Re: [PATCH 1/9] add defrag flags

[Paul Jackson <pj@sgi.com>]

Dave wrote:
> I think Joel simply made an error in his description.

Looks like he made the same mistake in the actual code comments:

+/* Allocation type modifiers, group together if possible
+ * __GPF_USER: Allocation for user page or a buffer page
+ * __GFP_KERNRCLM: Short-lived or reclaimable kernel allocation
+ */
+#define __GFP_USER	0x40000u /* Kernel page that is easily reclaimable */
+#define __GFP_KERNRCLM	0x80000u /* User is a userspace user */

I'd guess you meant to write more like the following:

#define __GFP_USER   0x40000u /* Page for user address space */
#define __GFP_KERNRCLM 0x80000u /* Kernel page that is easily reclaimable */

And the block comment seems to needlessly repeat the inline comments,
add a dubious claim, and omit the interesting stuff ...  In other words:

    Does it actually matter if these two bits are grouped, or not?  I
    suspect that some of your other code, such as shifting the gfpmask by
    RCLM_SHIFT bits, _requires_ that these two bits be adjacent.  So the
    "if possible" in the comment above is misleading.

    And I suspect that gfp.h should contain the RCLM_SHIFT define, or
    at least mention in comment that RCLM_SHIFT depends on the position
    of the above two __GFP_* bits.

    And I don't see any mention in the comments in gfp.h that these
    two bits, in tandem, have an additional meaning - both bits off
    means, I guess, not reclaimable, well at least not easily.

My HARDWALL patch appears to already be in Linus's kernel, so you
probably also need to do a global substitute of all instances in
the kernel of __GFP_HARDWALL, replacing it with __GFP_USER.  Here
is the list of files I see affected, with a count of the number of
__GFP_HARDWALL strings in each:

    include/linux/gfp.h:4
    kernel/cpuset.c:6
    mm/page_alloc.c:2
    mm/vmscan.c:4

The comment in the next line looks like it needs to be changed to match
the code change:

+#define __GFP_BITS_SHIFT 21	/* Room for 20 __GFP_FOO bits */

On the other hand, why did you change __GFP_BITS_SHIFT?  Isn't 20
enough - just enough?

Why was the flag change in fs/buffer.c:grow_dev_page() to add the
__GFP_USER bit, not to add the __GFP_KERNRCLM bit?  I don't know that
code - perhaps the answer is simply that the resulting page ends up in
user space.

Aha - I just read one of the comments above that I cut+pasted.
It says that __GFP_USER means user *OR* buffer page.  That certainly
explains the fs/buffer.c code using __GFP_USER.  But it causes me to
wonder if we can equate __GFP_USER with __GFP_HARDWALL.  I'm reluctant,
but more on principal than concrete experience, to modify the meaning
of hardwall cpusets to constrain both user address space pages *AND*
buffer pages.  How open would you be to making buffers __GFP_KERNRCLM
instead of __GFP_USER?

If you have good reason to keep __GFP_USER meanin either user or buffer,
then perhaps the name __GFP_USER is misleading.

What sort of performance claims can you make for this change?  How does
it impact kernel text size?  Could we see a diffstat for the entire
patchset?  Under what sort of loads or conditions would you expect
this patchset to do more harm than good?

-- 
                  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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