linux-kernel - Re: [RFC PATCH] LTTng instrumentation mm (using page_to

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Date:	Wed, 28 Nov 2007 22:25:52 -0800
From:	Dave Hansen <haveblue@...ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, mbligh@...gle.com
Subject: Re: [RFC PATCH] LTTng instrumentation mm (using page_to_pfn)

On Wed, 2007-11-28 at 21:34 -0500, Mathieu Desnoyers wrote:
> Before I start digging deeper in checking whether it is already
> instrumented by the fs instrumentation (and would therefore be
> redundant), is there a particular data structure from mm/ that you
> suggest taking the swap file number and location in swap from ?

page_private() at this point stores a swp_entry_t.  There are swp_type()
and swp_offset() helpers to decode the two bits you need after you've
turned page_private() into a swp_entry_t.  See how get_swap_bio()
creates a temporary swp_entry_t from the page_private() passed into it,
then uses swp_type/offset() on it?

I don't know if there is some history behind it, but it doesn't make a
whole ton of sense to me to be passing page_private(page) into
get_swap_bio() (which happens from its only two call sites).  It just
kinda obfuscates where 'index' came from.

It think we probably could just be doing

	swp_entry_t entry = { .val = page_private(page), };

in get_swap_bio() and not passing page_private().  We have the page in
there already, so we don't need to pass a derived value like
page_private().  At the least, it'll save some clutter in the function
declaration.  

Or, make a helper:

static swp_entry_t page_swp_entry(struct page *page)
{
	swp_entry_t entry;
	VM_BUG_ON(!PageSwapCache(page));
	entry.val = page_private(page);
	return entry;
}

I see at least 4 call sites that could use this.  The try_to_unmap_one()
caller would trip over the debug check, so you'd have to move the call
inside of the if(PageSwapCache(page)) statement.

-- Dave

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