Re: [PATCH 02/15] mm: frontswap: lazy initialization to allow tmem backends to build/run as modules

[Ric Mason <ric.masonn@gmail.com>]

On Mon, 2013-02-04 at 13:53 +0800, Bob Liu wrote:
> On Sun, Feb 3, 2013 at 3:07 PM, Ric Mason <ric.masonn@gmail.com> wrote:
> > Hi Konrad,
> > On Fri, 2013-02-01 at 15:22 -0500, Konrad Rzeszutek Wilk wrote:
> >> From: Dan Magenheimer <dan.magenheimer@oracle.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.
> >
> > Do you mean __frontswap_store? It seems that just add fail count if
> > backend doesn't register, why you said that the creation of tmem_pools
> > will delay until this time?
> >
> >>
> >> Signed-off-by: Stefan Hengelein <ilendir@googlemail.com>
> >> Signed-off-by: Florian Schmaus <fschmaus@gmail.com>
> >> Signed-off-by: Andor Daam <andor.daam@googlemail.com>
> >> Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
> >> [v1: Fixes per Seth Jennings suggestions]
> >> [v2: Removed FRONTSWAP_HAS_.. ]
> >> [v3: Fix up per Bob Liu <lliubbo@gmail.com> recommendations]
> >> [v4: Fix up per Andrew's comments]
> >> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> >> ---
> >>  mm/frontswap.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++-------
> >>  1 file changed, 85 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/mm/frontswap.c b/mm/frontswap.c
> >> index 2890e67..c05a9db 100644
> >> --- a/mm/frontswap.c
> >> +++ b/mm/frontswap.c
> >> @@ -80,6 +80,46 @@ static inline void inc_frontswap_succ_stores(void) { }
> >>  static inline void inc_frontswap_failed_stores(void) { }
> >>  static inline void inc_frontswap_invalidates(void) { }
> >>  #endif
> >> +
> >> +/*
> >> + * Due to the asynchronous nature of the backends loading potentially
> >> + * _after_ the swap system has been activated, we have chokepoints
> >> + * on all frontswap functions to not call the backend until the backend
> >> + * has registered.
> >> + *
> >> + * Specifically when no backend is registered (nobody called
> >> + * frontswap_register_ops) all calls to frontswap_init (which is done via
> >> + * swapon -> enable_swap_info -> frontswap_init) are registered and remembered
> >> + * (via the setting of need_init bitmap) but fail to create tmem_pools. When a
> >> + * backend registers with frontswap at some later point the previous
> >> + * calls to frontswap_init are executed (by iterating over the need_init
> >> + * bitmap) to create tmem_pools and set the respective poolids. All of that is
> >> + * guarded by us using atomic bit operations on the 'need_init' bitmap.
> >> + *
> >> + * This would not guards us against the user deciding to call swapoff right as
> >> + * we are calling the backend to initialize (so swapon is in action).
> >> + * Fortunatly for us, the swapon_mutex has been taked by the callee so we are
> >> + * OK. The other scenario where calls to frontswap_store (called via
> >> + * swap_writepage) is racing with frontswap_invalidate_area (called via
> >> + * swapoff) is again guarded by the swap subsystem.
> >> + *
> >> + * While no backend is registered all calls to frontswap_[store|load|
> >> + * invalidate_area|invalidate_page] are ignored or fail.
> >> + *
> >> + * The time between the backend being registered and the swap file system
> >> + * calling the backend (via the frontswap_* functions) is indeterminate as
> >> + * backend_registered is not atomic_t (or a value guarded by a spinlock).
> >> + * That is OK as we are comfortable missing some of these calls to the newly
> >> + * registered backend.
> >> + *
> >> + * Obviously the opposite (unloading the backend) must be done after all
> >> + * the frontswap_[store|load|invalidate_area|invalidate_page] start
> >> + * ignorning or failing the requests - at which point backend_registered
> >> + * would have to be made in some fashion atomic.
> >> + */
> >> +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 +127,22 @@ 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))
> >> +                     (*frontswap_ops.init)(i);
> >> +     }
> >> +     /*
> >> +      * We MUST have backend_registered 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.
> >> +      */
> >> +     barrier();
> >> +     backend_registered = true;
> >>       return old;
> >>  }
> >>  EXPORT_SYMBOL(frontswap_register_ops);
> >> @@ -119,10 +172,17 @@ void __frontswap_init(unsigned type)
> >>  {
> >>       struct swap_info_struct *sis = swap_info[type];
> >>
> >> -     BUG_ON(sis == NULL);
> >> -     if (sis->frontswap_map == NULL)
> >> -             return;
> >> -     frontswap_ops.init(type);
> >> +     if (backend_registered) {
> >> +             BUG_ON(sis == NULL);
> >> +             if (sis->frontswap_map == NULL)
> >> +                     return;
> >> +             (*frontswap_ops.init)(type);
> >> +     }
> >> +     else {
> >> +             BUG_ON(type > MAX_SWAPFILES);
> >> +             set_bit(type, need_init);
> >> +     }
> >> +
> >>  }
> >>  EXPORT_SYMBOL(__frontswap_init);
> >>
> >> @@ -147,6 +207,11 @@ 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))
> >> @@ -186,6 +251,9 @@ 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 +277,9 @@ 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);
> >> @@ -226,12 +297,15 @@ 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 +438,7 @@ static int __init init_frontswap(void)
> >>       debugfs_create_u64("invalidates", S_IRUGO,
> >>                               root, &frontswap_invalidates);
> >>  #endif
> >> +     frontswap_enabled = 1;
> >
> > Why has this change?
> >
> 
> If don't set frontswap_enabled to 1,  frontswap_init() will return
> without record the swap type.
> 

Yup, thanks.



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