From: Guillaume Nault <gnault@redhat.com>
To: Eric Dumazet <edumazet@google.com>
Cc: "David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
netdev <netdev@vger.kernel.org>,
Chuck Lever <chuck.lever@oracle.com>,
Jeff Layton <jlayton@kernel.org>,
Trond Myklebust <trond.myklebust@hammerspace.com>,
Anna Schumaker <anna@kernel.org>,
Steve French <sfrench@samba.org>,
Josef Bacik <josef@toxicpanda.com>,
Scott Mayhew <smayhew@redhat.com>,
Benjamin Coddington <bcodding@redhat.com>,
Tejun Heo <tj@kernel.org>
Subject: Re: [RFC net] Should sk_page_frag() also look at the current GFP context?
Date: Fri, 8 Jul 2022 19:51:47 +0200 [thread overview]
Message-ID: <20220708175147.GA3166@debian.home> (raw)
In-Reply-To: <CANn89i+=GyHjkrHMZAftB-toEhi9GcAQom1_bpT+S0qMvCz0DQ@mail.gmail.com>
On Thu, Jul 07, 2022 at 06:29:03PM +0200, Eric Dumazet wrote:
> On Fri, Jul 1, 2022 at 8:41 PM Guillaume Nault <gnault@redhat.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@redhat.com>
>
> Can you provide a Fixes: tag ?
Fixes: a1231fda7e94 ("SUNRPC: Set memalloc_nofs_save() on all rpciod/xprtiod jobs")
> > ---
> > 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.
current_gfp_context() looked quite elegant to me as it avoided the need
to duplicate the NOFS/NOIO flag in the socket. But I understand the
performance concern.
> Not sure if we have available holes.
Nothing in the same cache line at least. There's a 1 bit hole in
struct sock_common after skc_net_refcnt. And it should be hot because
of sk->sk_state. We could add a "skc_use_task_frag" bit there, but I'm
not sure if it's worth using this last available bit for this.
Otherwise, the next available hole is right after sk_bind_phc.
According to pahole, it's two cache lines away from sk_allocation on my
x86_64 build, but that will depend of the size of spinlock_t and thus
on CONFIG_ options. It doesn't look very natural to add a no-reclaim
bit there.
Or maybe we could base the test on sk_kern_sock since the problem
happens on kernel sockets. But that looks like a hack to me, and it
might impact MPTCP, which also creates kernel TCP sockets but shouldn't
have the same constraints as NFS.
> > +
> > + if ((gfp_mask & ( | __GFP_MEMALLOC | __GFP_FS)) ==
> > (__GFP_DIRECT_RECLAIM | __GFP_FS))
> > return ¤t->task_frag;
> >
> > --
> > 2.21.3
> >
>
next prev parent reply other threads:[~2022-07-08 17:51 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-01 18:41 [RFC net] Should sk_page_frag() also look at the current GFP context? Guillaume Nault
2022-07-07 15:31 ` Benjamin Coddington
2022-07-07 16:29 ` Eric Dumazet
2022-07-08 17:51 ` Guillaume Nault [this message]
2022-07-08 18:10 ` Benjamin Coddington
2022-07-08 20:04 ` Trond Myklebust
2022-07-11 14:07 ` Benjamin Coddington
2022-07-11 15:31 ` Eric Dumazet
2022-09-20 18:50 ` Guillaume Nault
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