannot be allocated, one needs to drain PCP lists. This assumes the code is
> trying to get CMA pages from buddy, which is not how CMA memory is designed
> to be used like David mentioned above.

Right. And alloc_contig_range_noprof() already does a 
drain_all_pages(cc.zone).

-- 
Cheers,

David / dhildenb

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Juan Yescas]

On Mon, Aug 4, 2025 at 11:50 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 04.08.25 20:20, Juan Yescas wrote:
> > Hi David/Zi,
> >
> > Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
> >
> > There are many devices that need fast allocation of MIGRATE_CMA pages,
> > and they have to get them from the buddy allocator, which is a bit
> > slower in comparison to the PCP lists.
> >
> > We also have cases where the MIGRATE_CMA memory requirements are big.
> > For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
> > These cases would benefit if we have THPs for CMAs.
> >
> > Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
>
> Remember how CMA memory is used:
>
> The owner allocates it through cma_alloc() and friends, where the CMA
> allocator will try allocating *specific physical memory regions* using
> alloc_contig_range(). It doesn't just go ahead and pick a random CMA
> page from the buddy (or PCP) lists. Doesn't work (just imagine having
> different CMA areas etc).
>
> Anybody else is free to use CMA pages for MOVABLE allocations. So we
> treat them as being MOVABLE on the PCP.
>
> Having a separate CMA PCP list doesn't solve or speedup anything, really.
>

Thanks David for the quick overview.

> I still have no clue what this patch here tried to solve: it doesn't
> make any sense.
>

The story started with this out of tree patch that is part of Android.

https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u

This patch introduced the __GFP_CMA flag that allocates pages from
MIGRATE_MOVABLE
or MIGRATE_CMA. What it happens then, it is that the MIGRATE_MOVABLE
pages in the
PCP lists were consumed pretty fast. To solve this issue, the PCP
MIGRATE_CMA list was added.
This list is initialized by rmqueue_bulk() when it is empty. That's
how we end up with the PCP MIGRATE_CMA list
in Android. In addition to this, the THP list for MIGRATE_MOVABLE was
allowed to contain
MIGRATE_CMA pages. This is causing THP MIGRATE_CMA pages to be used
for THP MIGRATE_MOVABLE
making later allocations from THP MIGRATE_CMA to fail.

These workarounds are mainly because we need to solve this issue upstream:

- When devices reserve big blocks of MIGRATE_CMA pages, the
underutilized MIGRATE_CMA
can fall back to MIGRATE_MOVABLE and these pages can be pinned, so if
we require MIGRATE_CMA
pages, the allocations might fail.

I remember that you presented the problem in LPC. Were you able to
make some progress on that?

Thanks
Juan





> --
> Cheers,
>
> David / dhildenb
>

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Juan Yescas]

On Mon, Aug 4, 2025 at 12:00 PM Zi Yan <ziy@nvidia.com> wrote:
>
> On 4 Aug 2025, at 14:49, David Hildenbrand wrote:
>
> > On 04.08.25 20:20, Juan Yescas wrote:
> >> Hi David/Zi,
> >>
> >> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
> >>
> >> There are many devices that need fast allocation of MIGRATE_CMA pages,
> >> and they have to get them from the buddy allocator, which is a bit
> >> slower in comparison to the PCP lists.
> >>
> >> We also have cases where the MIGRATE_CMA memory requirements are big.
> >> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
> >> These cases would benefit if we have THPs for CMAs.
> >>
> >> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
> >
> > Remember how CMA memory is used:
> >
> > The owner allocates it through cma_alloc() and friends, where the CMA allocator will try allocating *specific physical memory regions* using alloc_contig_range(). It doesn't just go ahead and pick a random CMA page from the buddy (or PCP) lists. Doesn't work (just imagine having different CMA areas etc).
>
> Yeah, unless some code is relying on gfp_to_alloc_flags_cma() to get ALLOC_CMA
> to try to get CMA pages from buddy.
>
> >
> > Anybody else is free to use CMA pages for MOVABLE allocations. So we treat them as being MOVABLE on the PCP.
> >
> > Having a separate CMA PCP list doesn't solve or speedup anything, really.
>
> It can be slower when small CMA pages are on PCP lists and large CMA pages
> cannot be allocated, one needs to drain PCP lists. This assumes the code is
> trying to get CMA pages from buddy, which is not how CMA memory is designed
> to be used like David mentioned above.
>

Thanks Zi for confirming!

