[PATCH] mm: clarify why we want kmalloc before falling backto vmallock

[PATCH] mm: clarify why we want kmalloc before falling backto vmallock

[Michal Hocko]

From: Michal Hocko <mhocko@suse.com>

While converting drm_[cm]alloc* helpers to kvmalloc* variants Chris
Wilson has wondered why we want to try kmalloc before vmalloc fallback
even for larger allocations requests. Let's clarify that one larger
physically contiguous block is less likely to fragment memory than many
scattered pages which can prevent more large blocks from being created.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/util.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index 464df3489903..87499f8119f2 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -357,7 +357,10 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
 	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
 
 	/*
-	 * Make sure that larger requests are not too disruptive - no OOM
+	 * We want to attempt a large physically contiguous block first because
+	 * it is less likely to fragment multiple larger blocks and therefore
+	 * contribute to a long term fragmentation less than vmalloc fallback.
+	 * However make sure that larger requests are not too disruptive - no OOM
 	 * killer and no allocation failure warnings as we have a fallback
 	 */
 	if (size > PAGE_SIZE) {
-- 
2.11.0

Re: [PATCH] mm: clarify why we want kmalloc before falling backto vmallock

[Chris Wilson]

subject s/vmallock/vmalloc/

On Wed, May 17, 2017 at 10:09:32AM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> While converting drm_[cm]alloc* helpers to kvmalloc* variants Chris
> Wilson has wondered why we want to try kmalloc before vmalloc fallback
> even for larger allocations requests. Let's clarify that one larger
> physically contiguous block is less likely to fragment memory than many
> scattered pages which can prevent more large blocks from being created.
> 
> Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

It helped me understand the decisions made by the code, so
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

Re: [PATCH] mm: clarify why we want kmalloc before falling backto vmallock

[Vlastimil Babka]

On 05/17/2017 10:09 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> While converting drm_[cm]alloc* helpers to kvmalloc* variants Chris
> Wilson has wondered why we want to try kmalloc before vmalloc fallback
> even for larger allocations requests. Let's clarify that one larger
> physically contiguous block is less likely to fragment memory than many
> scattered pages which can prevent more large blocks from being created.
> 
> Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

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

> ---
>  mm/util.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 464df3489903..87499f8119f2 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -357,7 +357,10 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
>  	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
>  
>  	/*
> -	 * Make sure that larger requests are not too disruptive - no OOM
> +	 * We want to attempt a large physically contiguous block first because
> +	 * it is less likely to fragment multiple larger blocks and therefore
> +	 * contribute to a long term fragmentation less than vmalloc fallback.
> +	 * However make sure that larger requests are not too disruptive - no OOM
>  	 * killer and no allocation failure warnings as we have a fallback
>  	 */
>  	if (size > PAGE_SIZE) {
> 

Re: [PATCH] mm: clarify why we want kmalloc before falling backto vmallock

[John Hubbard]

On 05/17/2017 01:09 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> While converting drm_[cm]alloc* helpers to kvmalloc* variants Chris
> Wilson has wondered why we want to try kmalloc before vmalloc fallback
> even for larger allocations requests. Let's clarify that one larger
> physically contiguous block is less likely to fragment memory than many
> scattered pages which can prevent more large blocks from being created.
> 
> Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>   mm/util.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 464df3489903..87499f8119f2 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -357,7 +357,10 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
>   	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
>   
>   	/*
> -	 * Make sure that larger requests are not too disruptive - no OOM
> +	 * We want to attempt a large physically contiguous block first because
> +	 * it is less likely to fragment multiple larger blocks and therefore
> +	 * contribute to a long term fragmentation less than vmalloc fallback.
> +	 * However make sure that larger requests are not too disruptive - no OOM
>   	 * killer and no allocation failure warnings as we have a fallback
>   	 */

Thanks for adding this, it's great to have. Here's a slightly polished version of your words, if you 
like:

	/*
	 * We want to attempt a large physically contiguous block first because
	 * it is less likely to fragment multiple larger blocks. This approach
	 * therefore contributes less to long term fragmentation than a vmalloc
	 * fallback would. However, make sure that larger requests are not too
	 * disruptive: no OOM killer and no allocation failure warnings, as we
	 * have a fallback.
	 */

thanks,
john h

>   	if (size > PAGE_SIZE) {
> -- 
> 2.11.0
> 
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Re: [PATCH] mm: clarify why we want kmalloc before falling backto vmallock

[Michal Hocko]

On Fri 19-05-17 17:46:58, John Hubbard wrote:
> On 05/17/2017 01:09 AM, Michal Hocko wrote:
> >From: Michal Hocko <mhocko@suse.com>
> >
> >While converting drm_[cm]alloc* helpers to kvmalloc* variants Chris
> >Wilson has wondered why we want to try kmalloc before vmalloc fallback
> >even for larger allocations requests. Let's clarify that one larger
> >physically contiguous block is less likely to fragment memory than many
> >scattered pages which can prevent more large blocks from being created.
> >
> >Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
> >Signed-off-by: Michal Hocko <mhocko@suse.com>
> >---
> >  mm/util.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> >diff --git a/mm/util.c b/mm/util.c
> >index 464df3489903..87499f8119f2 100644
> >--- a/mm/util.c
> >+++ b/mm/util.c
> >@@ -357,7 +357,10 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
> >  	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
> >  	/*
> >-	 * Make sure that larger requests are not too disruptive - no OOM
> >+	 * We want to attempt a large physically contiguous block first because
> >+	 * it is less likely to fragment multiple larger blocks and therefore
> >+	 * contribute to a long term fragmentation less than vmalloc fallback.
> >+	 * However make sure that larger requests are not too disruptive - no OOM
> >  	 * killer and no allocation failure warnings as we have a fallback
> >  	 */
> 
> Thanks for adding this, it's great to have. Here's a slightly polished
> version of your words, if you like:
> 
> 	/*
> 	 * We want to attempt a large physically contiguous block first because
> 	 * it is less likely to fragment multiple larger blocks. This approach
> 	 * therefore contributes less to long term fragmentation than a vmalloc
> 	 * fallback would. However, make sure that larger requests are not too
> 	 * disruptive: no OOM killer and no allocation failure warnings, as we
> 	 * have a fallback.
> 	 */

Looks ok to me.

-- 
Michal Hocko
SUSE Labs