linux-kernel - Re: [v4][PATCH 2/6] mm: swap: make 'struct page' and swp_entry_t variants of swapcache

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Date:	Mon, 03 Jun 2013 08:55:08 -0700
From:	Dave Hansen <dave@...1.net>
To:	Minchan Kim <minchan@...nel.org>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mgorman@...e.de,
	tim.c.chen@...ux.intel.com
Subject: Re: [v4][PATCH 2/6] mm: swap: make 'struct page' and swp_entry_t
 variants of swapcache_free().

On 06/02/2013 11:13 PM, Minchan Kim wrote:
> I lost from this description.
> 
> Old behavior
> 
> delete_from_swap_cache
>         swap_address_space
>         __delete_from_swap_cache
>                 swap_address_space
> 
> 
> New behavior
> 
> delete_from_swap_cache
>         __delete_from_swap_cache
>                 swap_address_space
>                 
> So you removes a swap_address_space, not adding a extra call.
> Am I missing something?

I think I got the page->swp_entry_t lookup confused with the
page->swap_address_space lookup when I was writing the description.  The
bit that you missed is that I _added_ a page_mapping() call, which calls
swap_address_space() internally:

Old behavior:

delete_from_swap_cache
        swap_address_space
        __delete_from_swap_cache
                swap_address_space

New behavior:

delete_from_swap_cache
	page_mapping
		swap_address_space
        __delete_from_swap_cache
                swap_address_space

--

New description (last paragraph changed).  Andrew, I'll resend the
series since there are a few of these cleanups.

From: Dave Hansen <dave.hansen@...ux.intel.com>

swapcache_free() takes two arguments:

	void swapcache_free(swp_entry_t entry, struct page *page)

Most of its callers (5/7) are from error handling paths haven't even
instantiated a page, so they pass page=NULL.  Both of the callers
that call in with a 'struct page' create and pass in a temporary
swp_entry_t.

Now that we are deferring clearing page_private() until after
swapcache_free() has been called, we can just create a variant
that takes a 'struct page' and does the temporary variable in
the helper.

That leaves all the other callers doing

	swapcache_free(entry, NULL)

so create another helper for them that makes it clear that they
need only pass in a swp_entry_t.

One downside here is that delete_from_swap_cache() now calls
swap_address_space() via page_mapping() instead of calling
swap_address_space() directly.  In doing so, it removes one more
case of the swap cache code being special-cased, which is a good
thing in my book.

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