netdev - Re: [PATCH v3 1/1] xdp: avoid calling kfree twice

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Message-ID: <b3cd5ccb-e1cc-f091-8330-dba6c58b2fc3@iogearbox.net>
Date:   Fri, 11 Dec 2020 00:11:03 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Zhu Yanjun <yanjun.zhu@...el.com>, zyjzyj2000@...il.com,
        bjorn.topel@...el.com, magnus.karlsson@...el.com,
        netdev@...r.kernel.org, jonathan.lemon@...il.com
Cc:     Ye Dong <dong.ye@...el.com>
Subject: Re: [PATCH v3 1/1] xdp: avoid calling kfree twice

On 12/11/20 5:26 AM, Zhu Yanjun wrote:
> In the function xdp_umem_pin_pages, if npgs != umem->npgs and
> npgs >= 0, the function xdp_umem_unpin_pages is called. In this
> function, kfree is called to handle umem->pgs, and then in the
> function xdp_umem_pin_pages, kfree is called again to handle
> umem->pgs. Eventually, to umem->pgs, kfree is called twice.
> 
> Since umem->pgs is set to NULL after the first kfree, the second
> kfree would not trigger call trace.

This can still be misinterpreted imho; maybe lets simplify, for example:

   [bpf-next] xdp: avoid unnecessary second call to kfree

   For the case when in xdp_umem_pin_pages() the call to pin_user_pages()
   wasn't able to pin all the requested number of pages in memory (but some)
   then we error out by cleaning up the ones that got pinned through a call
   to xdp_umem_unpin_pages() and later on we free kfree(umem->pgs) itself.

   This is unneeded since xdp_umem_unpin_pages() itself already does the
   kfree(umem->pgs) internally with subsequent setting umem->pgs to NULL, so
   that in xdp_umem_pin_pages() the second kfree(umem->pgs) becomes entirely
   unnecessary for this case. Therefore, clean the error handling up.

> Fixes: c0c77d8fb787 ("xsk: add user memory registration support sockopt")

Why do we need a Fixes tag here? It's a _cleanup_, not a bug/fix technically.

> CC: Ye Dong <dong.ye@...el.com>
> Acked-by: Björn Töpel <bjorn.topel@...el.com>
> Signed-off-by: Zhu Yanjun <yanjun.zhu@...el.com>
> ---
>   net/xdp/xdp_umem.c | 17 +++++------------
>   1 file changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> index 56a28a686988..01b31c56cead 100644
> --- a/net/xdp/xdp_umem.c
> +++ b/net/xdp/xdp_umem.c
> @@ -97,7 +97,6 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
>   {
>   	unsigned int gup_flags = FOLL_WRITE;
>   	long npgs;
> -	int err;
>   
>   	umem->pgs = kcalloc(umem->npgs, sizeof(*umem->pgs),
>   			    GFP_KERNEL | __GFP_NOWARN);
> @@ -112,20 +111,14 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
>   	if (npgs != umem->npgs) {
>   		if (npgs >= 0) {
>   			umem->npgs = npgs;
> -			err = -ENOMEM;
> -			goto out_pin;
> +			xdp_umem_unpin_pages(umem);
> +			return -ENOMEM;
>   		}
> -		err = npgs;
> -		goto out_pgs;
> +		kfree(umem->pgs);
> +		umem->pgs = NULL;
> +		return (int)npgs;
>   	}
>   	return 0;
> -
> -out_pin:
> -	xdp_umem_unpin_pages(umem);
> -out_pgs:
> -	kfree(umem->pgs);
> -	umem->pgs = NULL;
> -	return err;
>   }
>   
>   static int xdp_umem_account_pages(struct xdp_umem *umem)
> 

While at it, maybe we could also simplify the if (npgs != umem->npgs) a bit to
get rid of the indent, something like:

diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
index 56a28a686988..fa4dd16cced5 100644
--- a/net/xdp/xdp_umem.c
+++ b/net/xdp/xdp_umem.c
@@ -97,7 +97,6 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
  {
  	unsigned int gup_flags = FOLL_WRITE;
  	long npgs;
-	int err;

  	umem->pgs = kcalloc(umem->npgs, sizeof(*umem->pgs),
  			    GFP_KERNEL | __GFP_NOWARN);
@@ -108,24 +107,17 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
  	npgs = pin_user_pages(address, umem->npgs,
  			      gup_flags | FOLL_LONGTERM, &umem->pgs[0], NULL);
  	mmap_read_unlock(current->mm);
-
-	if (npgs != umem->npgs) {
-		if (npgs >= 0) {
-			umem->npgs = npgs;
-			err = -ENOMEM;
-			goto out_pin;
-		}
-		err = npgs;
-		goto out_pgs;
+	if (npgs == umem->npgs)
+		return 0;
+	if (npgs >= 0) {
+		umem->npgs = npgs;
+		xdp_umem_unpin_pages(umem);
+		return -ENOMEM;
  	}
-	return 0;

-out_pin:
-	xdp_umem_unpin_pages(umem);
-out_pgs:
  	kfree(umem->pgs);
  	umem->pgs = NULL;
-	return err;
+	return npgs;
  }

  static int xdp_umem_account_pages(struct xdp_umem *umem)