linux-kernel - Re: [PATCHv3] frontswap: allow multiple backends

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Date:	Tue, 2 Jun 2015 15:19:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dan Streetman <ddstreet@...e.org>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCHv3] frontswap: allow multiple backends

On Tue,  2 Jun 2015 18:08:46 -0400 Dan Streetman <ddstreet@...e.org> wrote:

> Change frontswap single pointer to a singly linked list of frontswap
> implementations.  Update Xen tmem implementation as register no longer
> returns anything.
> 
> Frontswap only keeps track of a single implementation; any implementation
> that registers second (or later) will replace the previously registered
> implementation, and gets a pointer to the previous implementation that
> the new implementation is expected to pass all frontswap functions to
> if it can't handle the function itself.  However that method doesn't
> really make much sense, as passing that work on to every implementation
> adds unnecessary work to implementations; instead, frontswap should
> simply keep a list of all registered implementations and try each
> implementation for any function.  Most importantly, neither of the
> two currently existing frontswap implementations in the kernel actually
> do anything with any previous frontswap implementation that they
> replace when registering.
> 
> This allows frontswap to successfully manage multiple implementations
> by keeping a list of them all.
> 

offtopic trivia: this

--- a/mm/frontswap.c~frontswap-allow-multiple-backends-fix
+++ a/mm/frontswap.c
@@ -111,14 +111,11 @@ static inline void inc_frontswap_invalid
  */
 void frontswap_register_ops(struct frontswap_ops *ops)
 {
-	DECLARE_BITMAP(a, MAX_SWAPFILES);
-	DECLARE_BITMAP(b, MAX_SWAPFILES);
+	DECLARE_BITMAP(a, MAX_SWAPFILES) = { };
+	DECLARE_BITMAP(b, MAX_SWAPFILES) = { };
 	struct swap_info_struct *si;
 	unsigned int i;
 
-	bitmap_zero(a, MAX_SWAPFILES);
-	bitmap_zero(b, MAX_SWAPFILES);
-
 	spin_lock(&swap_lock);
 	plist_for_each_entry(si, &swap_active_head, list) {
 		if (!WARN_ON(!si->frontswap_map))

saves 64 bytes of text with my gcc.


It shouldn't be open-coded here, but a new macro in bitmap.h could be
useful, assuming it's a win for other sizes of bitmaps.
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