Re: WARNING in sk_stream_kill_queues (5)

Marco Elver
2020-12-10T19:02:58Z

On Thu, 10 Dec 2020 at 18:14, Eric Dumazet <edumazet@google.com> wrote:
> On Thu, Dec 10, 2020 at 5:51 PM Marco Elver <elver@google.com> wrote:
[...]
> > So I started putting gdb to work, and whenever I see an allocation
> > exactly like the above that goes through tso_fragment() a warning
> > immediately follows.
> >
> > Long story short, I somehow synthesized this patch that appears to fix
> > things, but I can't explain why exactly:
> >
> > | --- a/net/core/skbuff.c
> > | +++ b/net/core/skbuff.c
> > | @@ -1679,13 +1679,6 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> > |
> > | skb_metadata_clear(skb);
> > |
> > | - /* It is not generally safe to change skb->truesize.
> > | - * For the moment, we really care of rx path, or
> > | - * when skb is orphaned (not attached to a socket).
> > | - */
> > | - if (!skb->sk || skb->destructor == sock_edemux)
> > | - skb->truesize += size - osize;
> > | -
> > | return 0;
> > |
> > | nofrags:
> >
> > Now, here are the breadcrumbs I followed:
> >
> >
> > 1. Breakpoint on kfence_ksize() -- first allocation that matches the above:
> >
> > | #0 __kfence_ksize (s=18446612700164612096) at mm/kfence/core.c:726
> > | #1 0xffffffff816fbf30 in kfence_ksize (addr=0xffff888436856000) at mm/kfence/core.c:737
> > | #2 0xffffffff816217cf in ksize (objp=0xffff888436856000) at mm/slab_common.c:1178
> > | #3 0xffffffff84896911 in __alloc_skb (size=914710528, gfp_mask=2592, flags=0, node=-1) at net/core/skbuff.c:217
> > | #4 0xffffffff84d0ba73 in alloc_skb_fclone (priority=<optimized out>, size=<optimized out>) at ./include/linux/skbuff.h:1144
> > | #5 sk_stream_alloc_skb (sk=0xffff8881176cc000, size=0, gfp=2592, force_schedule=232) at net/ipv4/tcp.c:888
> > | #6 0xffffffff84d41c36 in tso_fragment (gfp=<optimized out>, mss_now=<optimized out>, len=<optimized out>,
> > | skb=<optimized out>, sk=<optimized out>) at net/ipv4/tcp_output.c:2124
> > | #7 tcp_write_xmit (sk=0xffff8881176cc000, mss_now=21950, nonagle=3096, push_one=-1996874776, gfp=0)
> > | at net/ipv4/tcp_output.c:2674
> > | #8 0xffffffff84d43e48 in __tcp_push_pending_frames (sk=0xffff8881176cc000, cur_mss=337, nonagle=0)
> > | at ./include/net/sock.h:918
> > | #9 0xffffffff84d3259c in tcp_push_pending_frames (sk=<optimized out>) at ./include/net/tcp.h:1864
> > | #10 tcp_data_snd_check (sk=<optimized out>) at net/ipv4/tcp_input.c:5374
> > | #11 tcp_rcv_established (sk=0xffff8881176cc000, skb=0x0 <fixed_percpu_data>) at net/ipv4/tcp_input.c:5869
> > | #12 0xffffffff84d56731 in tcp_v4_do_rcv (sk=0xffff8881176cc000, skb=0xffff888117f52ea0) at net/ipv4/tcp_ipv4.c:1668
> > | [...]
> >
> > Set watchpoint on skb->truesize:
> >
> > | (gdb) frame 3
> > | #3 0xffffffff84896911 in __alloc_skb (size=914710528, gfp_mask=2592, flags=0, node=-1) at net/core/skbuff.c:217
> > | 217 size = SKB_WITH_OVERHEAD(ksize(data));
> > | (gdb) p &skb->truesize
> > | $5 = (unsigned int *) 0xffff888117f55f90
> > | (gdb) awatch *0xffff888117f55f90
> > | Hardware access (read/write) watchpoint 6: *0xffff888117f55f90
> >
> > 2. Some time later, we see that the skb with kfence-allocated data
> > is cloned:
> >
> > | Thread 7 hit Hardware access (read/write) watchpoint 6: *0xffff888117f55f90
> > |
> > | Value = 1570
> > | 0xffffffff84886947 in __skb_clone (n=0xffff888117f55fa0, skb=0xffff888117f55ec0) at net/core/skbuff.c:1002
> > | 1002 C(truesize);
> > | (gdb) bt
> > | #0 0xffffffff84886947 in __skb_clone (n=0xffff888117f55fa0, skb=0xffff888117f55ec0) at net/core/skbuff.c:1002
> > | #1 0xffffffff8488bfb9 in skb_clone (skb=0xffff888117f55ec0, gfp_mask=2592) at net/core/skbuff.c:1454
> > | #2 0xffffffff84d3cd1c in __tcp_transmit_skb (sk=0xffff8881176cc000, skb=0xffff888117f55ec0, clone_it=0, gfp_mask=2592,
> > | rcv_nxt=0) at net/ipv4/tcp_output.c:1267
> > | #3 0xffffffff84d4125b in tcp_transmit_skb (gfp_mask=<optimized out>, clone_it=<optimized out>, skb=<optimized out>,
> > | sk=<optimized out>) at ./include/linux/tcp.h:439
> > | #4 tcp_write_xmit (sk=0xffff8881176cc000, mss_now=392485600, nonagle=1326, push_one=-1996875104, gfp=0)
> > | at net/ipv4/tcp_output.c:2688
> > | #5 0xffffffff84d43e48 in __tcp_push_pending_frames (sk=0xffff8881176cc000, cur_mss=337, nonagle=0)
