[PATCH] xen/events: replace __get_free_pages() with kmalloc()

[PATCH] xen/events: replace __get_free_pages() with kmalloc()

[Mike Rapoport (Microsoft)]

set_evtchn_to_irq() allocates a page for the event channel to IRQ
number software lookup table.

This buffer can be allocated with kmalloc() as there's nothing special
about it to go directly to the page allocator.

kmalloc() provides a better API that does not require ugly casts and
kfree() does not need to know the size of the freed object.

Performance difference between kmalloc() and __get_free_pages() is not
measurable as both allocators take an object/page from a per-CPU list for
fast path allocations.

For the slow path the performance is anyway determined by the amount of
reclaim involved rather than by what allocator is used.

Replace use of __get_free_pages() with kmalloc() and free_pages() with
kfree().

Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
This is a (small) part of larger work of replacing page allocator calls
with kmalloc.

Largely, anything that doesn't need struct page (or a memdesc in the
future) should just use kmalloc() or kvmalloc() to allocate memory.
Removing needless calls to page allocator helps with memdesc (aka project
folio) conversion and gives a nicer API without casts.
---
 drivers/xen/events/events_base.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
index fd16d652c81f2..8c377493429b7 100644
--- a/drivers/xen/events/events_base.c
+++ b/drivers/xen/events/events_base.c
@@ -230,7 +230,7 @@ static int set_evtchn_to_irq(evtchn_port_t evtchn, unsigned int irq)
 		if (irq == -1)
 			return 0;
 
-		evtchn_row = (int *) __get_free_pages(GFP_KERNEL, 0);
+		evtchn_row = kmalloc(PAGE_SIZE, GFP_KERNEL);
 		if (evtchn_row == NULL)
 			return -ENOMEM;
 
@@ -241,7 +241,7 @@ static int set_evtchn_to_irq(evtchn_port_t evtchn, unsigned int irq)
 		 * thread was faster inserting it, we can drop ours.
 		 */
 		if (cmpxchg(&evtchn_to_irq[row], NULL, evtchn_row) != NULL)
-			free_page((unsigned long) evtchn_row);
+			kfree(evtchn_row);
 	}
 
 	WRITE_ONCE(evtchn_to_irq[row][col], irq);

---
base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
change-id: 20260624-b4-xen-a1cd629ce9f4

--
Sincerely yours,
Mike.

Re: [PATCH] xen/events: replace __get_free_pages() with kmalloc()

[Juergen Gross]

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On 08.11.26 09:42, Mike Rapoport (Microsoft) wrote:
> set_evtchn_to_irq() allocates a page for the event channel to IRQ
> number software lookup table.
> 
> This buffer can be allocated with kmalloc() as there's nothing special
> about it to go directly to the page allocator.
> 
> kmalloc() provides a better API that does not require ugly casts and
> kfree() does not need to know the size of the freed object.
> 
> Performance difference between kmalloc() and __get_free_pages() is not
> measurable as both allocators take an object/page from a per-CPU list for
> fast path allocations.
> 
> For the slow path the performance is anyway determined by the amount of
> reclaim involved rather than by what allocator is used.
> 
> Replace use of __get_free_pages() with kmalloc() and free_pages() with
> kfree().
> 
> Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Shouldn't you also replace __get_free_page() in xen_init_IRQ()? I don't see
why this is being omitted.


Juergen

> ---
> This is a (small) part of larger work of replacing page allocator calls
> with kmalloc.
> 
> Largely, anything that doesn't need struct page (or a memdesc in the
> future) should just use kmalloc() or kvmalloc() to allocate memory.
> Removing needless calls to page allocator helps with memdesc (aka project
> folio) conversion and gives a nicer API without casts.
> ---
>   drivers/xen/events/events_base.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
> index fd16d652c81f2..8c377493429b7 100644
> --- a/drivers/xen/events/events_base.c
> +++ b/drivers/xen/events/events_base.c
> @@ -230,7 +230,7 @@ static int set_evtchn_to_irq(evtchn_port_t evtchn, unsigned int irq)
>   		if (irq == -1)
>   			return 0;
>   
> -		evtchn_row = (int *) __get_free_pages(GFP_KERNEL, 0);
> +		evtchn_row = kmalloc(PAGE_SIZE, GFP_KERNEL);
>   		if (evtchn_row == NULL)
>   			return -ENOMEM;
>   
> @@ -241,7 +241,7 @@ static int set_evtchn_to_irq(evtchn_port_t evtchn, unsigned int irq)
>   		 * thread was faster inserting it, we can drop ours.
>   		 */
>   		if (cmpxchg(&evtchn_to_irq[row], NULL, evtchn_row) != NULL)
> -			free_page((unsigned long) evtchn_row);
> +			kfree(evtchn_row);
>   	}
>   
>   	WRITE_ONCE(evtchn_to_irq[row][col], irq);
> 
> ---
> base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
> change-id: 20260624-b4-xen-a1cd629ce9f4
> 
> --
> Sincerely yours,
> Mike.


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