On 07/25/2013 08:40 AM, Wanpeng Li wrote:

On Tue, Jul 23, 2013 at 12:18:06AM +0530, Srivatsa S. Bhat wrote:

quoted

The free-page stealing code in __rmqueue_fallback() is somewhat hard to
follow, and has an incredible amount of subtlety hidden inside!

First off, there is a minor bug in the reporting of change-of-ownership of
pageblocks. Under some conditions, we try to move upto 'pageblock_nr_pages'
no. of pages to the preferred allocation list. But we change the ownership
of that pageblock to the preferred type only if we manage to successfully
move atleast half of that pageblock (or if page_group_by_mobility_disabled
is set).

However, the current code ignores the latter part and sets the 'migratetype'
variable to the preferred type, irrespective of whether we actually changed
the pageblock migratetype of that block or not. So, the page_alloc_extfrag
tracepoint can end up printing incorrect info (i.e., 'change_ownership'
might be shown as 1 when it must have been 0).

So fixing this involves moving the update of the 'migratetype' variable to
the right place. But looking closer, we observe that the 'migratetype' variable
is used subsequently for checks such as "is_migrate_cma()". Obviously the
intent there is to check if the *fallback* type is MIGRATE_CMA, but since we
already set the 'migratetype' variable to start_migratetype, we end up checking
if the *preferred* type is MIGRATE_CMA!!

To make things more interesting, this actually doesn't cause a bug in practice,
because we never change *anything* if the fallback type is CMA.

So, restructure the code in such a way that it is trivial to understand what
is going on, and also fix the above mentioned bug. And while at it, also add a
comment explaining the subtlety behind the migratetype used in the call to
expand().

Greate catch!

Thank you :-)

Regards,
Srivatsa S. Bhat

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