netdev - Re: [RFC net] Should sk_page_frag() also look at the current GFP context?

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Message-ID: <CANn89i+=GyHjkrHMZAftB-toEhi9GcAQom1_bpT+S0qMvCz0DQ@mail.gmail.com>
Date:   Thu, 7 Jul 2022 18:29:03 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jeff Layton <jlayton@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Steve French <sfrench@...ba.org>,
        Josef Bacik <josef@...icpanda.com>,
        Scott Mayhew <smayhew@...hat.com>,
        Benjamin Coddington <bcodding@...hat.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [RFC net] Should sk_page_frag() also look at the current GFP context?

On Fri, Jul 1, 2022 at 8:41 PM Guillaume Nault <gnault@...hat.com> wrote:
>
> I'm investigating a kernel oops that looks similar to
> 20eb4f29b602 ("net: fix sk_page_frag() recursion from memory reclaim")
> and dacb5d8875cc ("tcp: fix page frag corruption on page fault").
>
> This time the problem happens on an NFS client, while the previous bzs
> respectively used NBD and CIFS. While NBD and CIFS clear __GFP_FS in
> their socket's ->sk_allocation field (using GFP_NOIO or GFP_NOFS), NFS
> leaves sk_allocation to its default value since commit a1231fda7e94
> ("SUNRPC: Set memalloc_nofs_save() on all rpciod/xprtiod jobs").
>
> To recap the original problems, in commit 20eb4f29b602 and dacb5d8875cc,
> memory reclaim happened while executing tcp_sendmsg_locked(). The code
> path entered tcp_sendmsg_locked() recursively as pages to be reclaimed
> were backed by files on the network. The problem was that both the
> outer and the inner tcp_sendmsg_locked() calls used current->task_frag,
> thus leaving it in an inconsistent state. The fix was to use the
> socket's ->sk_frag instead for the file system socket, so that the
> inner and outer calls wouln't step on each other's toes.
>
> But now that NFS doesn't modify ->sk_allocation anymore, sk_page_frag()
> sees sunrpc sockets as plain TCP ones and returns ->task_frag in the
> inner tcp_sendmsg_locked() call.
>
> Also it looks like the trend is to avoid GFS_NOFS and GFP_NOIO and use
> memalloc_no{fs,io}_save() instead. So maybe other network file systems
> will also stop setting ->sk_allocation in the future and we should
> teach sk_page_frag() to look at the current GFP flags. Or should we
> stick to ->sk_allocation and make NFS drop __GFP_FS again?
>
> Signed-off-by: Guillaume Nault <gnault@...hat.com>

Can you provide a Fixes: tag ?

> ---
>  include/net/sock.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 72ca97ccb460..b934c9851058 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -46,6 +46,7 @@
>  #include <linux/netdevice.h>
>  #include <linux/skbuff.h>      /* struct sk_buff */
>  #include <linux/mm.h>
> +#include <linux/sched/mm.h>
>  #include <linux/security.h>
>  #include <linux/slab.h>
>  #include <linux/uaccess.h>
> @@ -2503,14 +2504,17 @@ static inline void sk_stream_moderate_sndbuf(struct sock *sk)
>   * socket operations and end up recursing into sk_page_frag()
>   * while it's already in use: explicitly avoid task page_frag
>   * usage if the caller is potentially doing any of them.
> - * This assumes that page fault handlers use the GFP_NOFS flags.
> + * This assumes that page fault handlers use the GFP_NOFS flags
> + * or run under memalloc_nofs_save() protection.
>   *
>   * Return: a per task page_frag if context allows that,
>   * otherwise a per socket one.
>   */
>  static inline struct page_frag *sk_page_frag(struct sock *sk)
>  {
> -       if ((sk->sk_allocation & (__GFP_DIRECT_RECLAIM | __GFP_MEMALLOC | __GFP_FS)) ==
> +       gfp_t gfp_mask = current_gfp_context(sk->sk_allocation);

This is slowing down TCP sendmsg() fast path, reading current->flags,
possibly cold value.

I would suggest using one bit in sk, close to sk->sk_allocation to
make the decision,
instead of testing sk->sk_allocation for various flags.

Not sure if we have available holes.

> +
> +       if ((gfp_mask & ( | __GFP_MEMALLOC | __GFP_FS)) ==
>             (__GFP_DIRECT_RECLAIM | __GFP_FS))
>                 return &current->task_frag;
>
> --
> 2.21.3
>