Re: git: dd0e5c02ab13 - main - swap_pager: small improvement to find_least

On Tue, Jun 11, 2024 at 04:38:49PM +0000, Doug Moore wrote:
> The branch main has been updated by dougm:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=dd0e5c02ab13b9eb240d42a71a8f41a8b036bd33
> 
> commit dd0e5c02ab13b9eb240d42a71a8f41a8b036bd33
> Author:     Doug Moore <dougm@FreeBSD.org>
> AuthorDate: 2024-06-11 16:36:23 +0000
> Commit:     Doug Moore <dougm@FreeBSD.org>
> CommitDate: 2024-06-11 16:36:23 +0000
> 
>     swap_pager: small improvement to find_least
>     
>     Drop an unneeded test, a branch and a needless computation to save a
>     few instructions.
>     
>     Reviewed by:    kib
>     Differential Revision:  https://reviews.freebsd.org/D45558
> ---
>  sys/vm/swap_pager.c | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
> 
> diff --git a/sys/vm/swap_pager.c b/sys/vm/swap_pager.c
> index a07a38481490..15f227457bba 100644
> --- a/sys/vm/swap_pager.c

Doug,

Here's an easy way to reproduce the issue:

12:43 ~ $ sort /dev/zero &
[1] 3650
12:43 ~ $ sort /dev/zero &
[2] 3651
12:43 ~ $ sort /dev/zero &
[3] 3652
12:43 ~ $ panic: Negative index: -16
cpuid = 8
time = 1718534645
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe010855fa50
vpanic() at vpanic+0x13f/frame 0xfffffe010855fb80
panic() at panic+0x43/frame 0xfffffe010855fbe0
swap_pager_find_least() at swap_pager_find_least+0x105/frame 0xfffffe010855fc20
vm_object_collapse() at vm_object_collapse+0x68b/frame 0xfffffe010855fca0
vmspace_fork() at vmspace_fork+0xb68/frame 0xfffffe010855fd30
fork1() at fork1+0x4f8/frame 0xfffffe010855fda0
sys_fork() at sys_fork+0x54/frame 0xfffffe010855fe00
amd64_syscall() at amd64_syscall+0x158/frame 0xfffffe010855ff30
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe010855ff30
--- syscall (2, FreeBSD ELF64, fork), rip = 0xaba1c31565a, rsp = 0xaba1b0cecb8, rbp = 0xaba1b0cf220 ---
KDB: enter: panic
[ thread pid 3653 tid 100349 ]
Stopped at         $0,0x104d5c2(%rip)
db> x/s version
version:        FreeBSD 15.0-CURRENT #2 main-n270778-d26c565597b8-dirty: Sun Jun 16 12:32:39 CEST 2024\012    pho@mercat1.netperf.freebsd.org:/usr/src/sys/amd64/compile/PHO\012
db> 

--- a/sys/vm/swap_pager.c
+++ b/sys/vm/swap_pager.c
@@ -2303,6 +2303,7 @@ swap_pager_find_least(vm_object_t object, vm_pindex_t pindex)
        if (sb == NULL)
                return (object->size);
        for (i = pindex - sb->p; i < SWAP_META_PAGES; i++) {
+               KASSERT((i >= 0), ("Negative index: %d", i));
                if (sb->d[i] != SWAPBLK_NONE)
                        return (sb->p + i);
        }

- Peter