> >
> > I still have no clue what this patch here tried to solve: it doesn't make any sense.
>
>
> Best Regards,
> Yan, Zi

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Vlastimil Babka]

On 8/5/25 3:22 AM, Juan Yescas wrote:
> On Mon, Aug 4, 2025 at 11:50 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 04.08.25 20:20, Juan Yescas wrote:
>>> Hi David/Zi,
>>>
>>> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
>>>
>>> There are many devices that need fast allocation of MIGRATE_CMA pages,
>>> and they have to get them from the buddy allocator, which is a bit
>>> slower in comparison to the PCP lists.
>>>
>>> We also have cases where the MIGRATE_CMA memory requirements are big.
>>> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
>>> These cases would benefit if we have THPs for CMAs.
>>>
>>> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
>>
>> Remember how CMA memory is used:
>>
>> The owner allocates it through cma_alloc() and friends, where the CMA
>> allocator will try allocating *specific physical memory regions* using
>> alloc_contig_range(). It doesn't just go ahead and pick a random CMA
>> page from the buddy (or PCP) lists. Doesn't work (just imagine having
>> different CMA areas etc).
>>
>> Anybody else is free to use CMA pages for MOVABLE allocations. So we
>> treat them as being MOVABLE on the PCP.
>>
>> Having a separate CMA PCP list doesn't solve or speedup anything, really.
>>
> 
> Thanks David for the quick overview.
> 
>> I still have no clue what this patch here tried to solve: it doesn't
>> make any sense.
>>
> 
> The story started with this out of tree patch that is part of Android.
> 
> https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u
> 
> This patch introduced the __GFP_CMA flag that allocates pages from
> MIGRATE_MOVABLE
> or MIGRATE_CMA.

What kinds of allocations would then use __GFP_CMA? (let me try guess
one - zswap backend?)

> What it happens then, it is that the MIGRATE_MOVABLE
> pages in the
> PCP lists were consumed pretty fast. To solve this issue, the PCP
> MIGRATE_CMA list was added.
> This list is initialized by rmqueue_bulk() when it is empty. That's
> how we end up with the PCP MIGRATE_CMA list
> in Android. In addition to this, the THP list for MIGRATE_MOVABLE was
> allowed to contain
> MIGRATE_CMA pages. This is causing THP MIGRATE_CMA pages to be used
> for THP MIGRATE_MOVABLE
> making later allocations from THP MIGRATE_CMA to fail.

If you don't want THP's to use the large (THP-sized) MIGRATE_CMA pages,
what kind of such large allocations would be ok to use MIGRATE_CMA then?
> These workarounds are mainly because we need to solve this issue upstream:
> 
> - When devices reserve big blocks of MIGRATE_CMA pages, the
> underutilized MIGRATE_CMA
> can fall back to MIGRATE_MOVABLE and these pages can be pinned, so if
> we require MIGRATE_CMA
> pages, the allocations might fail.
> 
> I remember that you presented the problem in LPC. Were you able to
> make some progress on that?
> 
> Thanks
> Juan
> 
> 
> 
> 
> 
>> --
>> Cheers,
>>
>> David / dhildenb
>>

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[David Hildenbrand]

