Re: Problems in compressed cache development

[Rodrigo Castro <rcastro@linux.ime.usp.br>]

	I used GFP_ATOMIC flag to allocate with get_free_page (and
when I tried kmalloc). 
	Now I tried that with GFP_KERNEL and it worked!  We couldn't
understand why it did work. I am calling my function (compressed_copy)
from try_to_free_pages() that is a function from kswapd kernel
thread. By what we understood (reading some documentation about it),
it could result in a deadlock, because we would be trying to get a
free page and how the number of free pages (nr_free_page) was smaller
than the water mark (something like min_free_pages) and it wouldnt
have a concurrent process to free pages to our allocation. What is
wrong with our reasoning?

[]'s
-- 
Rodrigo Castro   <rcastro@linux.ime.usp.br>
Computer Science undergraduate student - University of Sao Paulo

Show me a sane man and I will cure him for you.
                -- C.G. Jung

On Fri, Jun 23, 2000 at 11:32:17AM -0300, Rodrigo Castro wrote:
> Hello,
> 
> 	I am an undergraduate student at University of Sao Paulo,
> Brazil and I am working on a compressed cache implementation for Linux
> kernel. Our group (there are two more students plus two professors) is
> working on version 2.2.14 and we are just crawling in our
> development. We've been spending such a long time studying memory
> management system and we've started working on source code for about
> two months. We implemented some functions to initialize a slab cache,
> this cache is supposed to be the heart of our system, and a function
> that copies the first ten pages that goes to swap (yeah, the first 10
> that go to disk). We did that by allocating a page using get_free_page
> and copying memory data with copy_page macro. Well, everything worked
> just fine until we put that to work. Our test machine had 22 Mb of
> free memory. We allocated 20 Mb with a test program, and after that,
> allocated 3 Mb in order to force swapping pages. What happened is our
> second test program (the one that allocated 3 Mb) has been killed by
> VM (message from kern.log: VM: killing process test). Well, we
> replaced get_free_page by kmalloc and we had the same problem. A
> sudden idea came to our mind that we should be updating some variable
> related to the free pages number, but we couldn't find which one would
> be this (these) variable(s). Well, I am writing to you 'cause I would
> like to know if you could have an idea of what may be happening, or
> what we could do to find a solution to that. We've been studying the
> code, and reading many books, but unsuccesfully. Could you give us a
> hand?
> 
> PS: We changed and allocated the 10 pages at initialization. Using
> that, our test program worked, but it would be really useful to know
> why we can't make it working allocating dynamically. 
> PPS: After killing our process, if we run it again, trying to allocate
> the 3 Mb, it works! Oh, the problem procedure is reproducible.
> 
> Thank you in advance,
> -- 
> Rodrigo Castro   <rcastro@linux.ime.usp.br>
> Computer Science undergraduate student - University of Sao Paulo
> 
> Show me a sane man and I will cure him for you.
>                 -- C.G. Jung
> 
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