Atomic operation for physically moving a page (for memory defragmentation)

[Ashwin Rao]

--- Valdis.Kletnieks@xxxxxx wrote:
> On Fri, 18 Jun 2004 17:37:12 PDT, Ashwin Rao
>said:
> > I want to copy a page from one physical location
> to
> > another (taking the appr. locks).
> 
> At the risk of sounding stupid, what problem are you
> trying to solve by copying
> a page? Not only (as you note) could the page be
> referenced by multiple
> processes, it could (conceivably) belong to a kernel
> slab or something, or be a
> buffer for an in-flight I/O request, or any number
> of other possibly-racy
> situations.
> 

The problem is the memory fragmentation. The code i am
writing is for the memory defragmentation as proposed
by Daniel Phillips, my project partner Alok mooley has
given mailed a simple prototype in the mid of feb.

> If it's only a specific *type* of page, or
> explaining why you're trying to do
> it, or what timing/etc constraints you have (if it's
> a sufficiently rare(*) case,
> it might make sense to just grab the BKL and copy
> the page with a memcpy().)
> 

The pages in the LRU list are selected. As these pages
can be swapped they can moved to another location in
the memory.

> (*) Yes, I know the BKL isn't something you want to
> grab if you can help it.

Isnt it a bad idea to take the BKL, the performance of
SMP systems will drastically be hampered.

> However, if we're on an unlikely error path or
> similar and other options aren't suitable...
The way we work is as follows
Initially a block is selected which can be moved i.e
pages on lru or free and the pages are moved to a
suitable free pages. The main problem arises during
the copying and updation process. All the ptes are to
updates. a method similar to try_to_unmap_one  is used
to identify the ptes and the physical address is
updated.

Maintaining atomicity in uniprocessor systems is easy
by preempt_enable and preempt_disable during the
operation. This implementation cannot be used for SMP
systems. 
Now during the time a page is copied/updatede if a
page is accessed the copied contents become invalid,
as updation is not done. Also during updation a
similar situation might arise.
The problem we are facing is to maintain the atomicity
of this operation on SMP boxes.

Ashwin


	
		
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