On 05.08.25 03:22, Juan Yescas wrote:
> On Mon, Aug 4, 2025 at 11:50 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 04.08.25 20:20, Juan Yescas wrote:
>>> Hi David/Zi,
>>>
>>> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
>>>
>>> There are many devices that need fast allocation of MIGRATE_CMA pages,
>>> and they have to get them from the buddy allocator, which is a bit
>>> slower in comparison to the PCP lists.
>>>
>>> We also have cases where the MIGRATE_CMA memory requirements are big.
>>> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
>>> These cases would benefit if we have THPs for CMAs.
>>>
>>> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
>>
>> Remember how CMA memory is used:
>>
>> The owner allocates it through cma_alloc() and friends, where the CMA
>> allocator will try allocating *specific physical memory regions* using
>> alloc_contig_range(). It doesn't just go ahead and pick a random CMA
>> page from the buddy (or PCP) lists. Doesn't work (just imagine having
>> different CMA areas etc).
>>
>> Anybody else is free to use CMA pages for MOVABLE allocations. So we
>> treat them as being MOVABLE on the PCP.
>>
>> Having a separate CMA PCP list doesn't solve or speedup anything, really.
>>
> 
> Thanks David for the quick overview.
> 
>> I still have no clue what this patch here tried to solve: it doesn't
>> make any sense.
>>
> 
> The story started with this out of tree patch that is part of Android.
> 
> https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u
> 
> This patch introduced the __GFP_CMA flag that allocates pages from
> MIGRATE_MOVABLE
> or MIGRATE_CMA. What it happens then, it is that the MIGRATE_MOVABLE
> pages in the
> PCP lists were consumed pretty fast. To solve this issue, the PCP
> MIGRATE_CMA list was added.
> This list is initialized by rmqueue_bulk() when it is empty. That's
> how we end up with the PCP MIGRATE_CMA list
> in Android. In addition to this, the THP list for MIGRATE_MOVABLE was
> allowed to contain
> MIGRATE_CMA pages. This is causing THP MIGRATE_CMA pages to be used
> for THP MIGRATE_MOVABLE
> making later allocations from THP MIGRATE_CMA to fail.

Okay, so this patch here really is not suitable for the upstream kernel 
as is. It's purely targeted at the OOT Android patch.

> 
> These workarounds are mainly because we need to solve this issue upstream:
> 
> - When devices reserve big blocks of MIGRATE_CMA pages, the
> underutilized MIGRATE_CMA
> can fall back to MIGRATE_MOVABLE and these pages can be pinned, so if
> we require MIGRATE_CMA
> pages, the allocations might fail.
> 
> I remember that you presented the problem in LPC. Were you able to
> make some progress on that?

There is the problem of CMA pages getting allocated by someone for a 
MOVABLE allocation, to then short-term pin it for DMA. Long-term 
pinnings are disallowed (we just recently fixed a bug where we 
accidentally allowed it).

One concern is that a steady stream of short-term pinnings can turn such 
pages unmovable. We discussed ideas on how to handle that, but there is 
no solution upstream yet.

-- 
Cheers,

David / dhildenb

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Juan Yescas]

On Tue, Aug 5, 2025 at 2:52 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 8/5/25 3:22 AM, Juan Yescas wrote:
> > On Mon, Aug 4, 2025 at 11:50 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 04.08.25 20:20, Juan Yescas wrote:
> >>> Hi David/Zi,
> >>>
> >>> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
> >>>
> >>> There are many devices that need fast allocation of MIGRATE_CMA pages,
> >>> and they have to get them from the buddy allocator, which is a bit
> >>> slower in comparison to the PCP lists.
> >>>
> >>> We also have cases where the MIGRATE_CMA memory requirements are big.
> >>> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
> >>> These cases would benefit if we have THPs for CMAs.
> >>>
> >>> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
> >>
> >> Remember how CMA memory is used:
> >>
> >> The owner allocates it through cma_alloc() and friends, where the CMA
> >> allocator will try allocating *specific physical memory regions* using
> >> alloc_contig_range(). It doesn't just go ahead and pick a random CMA
> >> page from the buddy (or PCP) lists. Doesn't work (just imagine having
> >> different CMA areas etc).
> >>
> >> Anybody else is free to use CMA pages for MOVABLE allocations. So we
> >> treat them as being MOVABLE on the PCP.
> >>
> >> Having a separate CMA PCP list doesn't solve or speedup anything, really.
> >>
> >
> > Thanks David for the quick overview.
> >
> >> I still have no clue what this patch here tried to solve: it doesn't
> >> make any sense.
> >>
> >
> > The story started with this out of tree patch that is part of Android.
> >
> > https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u
> >
> > This patch introduced the __GFP_CMA flag that allocates pages from
> > MIGRATE_MOVABLE
> > or MIGRATE_CMA.
>
> What kinds of allocations would then use __GFP_CMA? (let me try guess
> one - zswap backend?)
>
> > What it happens then, it is that the MIGRATE_MOVABLE
> > pages in the
> > PCP lists were consumed pretty fast. To solve this issue, the PCP
> > MIGRATE_CMA list was added.
> > This list is initialized by rmqueue_bulk() when it is empty. That's
> > how we end up with the PCP MIGRATE_CMA list
> > in Android. In addition to this, the THP list for MIGRATE_MOVABLE was
> > allowed to contain
> > MIGRATE_CMA pages. This is causing THP MIGRATE_CMA pages to be used
> > for THP MIGRATE_MOVABLE
> > making later allocations from THP MIGRATE_CMA to fail.
>
> If you don't want THP's to use the large (THP-sized) MIGRATE_CMA pages,
> what kind of such large allocations would be ok to use MIGRATE_CMA then?

