--- Valdis.Kletnieks@vt.edu wrote:

On Fri, 18 Jun 2004 17:37:12 PDT, Ashwin Rao
said:

quoted

I want to copy a page from one physical location

to

quoted

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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