[PATCH][net-next] vhost:net: allocate 32KB memory instead of 32K pages when page frag refill

[PATCH][net-next] vhost:net: allocate 32KB memory instead of 32K pages when page frag refill

[Li RongQing]

when page frag refills, 32K pages, 128MB memory is asked, it
hardly successes when system has memory stress

And such large memory size will cause the underflow of reference
bias, and make refcount of page chaos, since reference bias will
be decreased to negative before the allocated memory is used up

so 32KB memory is safe choice, meanwhile, remove a unnecessary
check

Fixes: e4dab1e6ea64 ("vhost_net: mitigate page reference counting during page frag refill")
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
---
 drivers/vhost/net.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index d919284f103b..b933a4a8e4ba 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -641,7 +641,7 @@ static bool tx_can_batch(struct vhost_virtqueue *vq, size_t total_len)
 	       !vhost_vq_avail_empty(vq->dev, vq);
 }
 
-#define SKB_FRAG_PAGE_ORDER     get_order(32768)
+#define SKB_FRAG_PAGE_ORDER    3
 
 static bool vhost_net_page_frag_refill(struct vhost_net *net, unsigned int sz,
 				       struct page_frag *pfrag, gfp_t gfp)
@@ -654,17 +654,17 @@ static bool vhost_net_page_frag_refill(struct vhost_net *net, unsigned int sz,
 
 	pfrag->offset = 0;
 	net->refcnt_bias = 0;
-	if (SKB_FRAG_PAGE_ORDER) {
-		/* Avoid direct reclaim but allow kswapd to wake */
-		pfrag->page = alloc_pages((gfp & ~__GFP_DIRECT_RECLAIM) |
-					  __GFP_COMP | __GFP_NOWARN |
-					  __GFP_NORETRY,
-					  SKB_FRAG_PAGE_ORDER);
-		if (likely(pfrag->page)) {
-			pfrag->size = PAGE_SIZE << SKB_FRAG_PAGE_ORDER;
-			goto done;
-		}
+
+	/* Avoid direct reclaim but allow kswapd to wake */
+	pfrag->page = alloc_pages((gfp & ~__GFP_DIRECT_RECLAIM) |
+				  __GFP_COMP | __GFP_NOWARN |
+				  __GFP_NORETRY,
+				  SKB_FRAG_PAGE_ORDER);
+	if (likely(pfrag->page)) {
+		pfrag->size = PAGE_SIZE << SKB_FRAG_PAGE_ORDER;
+		goto done;
 	}
+
 	pfrag->page = alloc_page(gfp);
 	if (likely(pfrag->page)) {
 		pfrag->size = PAGE_SIZE;
-- 
2.16.2

Re: [PATCH][net-next] vhost:net: allocate 32KB memory instead of 32K pages when page frag refill

[Jason Wang]

On 2018/11/23 上午10:04, Li RongQing wrote:
> when page frag refills, 32K pages, 128MB memory is asked, it
> hardly successes when system has memory stress


Looking at get_order(), it seems we get 3 after get_order(32768) since 
it accepts the size of block.

/**
  * get_order - Determine the allocation order of a memory size
  * @size: The size for which to get the order
...

define get_order(n)                                            \
(                                                               \
         __builtin_constant_p(n) ? (                             \
                 ((n) == 0UL) ? BITS_PER_LONG - PAGE_SHIFT :     \
                 (((n) < (1UL << PAGE_SHIFT)) ? 0 :              \
                  ilog2((n) - 1) - PAGE_SHIFT + 1)               \

                                           ^^^

         ) :                                                     \
         __get_order(n)                                          \
)


>
> And such large memory size will cause the underflow of reference
> bias, and make refcount of page chaos, since reference bias will
> be decreased to negative before the allocated memory is used up


Do you have reproducer for this issue?

Thanks


>
> so 32KB memory is safe choice, meanwhile, remove a unnecessary
> check
>
> Fixes: e4dab1e6ea64 ("vhost_net: mitigate page reference counting during page frag refill")
> Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
>   drivers/vhost/net.c | 22 +++++++++++-----------
>   1 file changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index d919284f103b..b933a4a8e4ba 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -641,7 +641,7 @@ static bool tx_can_batch(struct vhost_virtqueue *vq, size_t total_len)
>   	       !vhost_vq_avail_empty(vq->dev, vq);
>   }
>   
> -#define SKB_FRAG_PAGE_ORDER     get_order(32768)
> +#define SKB_FRAG_PAGE_ORDER    3
>   
>   static bool vhost_net_page_frag_refill(struct vhost_net *net, unsigned int sz,
>   				       struct page_frag *pfrag, gfp_t gfp)
> @@ -654,17 +654,17 @@ static bool vhost_net_page_frag_refill(struct vhost_net *net, unsigned int sz,
>   
>   	pfrag->offset = 0;
>   	net->refcnt_bias = 0;
> -	if (SKB_FRAG_PAGE_ORDER) {
> -		/* Avoid direct reclaim but allow kswapd to wake */
> -		pfrag->page = alloc_pages((gfp & ~__GFP_DIRECT_RECLAIM) |
> -					  __GFP_COMP | __GFP_NOWARN |
> -					  __GFP_NORETRY,
> -					  SKB_FRAG_PAGE_ORDER);
> -		if (likely(pfrag->page)) {
> -			pfrag->size = PAGE_SIZE << SKB_FRAG_PAGE_ORDER;
> -			goto done;
> -		}
> +
> +	/* Avoid direct reclaim but allow kswapd to wake */
> +	pfrag->page = alloc_pages((gfp & ~__GFP_DIRECT_RECLAIM) |
> +				  __GFP_COMP | __GFP_NOWARN |
> +				  __GFP_NORETRY,
> +				  SKB_FRAG_PAGE_ORDER);
> +	if (likely(pfrag->page)) {
> +		pfrag->size = PAGE_SIZE << SKB_FRAG_PAGE_ORDER;
> +		goto done;
>   	}
> +
>   	pfrag->page = alloc_page(gfp);
>   	if (likely(pfrag->page)) {
>   		pfrag->size = PAGE_SIZE;

答复: [PATCH][net-next] vhost:net: allocate 32KB memory instead of 32K pages when page frag refill

[Li,Rongqing]

On 2018/11/23 上午10:04, Li RongQing wrote:
> >when page frag refills, 32K pages, 128MB memory is asked, it hardly 
> >successes when system has memory stress


> Looking at get_order(), it seems we get 3 after get_order(32768) since it accepts the size of block.

You are right, I understood wrongly, 

Please drop this patch, sorry for the noise

-Q