The large THP MIGRATE_CMA allocs are used by GPUs or devices that require
large chunks of memory (have many cameras for example).


> > These workarounds are mainly because we need to solve this issue upstream:
> >
> > - When devices reserve big blocks of MIGRATE_CMA pages, the
> > underutilized MIGRATE_CMA
> > can fall back to MIGRATE_MOVABLE and these pages can be pinned, so if
> > we require MIGRATE_CMA
> > pages, the allocations might fail.
> >
> > I remember that you presented the problem in LPC. Were you able to
> > make some progress on that?
> >
> > Thanks
> > Juan
> >
> >
> >
> >
> >
> >> --
> >> Cheers,
> >>
> >> David / dhildenb
> >>
>

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Juan Yescas]

On Tue, Aug 5, 2025 at 2:58 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 05.08.25 03:22, Juan Yescas wrote:
> > On Mon, Aug 4, 2025 at 11:50 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 04.08.25 20:20, Juan Yescas wrote:
> >>> Hi David/Zi,
> >>>
> >>> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
> >>>
> >>> There are many devices that need fast allocation of MIGRATE_CMA pages,
> >>> and they have to get them from the buddy allocator, which is a bit
> >>> slower in comparison to the PCP lists.
> >>>
> >>> We also have cases where the MIGRATE_CMA memory requirements are big.
> >>> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
> >>> These cases would benefit if we have THPs for CMAs.
> >>>
> >>> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
> >>
> >> Remember how CMA memory is used:
> >>
> >> The owner allocates it through cma_alloc() and friends, where the CMA
> >> allocator will try allocating *specific physical memory regions* using
> >> alloc_contig_range(). It doesn't just go ahead and pick a random CMA
> >> page from the buddy (or PCP) lists. Doesn't work (just imagine having
> >> different CMA areas etc).
> >>
> >> Anybody else is free to use CMA pages for MOVABLE allocations. So we
> >> treat them as being MOVABLE on the PCP.
> >>
> >> Having a separate CMA PCP list doesn't solve or speedup anything, really.
> >>
> >
> > Thanks David for the quick overview.
> >
> >> I still have no clue what this patch here tried to solve: it doesn't
> >> make any sense.
> >>
> >
> > The story started with this out of tree patch that is part of Android.
> >
> > https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u
> >
> > This patch introduced the __GFP_CMA flag that allocates pages from
> > MIGRATE_MOVABLE
> > or MIGRATE_CMA. What it happens then, it is that the MIGRATE_MOVABLE
> > pages in the
> > PCP lists were consumed pretty fast. To solve this issue, the PCP
> > MIGRATE_CMA list was added.
> > This list is initialized by rmqueue_bulk() when it is empty. That's
> > how we end up with the PCP MIGRATE_CMA list
> > in Android. In addition to this, the THP list for MIGRATE_MOVABLE was
> > allowed to contain
> > MIGRATE_CMA pages. This is causing THP MIGRATE_CMA pages to be used
> > for THP MIGRATE_MOVABLE
> > making later allocations from THP MIGRATE_CMA to fail.
>
> Okay, so this patch here really is not suitable for the upstream kernel
> as is. It's purely targeted at the OOT Android patch.
>
Right, it is a temporary solution for the pinned MIGRATE_CMA pages.