> > | at ./include/net/sock.h:918
> > | #6 0xffffffff84d3259c in tcp_push_pending_frames (sk=<optimized out>) at ./include/net/tcp.h:1864
> > | #7 tcp_data_snd_check (sk=<optimized out>) at net/ipv4/tcp_input.c:5374
> > | #8 tcp_rcv_established (sk=0xffff8881176cc000, skb=0x0 <fixed_percpu_data>) at net/ipv4/tcp_input.c:5869
> > | #9 0xffffffff84d56731 in tcp_v4_do_rcv (sk=0xffff8881176cc000, skb=0xffff888117f57820) at net/ipv4/tcp_ipv4.c:1668
> > | #10 0xffffffff8487bf67 in sk_backlog_rcv (skb=<optimized out>, sk=<optimized out>) at ./include/net/sock.h:1010
> > [...]
> >
> >
> > 3. The original skb (that was cloned) has its truesize adjusted
> > after a pskb_expand_head():
> >
> > | Thread 2 hit Hardware access (read/write) watchpoint 6: *0xffff888117f55f90
> > |
> > | Old value = 1570
> > | New value = 1954
> >
> > ^^ the difference between the old and the new value is exactly
> > 384, which is also the final underflow of the sk_wmem_queued
> > that triggers the warning. Presumably if the original allocation
> > had been through kmalloc-1k and not KFENCE, the difference here
> > would have been 0, since ksize() of the original allocation in
> > step (1) would have been 1024, and not 640 (difference of 384).
> >
> > | 0xffffffff8488d84b in pskb_expand_head (skb=0xffff888117f55ec0, nhead=401956752, ntail=1954, gfp_mask=2298092192)
> > | at net/core/skbuff.c:1687
> > | 1687 skb->truesize += size - osize;
> > | (gdb) bt
> > | #0 0xffffffff8488d84b in pskb_expand_head (skb=0xffff888117f55ec0, nhead=401956752, ntail=1954, gfp_mask=2298092192)
> > | at net/core/skbuff.c:1687
> > | #1 0xffffffff8488de01 in skb_prepare_for_shift (skb=<optimized out>) at ./arch/x86/include/asm/atomic.h:29
> > | #2 skb_prepare_for_shift (skb=0xffff888117f55ec0) at net/core/skbuff.c:3276
> > | #3 0xffffffff848936b1 in skb_shift (tgt=0xffff888117f549c0, skb=0xffff888117f55ec0, shiftlen=674) at net/core/skbuff.c:3351
> > | #4 0xffffffff84d264de in tcp_skb_shift (shiftlen=<optimized out>, pcount=<optimized out>, from=<optimized out>,
> > | to=<optimized out>) at net/ipv4/tcp_input.c:1497
> > | #5 tcp_shift_skb_data (dup_sack=<optimized out>, end_seq=<optimized out>, start_seq=<optimized out>, state=<optimized out>,
> > | skb=<optimized out>, sk=<optimized out>) at net/ipv4/tcp_input.c:1605
> > | #6 tcp_sacktag_walk (skb=0xffff888117f55ec0, sk=0xffff8881176cc000, next_dup=0x894,
> > | state=0xffffffff88fa1aa0 <watchpoints+192>, start_seq=0, end_seq=401956752, dup_sack_in=false)
> > | at net/ipv4/tcp_input.c:1670
> > | #7 0xffffffff84d276de in tcp_sacktag_write_queue (sk=0xffff888117f55f90, ack_skb=0x1888117f55f90, prior_snd_una=2196,
> > | state=0xffffffff88fa1aa0 <watchpoints+192>) at net/ipv4/tcp_input.c:1931
> > | #8 0xffffffff84d2ca1d in tcp_ack (sk=0xffff8881176cc000, skb=0x1888117f55f90, flag=16643) at net/ipv4/tcp_input.c:3758
> > | #9 0xffffffff84d32387 in tcp_rcv_established (sk=0xffff8881176cc000, skb=0xffff888117f54020) at net/ipv4/tcp_input.c:5858
> > | #10 0xffffffff84d56731 in tcp_v4_do_rcv (sk=0xffff8881176cc000, skb=0xffff888117f54020) at net/ipv4/tcp_ipv4.c:1668
> > [...]
> >
> >
> > Any of this make sense?
>
> Very nice debugging !
>
> I guess we could fix this in skb_prepare_for_shift(), eventually
> caring for the truesize manipulation
> (or reverting the change done in pskb_expand_head(), since only kfence
> is having this issue.

Phew, good to hear I finally got lucky. :-)

Either option is fine, as long as it avoids this problem in future.
Hopefully it can be fixed for 5.11.

> (All TCP skbs in output path have the same allocation size for skb->head)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index e578544b2cc7110ec2f6bcf4c29d93e4b4b1ad14..798b51eeeaa4fbed65d41d9eab207dbbf438dab3
> 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3270,7 +3270,14 @@ EXPORT_SYMBOL(skb_split);
> */
> static int skb_prepare_for_shift(struct sk_buff *skb)
> {
> - return skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
> + unsigned int ret = 0, save;
> +
> + if (skb_cloned(skb)) {
> + save = skb->truesize;
> + ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
> + skb->truesize = save;
> + }
> + return ret;
> }

FWIW,

Tested-by: Marco Elver <elver@google.com>

Thanks,
-- Marco