Re: [Fwd: [PATCH 2/4] cpusets new __GFP_HARDWALL flag]

[Paul Jackson <pj@sgi.com>]

Mel wrote:
> Well, what would people feel is obvious?

The lines that you (Mel) add that I am puzzling over ways to clarify are
these added lines in gfp.h:

    +#define __GFP_KERNRCLM  0x20000u  /* Kernel page that is easily reclaimable */
    +#define __GFP_USERRCLM  0x40000u  /* User is a userspace user */

    +#define __GFP_TYPE_SHIFT 17     /* Translate RCLM flags to array index */

and perhaps these added lines in mmzone.h:

    +/* Page allocations are divided into these types */
    +#define ALLOC_TYPES 4
    +#define ALLOC_KERNNORCLM 0
    +#define ALLOC_KERNRCLM 1
    +#define ALLOC_USERRCLM 2
    +#define ALLOC_FALLBACK 3
    +
    +/* Number of bits required to encode the type */
    +#define BITS_PER_ALLOC_TYPE 2

It didn't jump out at me, first pass, that these two GFP bits
were a 2 bit field, not 2 separate and independent bits.  The name
GFP_TYPE_SHIFT is vague.  There are some redundant (interdependent)
defines here.

How about (just brainstorming here) something like the following:

    #define __GFP_RCLM_BITS 0x60000u	/* page reclaim types: see RCLM_* defines */

    /*
     * Reduce buddy heap fragmentation by keeping pages with similar
     * reclaimability behavior together.  The two bit field __GFP_RECLAIMBITS
     * enumerates the following 4 kinds of page reclaimability:
     */
    #define RCLM_NONRECLAIMABLE 0	/* nonreclaimable kernel pages */
    #define RCLM_KERNEL 1		/* reclaimable kernel pages */
    #define RCLM_USER 2			/* reclaimable user pages */
    #define RCLM_FALLBACK 3		/* mark alloc requests when memory low */

    #define RCLM_SHIFT 17		/* Shift __GFP_RECLAIMBITS to RCLM_* values */

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401
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