> >
> > These workarounds are mainly because we need to solve this issue upstream:
> >
> > - When devices reserve big blocks of MIGRATE_CMA pages, the
> > underutilized MIGRATE_CMA
> > can fall back to MIGRATE_MOVABLE and these pages can be pinned, so if
> > we require MIGRATE_CMA
> > pages, the allocations might fail.
> >
> > I remember that you presented the problem in LPC. Were you able to
> > make some progress on that?
>
> There is the problem of CMA pages getting allocated by someone for a
> MOVABLE allocation, to then short-term pin it for DMA. Long-term
> pinnings are disallowed (we just recently fixed a bug where we
> accidentally allowed it).
>
Nice, it is great the issue got caught and fixed upstream :)

> One concern is that a steady stream of short-term pinnings can turn such
> pages unmovable. We discussed ideas on how to handle that, but there is
> no solution upstream yet.

Are there any plans to continue the discussion? Is it in the priority
list? Maybe
a topic we can discuss in LPC Japan?

Thanks
Juan

>
> --
> Cheers,
>
> David / dhildenb
>

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Juan Yescas]

On Tue, Aug 5, 2025 at 2:52 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 8/5/25 3:22 AM, Juan Yescas wrote:
> > On Mon, Aug 4, 2025 at 11:50 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 04.08.25 20:20, Juan Yescas wrote:
> >>> Hi David/Zi,
> >>>
> >>> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
> >>>
> >>> There are many devices that need fast allocation of MIGRATE_CMA pages,
> >>> and they have to get them from the buddy allocator, which is a bit
> >>> slower in comparison to the PCP lists.
> >>>
> >>> We also have cases where the MIGRATE_CMA memory requirements are big.
> >>> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
> >>> These cases would benefit if we have THPs for CMAs.
> >>>
> >>> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
> >>
> >> Remember how CMA memory is used:
> >>
> >> The owner allocates it through cma_alloc() and friends, where the CMA
> >> allocator will try allocating *specific physical memory regions* using
> >> alloc_contig_range(). It doesn't just go ahead and pick a random CMA
> >> page from the buddy (or PCP) lists. Doesn't work (just imagine having
> >> different CMA areas etc).
> >>
> >> Anybody else is free to use CMA pages for MOVABLE allocations. So we
> >> treat them as being MOVABLE on the PCP.
> >>
> >> Having a separate CMA PCP list doesn't solve or speedup anything, really.
> >>
> >
> > Thanks David for the quick overview.
> >
> >> I still have no clue what this patch here tried to solve: it doesn't
> >> make any sense.
> >>
> >
> > The story started with this out of tree patch that is part of Android.
> >
> > https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u
> >
> > This patch introduced the __GFP_CMA flag that allocates pages from
> > MIGRATE_MOVABLE
> > or MIGRATE_CMA.
>
> What kinds of allocations would then use __GFP_CMA?

The __GFP_CMA allocations are used to allocate userspace anonymous memory. This
was done initially in the alloc_zeroed_user_highpage_movable()
function, now it is done
in vma_alloc_zeroed_movable_folio().

> (let me try guess
> one - zswap backend?)

Yep, the __GFP_CMA pages are also used in zram block driver.

