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SubjectRe: [PATCHv2] frontswap: allow multiple backends
On Mon,  1 Jun 2015 10:22:24 -0400 Dan Streetman <ddstreet@ieee.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.
>
> ...
>
> -struct frontswap_ops *frontswap_register_ops(struct frontswap_ops *ops)
> +void frontswap_register_ops(struct frontswap_ops *ops)
> {
> - struct frontswap_ops *old = frontswap_ops;
> - int i;
> -
> - for (i = 0; i < MAX_SWAPFILES; i++) {
> - if (test_and_clear_bit(i, need_init)) {
> - struct swap_info_struct *sis = swap_info[i];
> - /* __frontswap_init _should_ have set it! */
> - if (!sis->frontswap_map)
> - return ERR_PTR(-EINVAL);
> - ops->init(i);
> - }
> + DECLARE_BITMAP(a, MAX_SWAPFILES);
> + DECLARE_BITMAP(b, MAX_SWAPFILES);
> + struct swap_info_struct *si;
> + unsigned int i;
> +
> + spin_lock(&swap_lock);
> + plist_for_each_entry(si, &swap_active_head, list) {
> + if (!WARN_ON(!si->frontswap_map))
> + set_bit(si->type, a);

umm, DECLARE_BITMAP() doesn't initialise the storage. Either this
patch wasn't tested very well or you should buy me a lottery ticket!

> }
> - /*
> - * We MUST have frontswap_ops set _after_ the frontswap_init's
> - * have been called. Otherwise __frontswap_store might fail. Hence
> - * the barrier to make sure compiler does not re-order us.
> + spin_unlock(&swap_lock);
> +
> + /* the new ops needs to know the currently active swap devices */
> + for_each_set_bit(i, a, MAX_SWAPFILES)
> + ops->init(i);
> +
> + /* setting frontswap_ops must happen after the ops->init() calls
> + * above; cmpxchg implies smp_mb() which will ensure the init is
> + * complete at this point
> + */

Like this, please:

/*
* Setting ...

and sentences start with capital letters ;)




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