Re: [RFC] mm: Distinguish between mlocked and pinned pages

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 10 Aug 2011 15:21:47 -0500 (CDT)
Christoph Lameter <cl@linux.com> wrote:

> Some kernel components pin user space memory (infiniband and perf)
> (by increasing the page count) and account that memory as "mlocked".
> 
> The difference between mlocking and pinning is:
> 
> A. mlocked pages are marked with PG_mlocked and are exempt from
>    swapping. Page migration may move them around though.
>    They are kept on a special LRU list.
> 
> B. Pinned pages cannot be moved because something needs to
>    directly access physical memory. They may not be on any
>    LRU list.
> 
> I recently saw an mlockalled process where mm->locked_vm became
> bigger than the virtual size of the process (!) because some
> memory was accounted for twice:
> 
> Once when the page was mlocked and once when the Infiniband
> layer increased the refcount because it needt to pin the RDMA
> memory.
> 
> This patch introduces a separate counter for pinned pages and
> accounts them seperately.
> 

Sounds reasonable.  But how do we prevent future confusion?  We should
carefully define these terms in an obvious place, please.

> --- linux-2.6.orig/include/linux/mm_types.h	2011-08-10 14:08:42.000000000 -0500
> +++ linux-2.6/include/linux/mm_types.h	2011-08-10 14:09:02.000000000 -0500
> @@ -281,7 +281,7 @@ struct mm_struct {
>  	unsigned long hiwater_rss;	/* High-watermark of RSS usage */
>  	unsigned long hiwater_vm;	/* High-water virtual memory usage */
> 
> -	unsigned long total_vm, locked_vm, shared_vm, exec_vm;
> +	unsigned long total_vm, locked_vm, pinned_vm, shared_vm, exec_vm;
>  	unsigned long stack_vm, reserved_vm, def_flags, nr_ptes;
>  	unsigned long start_code, end_code, start_data, end_data;
>  	unsigned long start_brk, brk, start_stack;

This is an obvious place.  Could I ask that you split all these up into
one-definition-per-line and we can start in on properly documenting
each field?

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