Thanks
Juan

> > What it happens then, it is that the MIGRATE_MOVABLE
> > pages in the
> > PCP lists were consumed pretty fast. To solve this issue, the PCP
> > MIGRATE_CMA list was added.
> > This list is initialized by rmqueue_bulk() when it is empty. That's
> > how we end up with the PCP MIGRATE_CMA list
> > in Android. In addition to this, the THP list for MIGRATE_MOVABLE was
> > allowed to contain
> > MIGRATE_CMA pages. This is causing THP MIGRATE_CMA pages to be used
> > for THP MIGRATE_MOVABLE
> > making later allocations from THP MIGRATE_CMA to fail.
>
> If you don't want THP's to use the large (THP-sized) MIGRATE_CMA pages,
> what kind of such large allocations would be ok to use MIGRATE_CMA then?
> > These workarounds are mainly because we need to solve this issue upstream:
> >
> > - When devices reserve big blocks of MIGRATE_CMA pages, the
> > underutilized MIGRATE_CMA
> > can fall back to MIGRATE_MOVABLE and these pages can be pinned, so if
> > we require MIGRATE_CMA
> > pages, the allocations might fail.
> >
> > I remember that you presented the problem in LPC. Were you able to
> > make some progress on that?
> >
> > Thanks
> > Juan
> >
> >
> >
> >
> >
> >> --
> >> Cheers,
> >>
> >> David / dhildenb
> >>
>

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[David Hildenbrand]

On 05.08.25 18:57, Juan Yescas wrote:
> On Tue, Aug 5, 2025 at 2:58 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 05.08.25 03:22, Juan Yescas wrote:
>>> On Mon, Aug 4, 2025 at 11:50 AM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 04.08.25 20:20, Juan Yescas wrote:
>>>>> Hi David/Zi,
>>>>>
>>>>> Is there any reason why the MIGRATE_CMA pages are not in the PCP lists?
>>>>>
>>>>> There are many devices that need fast allocation of MIGRATE_CMA pages,
>>>>> and they have to get them from the buddy allocator, which is a bit
>>>>> slower in comparison to the PCP lists.
>>>>>
>>>>> We also have cases where the MIGRATE_CMA memory requirements are big.
>>>>> For example, GPUs need MIGRATE_CMA memory in the ranges of 30MiB to 500MiBs.
>>>>> These cases would benefit if we have THPs for CMAs.
>>>>>
>>>>> Could we add the support for MIGRATE_CMA pages on the PCP and THP lists?
>>>>
>>>> Remember how CMA memory is used:
>>>>
>>>> The owner allocates it through cma_alloc() and friends, where the CMA
>>>> allocator will try allocating *specific physical memory regions* using
>>>> alloc_contig_range(). It doesn't just go ahead and pick a random CMA
>>>> page from the buddy (or PCP) lists. Doesn't work (just imagine having
>>>> different CMA areas etc).
>>>>
>>>> Anybody else is free to use CMA pages for MOVABLE allocations. So we
>>>> treat them as being MOVABLE on the PCP.
>>>>
>>>> Having a separate CMA PCP list doesn't solve or speedup anything, really.
>>>>
>>>
>>> Thanks David for the quick overview.
>>>
>>>> I still have no clue what this patch here tried to solve: it doesn't
>>>> make any sense.
>>>>
>>>
>>> The story started with this out of tree patch that is part of Android.
>>>
>>> https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u
>>>
>>> This patch introduced the __GFP_CMA flag that allocates pages from
>>> MIGRATE_MOVABLE
>>> or MIGRATE_CMA. What it happens then, it is that the MIGRATE_MOVABLE
>>> pages in the
>>> PCP lists were consumed pretty fast. To solve this issue, the PCP
>>> MIGRATE_CMA list was added.
>>> This list is initialized by rmqueue_bulk() when it is empty. That's
>>> how we end up with the PCP MIGRATE_CMA list
>>> in Android. In addition to this, the THP list for MIGRATE_MOVABLE was
>>> allowed to contain
>>> MIGRATE_CMA pages. This is causing THP MIGRATE_CMA pages to be used
>>> for THP MIGRATE_MOVABLE
>>> making later allocations from THP MIGRATE_CMA to fail.
>>
>> Okay, so this patch here really is not suitable for the upstream kernel
>> as is. It's purely targeted at the OOT Android patch.
>>
> Right, it is a temporary solution for the pinned MIGRATE_CMA pages.
> 
>>>
>>> These workarounds are mainly because we need to solve this issue upstream:
>>>
>>> - When devices reserve big blocks of MIGRATE_CMA pages, the
>>> underutilized MIGRATE_CMA
>>> can fall back to MIGRATE_MOVABLE and these pages can be pinned, so if
>>> we require MIGRATE_CMA
>>> pages, the allocations might fail.
>>>
>>> I remember that you presented the problem in LPC. Were you able to
>>> make some progress on that?
>>
>> There is the problem of CMA pages getting allocated by someone for a
>> MOVABLE allocation, to then short-term pin it for DMA. Long-term
>> pinnings are disallowed (we just recently fixed a bug where we
>> accidentally allowed it).
>>
> Nice, it is great the issue got caught and fixed upstream :)
> 
>> One concern is that a steady stream of short-term pinnings can turn such
>> pages unmovable. We discussed ideas on how to handle that, but there is
>> no solution upstream yet.
> 
> Are there any plans to continue the discussion? Is it in the priority
> list?

