Re: [PATCH 2/5] mm: frontswap: lazy initialization to allow tmembackends to build/run as modules

[Konrad Rzeszutek Wilk]

On Wed, Oct 31, 2012 at 12:05:32PM -0500, Seth Jennings wrote:
> On 10/31/2012 10:07 AM, Dan Magenheimer wrote:
> > 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.
> > 
> > Signed-off-by: Stefan Hengelein <ilendir@xxxxxxxxxxxxxx>
> > Signed-off-by: Florian Schmaus <fschmaus@xxxxxxxxx>
> > Signed-off-by: Andor Daam <andor.daam@xxxxxxxxxxxxxx>
> > Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
> > ---
> >  include/linux/frontswap.h |    1 +
> >  mm/frontswap.c            |   70 +++++++++++++++++++++++++++++++++++++++-----
> >  2 files changed, 63 insertions(+), 8 deletions(-)
> > 
> > diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
> > index 3044254..ef6ada6 100644
> > --- a/include/linux/frontswap.h
> > +++ b/include/linux/frontswap.h
> > @@ -23,6 +23,7 @@ extern void frontswap_writethrough(bool);
> >  extern void frontswap_tmem_exclusive_gets(bool);
> > 
> >  extern void __frontswap_init(unsigned type);
> > +#define FRONTSWAP_HAS_LAZY_INIT
> >  extern int __frontswap_store(struct page *page);
> >  extern int __frontswap_load(struct page *page);
> >  extern void __frontswap_invalidate_page(unsigned, pgoff_t);
> > diff --git a/mm/frontswap.c b/mm/frontswap.c
> > index 2890e67..523a19b 100644
> > --- a/mm/frontswap.c
> > +++ b/mm/frontswap.c
> > @@ -80,6 +80,19 @@ 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
> > + * 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.
> > + * While no backend is registered all "puts", "gets" and "flushes" are
> > + * ignored or fail.
> > + */
> > +#define MAX_INITIALIZABLE_SD 32
> 
> MAX_INITIALIZABLE_SD should just be MAX_SWAPFILES
> 
> > +static int sds[MAX_INITIALIZABLE_SD];
> 
> Rather than store and array of enabled types indexed by type, why not
> an array of booleans indexed by type.  Or a bitfield if you really
> want to save space.
> 
> > +static int backend_registered;
> 
> (backend_registered) is equivalent to checking (frontswap_ops != NULL)
> right?
> 
> > +
> >  /*
> >   * Register operations for frontswap, returning previous thus allowing
> >   * detection of multiple backends and possible nesting.
> > @@ -87,9 +100,16 @@ 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;
> > +
> > +	backend_registered = 1;
> > +	for (i = 0; i < MAX_INITIALIZABLE_SD; i++) {
> > +		if (sds[i] != -1)
> > +			(*frontswap_ops.init)(sds[i]);
> > +	}
> >  	return old;
> >  }
> >  EXPORT_SYMBOL(frontswap_register_ops);
> > @@ -122,7 +142,10 @@ void __frontswap_init(unsigned type)
> >  	BUG_ON(sis == NULL);
> >  	if (sis->frontswap_map == NULL)
> >  		return;
> > -	frontswap_ops.init(type);
> > +	if (backend_registered) {
> > +		(*frontswap_ops.init)(type);
> > +		sds[type] = type;
> 
> This is weird, storing the type in an array indexed by type.  Hence my
> suggestion above about an array of booleans or a bitfield.
> 
> > +	}
> >  }
> >  EXPORT_SYMBOL(__frontswap_init);
> > 
> > @@ -147,10 +170,20 @@ int __frontswap_store(struct page *page)
> >  	struct swap_info_struct *sis = swap_info[type];
> >  	pgoff_t offset = swp_offset(entry);
> > 
> > +	if (!backend_registered) {
> > +		inc_frontswap_failed_stores();
> > +		return ret;
> > +	}
> > +
> >  	BUG_ON(!PageLocked(page));
> >  	BUG_ON(sis == NULL);
> >  	if (frontswap_test(sis, offset))
> >  		dup = 1;
> > +	if (type < MAX_INITIALIZABLE_SD && sds[type] == -1) {
> > +		/* lazy init call to handle post-boot insmod backends*/
> > +		(*frontswap_ops.init)(type);
> > +		sds[type] = type;
> > +	}
> >  	ret = frontswap_ops.store(type, offset, page);
> >  	if (ret == 0) {
> >  		frontswap_set(sis, offset);
> > @@ -186,6 +219,9 @@ int __frontswap_load(struct page *page)
> >  	struct swap_info_struct *sis = swap_info[type];
> >  	pgoff_t offset = swp_offset(entry);
> > 
> > +	if (!backend_registered)
> > +		return ret;
> > +
> >  	BUG_ON(!PageLocked(page));
> >  	BUG_ON(sis == NULL);
> >  	if (frontswap_test(sis, offset))
> > @@ -209,6 +245,9 @@ void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
> >  {
> >  	struct swap_info_struct *sis = swap_info[type];
> > 
> > +	if (!backend_registered)
> > +		return;
> > +
> >  	BUG_ON(sis == NULL);
> >  	if (frontswap_test(sis, offset)) {
> >  		frontswap_ops.invalidate_page(type, offset);
> > @@ -225,13 +264,23 @@ EXPORT_SYMBOL(__frontswap_invalidate_page);
> >  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));
> > +	int i;
> > +
> > +	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));
> > +	} else {
> > +		for (i = 0; i < MAX_INITIALIZABLE_SD; i++) {
> > +			if (sds[i] == type) {
> 
> Additional weirdness with sds.  It seems this whole for loop could
> just be reduced to:
> 
> sds[type] = -1;


How does this look? (I hadn't actually tested it, but did compile test
it)