Re: [PATCH] SUNRPC: Fix svc_flush_dcache()

[He Zhe]

On 9/21/20 3:51 AM, Chuck Lever wrote:
> On platforms that implement flush_dcache_page(), a large NFS WRITE
> triggers the WARN_ONCE in bvec_iter_advance():
>
> Sep 20 14:01:05 klimt.1015granger.net kernel: Attempted to advance past end of bvec iter
> Sep 20 14:01:05 klimt.1015granger.net kernel: WARNING: CPU: 0 PID: 1032 at include/linux/bvec.h:101 bvec_iter_advance.isra.0+0xa7/0x158 [sunrpc]
>
> Sep 20 14:01:05 klimt.1015granger.net kernel: Call Trace:
> Sep 20 14:01:05 klimt.1015granger.net kernel:  svc_tcp_recvfrom+0x60c/0x12c7 [sunrpc]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? bvec_iter_advance.isra.0+0x158/0x158 [sunrpc]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? del_timer_sync+0x4b/0x55
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? test_bit+0x1d/0x27 [sunrpc]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  svc_recv+0x1193/0x15e4 [sunrpc]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? try_to_freeze.isra.0+0x6f/0x6f [sunrpc]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? refcount_sub_and_test.constprop.0+0x13/0x40 [sunrpc]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? svc_xprt_put+0x1e/0x29f [sunrpc]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? svc_send+0x39f/0x3c1 [sunrpc]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  nfsd+0x282/0x345 [nfsd]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? __kthread_parkme+0x74/0xba
> Sep 20 14:01:05 klimt.1015granger.net kernel:  kthread+0x2ad/0x2bc
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? nfsd_destroy+0x124/0x124 [nfsd]
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? test_bit+0x1d/0x27
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ? kthread_mod_delayed_work+0x115/0x115
> Sep 20 14:01:05 klimt.1015granger.net kernel:  ret_from_fork+0x22/0x30
>
> Reported-by: He Zhe <zhe.he@xxxxxxxxxxxxx>
> Fixes: ca07eda33e01 ("SUNRPC: Refactor svc_recvfrom()")
> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> ---
>  net/sunrpc/svcsock.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Hi Zhe-
>
> If you confirm this fixes your issue and there are no other
> objections or regressions, I can submit this for v5.9-rc.

I don't quite get why we add "seek" to "size". It seems this action does not
reflect the actual scenario and forcedly neutralizes the WARN_ONCE check in
bvec_iter_advance, so that it may "advance past end of bvec iter" and thus
introduces overflow.

Why don't we avoid this problem at the very begginning like my v1? That is, call
svc_flush_bvec only when we have received more than we want to seek.

        len = sock_recvmsg(svsk->sk_sock, &msg, MSG_DONTWAIT);
-       if (len > 0)
+       if (len > 0 && (size_t)len > (seek & PAGE_MASK))
                svc_flush_bvec(bvec, len, seek);


Regards,
Zhe

>
>
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index d5805fa1d066..c2752e2b9ce3 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -228,7 +228,7 @@ static int svc_one_sock_name(struct svc_sock *svsk, char *buf, int remaining)
>  static void svc_flush_bvec(const struct bio_vec *bvec, size_t size, size_t seek)
>  {
>  	struct bvec_iter bi = {
> -		.bi_size	= size,
> +		.bi_size	= size + seek,
>  	};
>  	struct bio_vec bv;
>  
>
>
>