Re: [PATCH] sunrpc: raise kernel RPC channel buffer size

[J. Bruce Fields]

On Fri, Oct 23, 2020 at 11:44:38AM +0200, Geert Uytterhoeven wrote:
> 	Hi Bruce, Roberto,
> 
> On Mon, 19 Oct 2020, J. Bruce Fields wrote:
> >On Mon, Oct 19, 2020 at 11:33:56AM +0200, Roberto Bergantinos Corpas wrote:
> >>Its possible that using AUTH_SYS and mountd manage-gids option a
> >>user may hit the 8k RPC channel buffer limit. This have been observed
> >>on field, causing unanswered RPCs on clients after mountd fails to
> >>write on channel :
> >>
> >>rpc.mountd[11231]: auth_unix_gid: error writing reply
> >>
> >>Userland nfs-utils uses a buffer size of 32k (RPC_CHAN_BUF_SIZE), so
> >>lets match those two.
> >
> >Thanks, applying.
> >
> >That should allow about 4000 group memberships.  If that doesn't do it
> >then maybe it's time to rethink....
> >
> >--b.
> >
> >>
> >>Signed-off-by: Roberto Bergantinos Corpas <rbergant@xxxxxxxxxx>
> >>---
> >> net/sunrpc/cache.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >>diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
> >>index baef5ee43dbb..08df4c599ab3 100644
> >>--- a/net/sunrpc/cache.c
> >>+++ b/net/sunrpc/cache.c
> >>@@ -908,7 +908,7 @@ static ssize_t cache_do_downcall(char *kaddr, const char __user *buf,
> >> static ssize_t cache_slow_downcall(const char __user *buf,
> >> 				   size_t count, struct cache_detail *cd)
> >> {
> >>-	static char write_buf[8192]; /* protected by queue_io_mutex */
> >>+	static char write_buf[32768]; /* protected by queue_io_mutex */
> >> 	ssize_t ret = -EINVAL;
> >>
> >> 	if (count >= sizeof(write_buf))
> 
> This is now commit 27a1e8a0f79e643d ("sunrpc: raise kernel RPC channel
> buffer size") upstream, and increases kernel size by 24 KiB, even if
> RPC is not used.
> 
> Can this buffer allocated dynamically instead? This code path seems to
> be a slow path anyway. If it's critical, perhaps this buffer can be
> allocated on first use?

Sure.  Actually it is using an allocated buffer typically, see
cache_downcall().

Looking back at the history....  That was added by Trond in 2009
(da77005f0d64 "SUNRPC: Remove the global temporary write buffer in
net/sunrpc/cache.c").  

Before that there's a pre-git change from 2004 which replaced a simple
kmalloc(PAGE_SIZE).

So I guess the point was to be careful about higher-order allocations,
but probably it was overkill.

How about making it a kvmalloc?

--b.