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

Thank you, Peter.  That was a stupid mistake and I have now undone it.

On 6/16/24 07:13, Peter Holm wrote:
> 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