Ohh, it's somewheeeeeere on the todo list :)

Do you (or one of your colleagues) have capacity to work on that? One 
idea was to flag folios as "pending on migration" and disallow any 
further short-term pins until migration is done. IIRC, there were other 
ideas (e.g., isolated pageblock).

> Maybe
> a topic we can discuss in LPC Japan?

Sounds good, feel free to propose this as a topic. I wills end out the 
announcement of the MM MC probably next week.

-- 
Cheers,

David / dhildenb

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Vlastimil Babka]

On 8/5/25 19:12, Juan Yescas wrote:
> On Tue, Aug 5, 2025 at 2:52 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> >
>> > Thanks David for the quick overview.
>> >
>> >> I still have no clue what this patch here tried to solve: it doesn't
>> >> make any sense.
>> >>
>> >
>> > The story started with this out of tree patch that is part of Android.
>> >
>> > https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u
>> >
>> > This patch introduced the __GFP_CMA flag that allocates pages from
>> > MIGRATE_MOVABLE
>> > or MIGRATE_CMA.
>>
>> What kinds of allocations would then use __GFP_CMA?
> 
> The __GFP_CMA allocations are used to allocate userspace anonymous memory. This
> was done initially in the alloc_zeroed_user_highpage_movable()
> function, now it is done
> in vma_alloc_zeroed_movable_folio().

So that means you perceive the risk of anonymous memory being temporarily
pinned and thwarting a alloc_contig_pages() device CMA allocation lower than
for file pages? The pinning can be a gup(), or a compaction migrating the
page, etc...

Re: [RFC PATCH] mm/page_alloc: Add PCP list for THP CMA

[Juan Yescas]

On Tue, Aug 5, 2025 at 2:09 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 8/5/25 19:12, Juan Yescas wrote:
> > On Tue, Aug 5, 2025 at 2:52 AM Vlastimil Babka <vbabka@suse.cz> wrote:
> >>
> >> >
> >> > Thanks David for the quick overview.
> >> >
> >> >> I still have no clue what this patch here tried to solve: it doesn't
> >> >> make any sense.
> >> >>
> >> >
> >> > The story started with this out of tree patch that is part of Android.
> >> >
> >> > https://lore.kernel.org/lkml/cover.1604282969.git.cgoldswo@codeaurora.org/T/#u
> >> >
> >> > This patch introduced the __GFP_CMA flag that allocates pages from
> >> > MIGRATE_MOVABLE
> >> > or MIGRATE_CMA.
> >>
> >> What kinds of allocations would then use __GFP_CMA?
> >
> > The __GFP_CMA allocations are used to allocate userspace anonymous memory. This
> > was done initially in the alloc_zeroed_user_highpage_movable()
> > function, now it is done
> > in vma_alloc_zeroed_movable_folio().
>
> So that means you perceive the risk of anonymous memory being temporarily
> pinned and thwarting a alloc_contig_pages() device CMA allocation lower than
> for file pages? The pinning can be a gup(), or a compaction migrating the
> page, etc...
>

I think that was the assumption when the patches were sent :(


>