linux-kernel - Re: [patch 5/9] Guest page hinting: mlocked pages.

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Date:	Wed, 13 Sep 2006 18:22:01 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	akpm@...l.org, nickpiggin@...oo.com.au, frankeh@...son.ibm.com,
	rhim@...gateh.edu
Subject: Re: [patch 5/9] Guest page hinting: mlocked pages.

Martin Schwidefsky wrote:
> diff -urpN linux-2.6/mm/memory.c linux-2.6-patched/mm/memory.c
> --- linux-2.6/mm/memory.c	2006-09-01 12:50:24.000000000 +0200
> +++ linux-2.6-patched/mm/memory.c	2006-09-01 12:50:24.000000000 +0200
> @@ -2523,6 +2523,31 @@ int make_pages_present(unsigned long add
>  	BUG_ON(addr >= end);
>  	BUG_ON(end > vma->vm_end);
>  	len = (end+PAGE_SIZE-1)/PAGE_SIZE-addr/PAGE_SIZE;
> +
> +	if (page_host_discards() && (vma->vm_flags & VM_LOCKED)) {
> +		int rlen = len;
> +		ret = 0;
> +		while (rlen > 0) {
> +			struct page *page_refs[32];
> +			int chunk, cret, i;
> +
> +			chunk = rlen < 32 ? rlen : 32;
> +			cret = get_user_pages(current, current->mm, addr,
> +					      chunk, write, 0,
> +					      page_refs, NULL);
> +			if (cret > 0) {
> +				for (i = 0; i < cret; i++)
> +					page_cache_release(page_refs[i]);
> +				ret += cret;
> +			}
> +			if (cret < chunk)
> +				return ret ? : cret;
> +			addr += 32*PAGE_SIZE;
> +			rlen -= 32;
> +		}
> +		return ret == len ? 0 : -1;
> +	}
> +
>  	ret = get_user_pages(current, current->mm, addr,
>  			len, write, 0, NULL, NULL);
>  	if (ret < 0)
>   

This seems like a bit of unneeded complexity.  Since you've already 
changed get_user_pages, why not add a follow flag to release the page 
cache, and simply pass it to get_user_pages, instead of trying to fetch 
the page list back.  In particular, write and force arguments to 
get_user_pages look very ripe for combining into a flags field.  Sure, 
get_user_pages now has a bit more work to do, but it is already a 
monster, and I think it would keep make_page_present much cleaner.

Zach

Zach
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