netdev - Re: [PATCH net] sunrpc: Use GFP_NOFS to prevent use of current->task

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Date:   Thu, 22 Sep 2022 10:15:51 -0400
From:   "Benjamin Coddington" <bcodding@...hat.com>
To:     "Guillaume Nault" <gnault@...hat.com>
Cc:     "David Miller" <davem@...emloft.net>,
        "Jakub Kicinski" <kuba@...nel.org>,
        "Paolo Abeni" <pabeni@...hat.com>,
        "Eric Dumazet" <edumazet@...gle.com>, 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>, linux-nfs@...r.kernel.org
Subject: Re: [PATCH net] sunrpc: Use GFP_NOFS to prevent use of
 current->task_frag.

On 21 Sep 2022, at 8:16, Guillaume Nault wrote:

> Commit a1231fda7e94 ("SUNRPC: Set memalloc_nofs_save() on all
> rpciod/xprtiod jobs") stopped setting sk->sk_allocation explicitly in
> favor of using memalloc_nofs_save()/memalloc_nofs_restore() critical
> sections.
>
> However, ->sk_allocation isn't used just by the memory allocator.
> In particular, sk_page_frag() uses it to figure out if it can return
> the page_frag from current or if it has to use the socket one.
> With ->sk_allocation set to the default GFP_KERNEL, sk_page_frag() now
> returns current->page_frag, which might already be in use in the
> current context if the call happens during memory reclaim.
>
> Fix this by setting ->sk_allocation to GFP_NOFS.
> Note that we can't just instruct sk_page_frag() to look at
> current->flags, because it could generate a cache miss, thus slowing
> down the TCP fast path.
>
> This is similar to the problems fixed by the following two commits:
>   * cifs: commit dacb5d8875cc ("tcp: fix page frag corruption on page
>     fault").
>   * nbd: commit 20eb4f29b602 ("net: fix sk_page_frag() recursion from
>     memory reclaim").
>
> Link: 
> https://lore.kernel.org/netdev/b4d8cb09c913d3e34f853736f3f5628abfd7f4b6.1656699567.git.gnault@redhat.com/
> Fixes: a1231fda7e94 ("SUNRPC: Set memalloc_nofs_save() on all 
> rpciod/xprtiod jobs")
> Signed-off-by: Guillaume Nault <gnault@...hat.com>

Looks good, and thanks for looking through all the options.

Reviewed-by: Benjamin Coddington <bcodding@...hat.com>

Ben