Re: [PATCH] get_nodes mask miscalculation

[Andi Kleen <ak@muc.de>]

On Mon, Aug 09, 2004 at 10:25:31PM -0700, Paul Jackson wrote:
> While I'm here, the statement that only the highest zone is policied
> actually applies only to MPOL_BIND, right?  The comment that asserts

That is correct. I can add a comment.

>  1) Change the man page wording, for each of get_mempolicy(2), mbind(2),
>     and set_mempolicy(2), to boldly state:
> 
> 	Beware:
> 		Pass in a value of maxnode that is * one more * than the
> 		number of nodes represented in nodemask.  If for example,
> 		nodemask represents 64 nodes, numbered 0 to 63, pass in a
> 		value of 65 for maxnodes.

Yes, I will clarify the manpages.

I see no problem with hardcoding 8 bits per byte. 

> 
>  2) Review, test, fix, and apply as fixed the following patch.  For extra
>     credit, get rid of the hard coded 8, 32 and 64 values in the compat stuff,
>     visible in the patch below.  I compiled the patch, once, on an ia64.
>     Otherwise totally untested.
> 
>     This patch:
> 	a) Notes the situation in a prominent "==> Beware <==".
> 	b) Consistently decrements maxnode immediately on each system call
> 	   entry (where someone reading the code might best notice).
> 	c) Otherwise treats maxnode consistently within the code.
> 	d) Addresses the MPOL_BIND max policy only comment.
> 	e) Addresses the harcoded numbers 64 and 8 in copy_nodes_to_user().

Yes, the 64 should be addressed agreed. That came from a misguided
attempt by me to not require compat_* functions (by making the 32bit
and 64bit ABI be the same), but that didn't work out.

> 
>     Yes - it's ugly.  The time that will be lost by those who try to use
>     this interface directly will be ugly too.
> 
>  3) Could you propose a strategy for fixing this?  It might take a couple

The only way would be to allocate new system call slots for the 
two calls. But I'm not convinced it is worth it.

Sorry, but I don't want to break binary compatibility. 

If it was really that bad you should have complained earlier (the
code was out long enough for review), now it is too late.

> If
>    you have to tell me that SGI has to put in a workaround for a year, on
>    any machine with _exactly_ 2049 nodes, such as padding the user nodemask
>    up to 2050 nodes, I'm prepared to deal with that <grin>.

There are many more users of these calls than SGI. And most of them are
using libnuma, which you are proposing to break. 


-Andi