svn commit: r338802 - head/sys/vm

[Shawn Webb]

On Wed, Sep 19, 2018 at 04:02:34PM +0000, Mateusz Guzik wrote:
> Author: mjg
> Date: Wed Sep 19 16:02:33 2018
> New Revision: 338802
> URL: https://svnweb.freebsd.org/changeset/base/338802
> 
> Log:
>   vm: check for empty kstack cache before locking
>   
>   The current cache logic checks the total number of stacks in the kernel,
>   which even on small boxes significantly exceeds the 128 limit (e.g. an
>   8-way box with zfs has almost 800 stacks allocated).
>   
>   Stacks are cached earlier for each main thread.
>   
>   As a result the code is rarely executed, but when it is then (on boxes like
>   the above) it always fails. Since there are no provisions made for NUMA and
>   release time is approaching, just do a quick check to avoid acquiring the
>   lock.
>   
>   Approved by:	re (kib)
> 
> Modified:
>   head/sys/vm/vm_glue.c
> 
> Modified: head/sys/vm/vm_glue.c
> ==============================================================================
> --- head/sys/vm/vm_glue.c	Wed Sep 19 15:39:16 2018	(r338801)
> +++ head/sys/vm/vm_glue.c	Wed Sep 19 16:02:33 2018	(r338802)
> @@ -327,7 +327,7 @@ vm_thread_new(struct thread *td, int pages)
>  	else if (pages > KSTACK_MAX_PAGES)
>  		pages = KSTACK_MAX_PAGES;
>  
> -	if (pages == kstack_pages) {
> +	if (pages == kstack_pages && kstack_cache != NULL) {
>  		mtx_lock(&kstack_cache_mtx);
>  		if (kstack_cache != NULL) {
>  			ks_ce = kstack_cache;

Since kstack_cache is guaranteed not to be NULL, can the second
conditional that checks for kstack_cache not being NULL be removed?

Thanks,

-- 
Shawn Webb
Cofounder and Security Engineer
HardenedBSD

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