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Message-ID: <CAA_GA1crg1ngNx2MAv-fJbgKYqSKmkapZHq=8F4QcNgFja1A-w@mail.gmail.com>
Date: Mon, 19 Nov 2012 08:53:46 +0800
From: Bob Liu <lliubbo@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
sjenning@...ux.vnet.ibm.com, dan.magenheimer@...cle.com,
devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, ngupta@...are.org, minchan@...nel.org,
mgorman@...e.de, fschmaus@...il.com, andor.daam@...glemail.com,
ilendir@...glemail.com
Subject: Re: [PATCH 2/8] mm: frontswap: lazy initialization to allow tmem
backends to build/run as modules
On Sat, Nov 17, 2012 at 7:16 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Wed, 14 Nov 2012 13:57:06 -0500
> Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
>
>> From: Dan Magenheimer <dan.magenheimer@...cle.com>
>>
>> With the goal of allowing tmem backends (zcache, ramster, Xen tmem) to be
>> built/loaded as modules rather than built-in and enabled by a boot parameter,
>> this patch provides "lazy initialization", allowing backends to register to
>> frontswap even after swapon was run. Before a backend registers all calls
>> to init are recorded and the creation of tmem_pools delayed until a backend
>> registers or until a frontswap put is attempted.
>>
>>
>> ...
>>
>> --- a/mm/frontswap.c
>> +++ b/mm/frontswap.c
>> @@ -80,6 +80,18 @@ static inline void inc_frontswap_succ_stores(void) { }
>> static inline void inc_frontswap_failed_stores(void) { }
>> static inline void inc_frontswap_invalidates(void) { }
>> #endif
>> +
>> +/*
>> + * When no backend is registered all calls to init are registered and
>
> What is "init"? Spell it out fully, please.
>
I think it's frontswap_init().
swapon will call frontswap_init() and in it we need to call init
function of backends with some parameters
like swap_type.
>> + * remembered but fail to create tmem_pools. When a backend registers with
>> + * frontswap the previous calls to init are executed to create tmem_pools
>> + * and set the respective poolids.
>
> Again, seems really hacky. Why can't we just change callers so they
> call things in the correct order?
>
I don't think so, because it asynchronous.
The original idea was to make backends like zcache/tmem modularization.
So that it's more convenient and flexible to use and testing.
But currently callers like swapon only invoke frontswap_init() once,
it fail if backend not registered.
We have no way to notify swap to call frontswap_init() again when
backend registered in some random time
in future.
>> + * While no backend is registered all "puts", "gets" and "flushes" are
>> + * ignored or fail.
>> + */
>> +static DECLARE_BITMAP(need_init, MAX_SWAPFILES);
>> +static bool backend_registered __read_mostly;
>> +
>> /*
>> * Register operations for frontswap, returning previous thus allowing
>> * detection of multiple backends and possible nesting.
>> @@ -87,9 +99,19 @@ static inline void inc_frontswap_invalidates(void) { }
>> struct frontswap_ops frontswap_register_ops(struct frontswap_ops *ops)
>> {
>> struct frontswap_ops old = frontswap_ops;
>> + int i;
>>
>> frontswap_ops = *ops;
>> frontswap_enabled = true;
>> +
>> + for (i = 0; i < MAX_SWAPFILES; i++) {
>> + if (test_and_clear_bit(i, need_init))
>
> ooh, that wasn't racy ;)
>
Hmm, i agree.
Seems some lock is needed, actually i think this code only support one
backend at the same.
So it's less risky.
>> + (*frontswap_ops.init)(i);
>> + }
>> + /* We MUST have backend_registered called _after_ the frontswap_init's
>> + * have been called. Otherwise __frontswap_store might fail. */
>
> Comment makes no sense - backend_registered is not a function.
>
> Also, let's lay the comments out conventionally please:
>
> /*
> * We MUST have backend_registered called _after_ the frontswap_init's
> * have been called. Otherwise __frontswap_store might fail.
> */
>
>
>> + barrier();
>> + backend_registered = true;
>> return old;
>> }
>> EXPORT_SYMBOL(frontswap_register_ops);
>>
>> ...
>>
>> @@ -226,12 +266,15 @@ void __frontswap_invalidate_area(unsigned type)
>> {
>> struct swap_info_struct *sis = swap_info[type];
>>
>> - BUG_ON(sis == NULL);
>> - if (sis->frontswap_map == NULL)
>> - return;
>> - frontswap_ops.invalidate_area(type);
>> - atomic_set(&sis->frontswap_pages, 0);
>> - memset(sis->frontswap_map, 0, sis->max / sizeof(long));
>> + if (backend_registered) {
>> + BUG_ON(sis == NULL);
>> + if (sis->frontswap_map == NULL)
>> + return;
>> + (*frontswap_ops.invalidate_area)(type);
>> + atomic_set(&sis->frontswap_pages, 0);
>> + memset(sis->frontswap_map, 0, sis->max / sizeof(long));
>> + }
>> + clear_bit(type, need_init);
>> }
>> EXPORT_SYMBOL(__frontswap_invalidate_area);
>>
>> @@ -364,6 +407,9 @@ static int __init init_frontswap(void)
>> debugfs_create_u64("invalidates", S_IRUGO,
>> root, &frontswap_invalidates);
>> #endif
>> + bitmap_zero(need_init, MAX_SWAPFILES);
>
> unneeded?
>
>> + frontswap_enabled = 1;
>> return 0;
>> }
>>
>> ...
>>
--
Thanks,
--Bob
--
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