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

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Date:   Sat, 12 Dec 2020 13:36:51 +0800
From:   Rain River <rain.1986.08.12@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     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,
        Ye Dong <dong.ye@...el.com>
Subject: Re: [PATCH v3 1/1] xdp: avoid calling kfree twice

On Fri, Dec 11, 2020 at 11:53 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> 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:

The original patch is just to make some cleanups. Please do not make
it so complicated.
If you like it, please file an official patch for code review.

>
> 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)