Messages in this thread | | | From | Marco Elver <> | Date | Mon, 14 Dec 2020 11:09:02 +0100 | Subject | Re: WARNING in sk_stream_kill_queues (5) |
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On Thu, 10 Dec 2020 at 20:01, Marco Elver <elver@google.com> wrote: > 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>
Has this patch, or similar, already been sent?
Thanks, -- Marco
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