Re: [Xen-devel] [PATCHv3] frontswap: allow multiple backends

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

On Tue,  2 Jun 2015 18:08:46 -0400 Dan Streetman <ddstreet@xxxxxxxx> 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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