Hi Henry, On Thu, Aug 01, 2019 at 06:53:32PM -0700, Henry Burns wrote: > In zs_destroy_pool() we call flush_work(&pool->free_work). However, we > have no guarantee that migration isn't happening in the background > at that time. > > Since migration can't directly free pages, it relies on free_work > being scheduled to free the pages. But there's nothing preventing an > in-progress migrate from queuing the work *after* > zs_unregister_migration() has called flush_work(). Which would mean > pages still pointing at the inode when we free it.
We already unregister shrinker so there is no upcoming async free call via shrinker so the only concern is zs_compact API direct call from the user. Is that what what you desribe from the description? If so, can't we add a flag to indicate destroy of the pool and global counter to indicate how many of zs_compact was nested? So, zs_unregister_migration in zs_destroy_pool can set the flag to prevent upcoming zs_compact call and wait until the global counter will be zero. Once it's done, finally flush the work. My point is it's not a per-class granuarity but global. Thanks. > > Since we know at destroy time all objects should be free, no new > migrations can come in (since zs_page_isolate() fails for fully-free > zspages). This means it is sufficient to track a "# isolated zspages" > count by class, and have the destroy logic ensure all such pages have > drained before proceeding. Keeping that state under the class > spinlock keeps the logic straightforward. > > Signed-off-by: Henry Burns <henrybu...@google.com> > --- > mm/zsmalloc.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++--- > 1 file changed, 65 insertions(+), 3 deletions(-) > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c > index efa660a87787..1f16ed4d6a13 100644 > --- a/mm/zsmalloc.c > +++ b/mm/zsmalloc.c > @@ -53,6 +53,7 @@ > #include <linux/zpool.h> > #include <linux/mount.h> > #include <linux/migrate.h> > +#include <linux/wait.h> > #include <linux/pagemap.h> > #include <linux/fs.h> > > @@ -206,6 +207,10 @@ struct size_class { > int objs_per_zspage; > /* Number of PAGE_SIZE sized pages to combine to form a 'zspage' */ > int pages_per_zspage; > +#ifdef CONFIG_COMPACTION > + /* Number of zspages currently isolated by compaction */ > + int isolated; > +#endif > > unsigned int index; > struct zs_size_stat stats; > @@ -267,6 +272,8 @@ struct zs_pool { > #ifdef CONFIG_COMPACTION > struct inode *inode; > struct work_struct free_work; > + /* A workqueue for when migration races with async_free_zspage() */ > + struct wait_queue_head migration_wait; > #endif > }; > > @@ -1917,6 +1924,21 @@ static void putback_zspage_deferred(struct zs_pool > *pool, > > } > > +static inline void zs_class_dec_isolated(struct zs_pool *pool, > + struct size_class *class) > +{ > + assert_spin_locked(&class->lock); > + VM_BUG_ON(class->isolated <= 0); > + class->isolated--; > + /* > + * There's no possibility of racing, since wait_for_isolated_drain() > + * checks the isolated count under &class->lock after enqueuing > + * on migration_wait. > + */ > + if (class->isolated == 0 && waitqueue_active(&pool->migration_wait)) > + wake_up_all(&pool->migration_wait); > +} > + > static void replace_sub_page(struct size_class *class, struct zspage *zspage, > struct page *newpage, struct page *oldpage) > { > @@ -1986,6 +2008,7 @@ static bool zs_page_isolate(struct page *page, > isolate_mode_t mode) > */ > if (!list_empty(&zspage->list) && !is_zspage_isolated(zspage)) { > get_zspage_mapping(zspage, &class_idx, &fullness); > + class->isolated++; > remove_zspage(class, zspage, fullness); > } > > @@ -2085,8 +2108,14 @@ static int zs_page_migrate(struct address_space > *mapping, struct page *newpage, > * Page migration is done so let's putback isolated zspage to > * the list if @page is final isolated subpage in the zspage. > */ > - if (!is_zspage_isolated(zspage)) > + if (!is_zspage_isolated(zspage)) { > + /* > + * We still hold the class lock while all of this is happening, > + * so we cannot race with zs_destroy_pool() > + */ > putback_zspage_deferred(pool, class, zspage); > + zs_class_dec_isolated(pool, class); > + } > > reset_page(page); > put_page(page); > @@ -2131,9 +2160,11 @@ static void zs_page_putback(struct page *page) > > spin_lock(&class->lock); > dec_zspage_isolation(zspage); > - if (!is_zspage_isolated(zspage)) > - putback_zspage_deferred(pool, class, zspage); > > + if (!is_zspage_isolated(zspage)) { > + putback_zspage_deferred(pool, class, zspage); > + zs_class_dec_isolated(pool, class); > + } > spin_unlock(&class->lock); > } > > @@ -2156,8 +2187,36 @@ static int zs_register_migration(struct zs_pool *pool) > return 0; > } > > +static bool class_isolated_are_drained(struct size_class *class) > +{ > + bool ret; > + > + spin_lock(&class->lock); > + ret = class->isolated == 0; > + spin_unlock(&class->lock); > + return ret; > +} > + > +/* Function for resolving migration */ > +static void wait_for_isolated_drain(struct zs_pool *pool) > +{ > + int i; > + > + /* > + * We're in the process of destroying the pool, so there are no > + * active allocations. zs_page_isolate() fails for completely free > + * zspages, so we need only wait for each size_class's isolated > + * count to hit zero. > + */ > + for (i = 0; i < ZS_SIZE_CLASSES; i++) { > + wait_event(pool->migration_wait, > + class_isolated_are_drained(pool->size_class[i])); > + } > +} > + > static void zs_unregister_migration(struct zs_pool *pool) > { > + wait_for_isolated_drain(pool); /* This can block */ > flush_work(&pool->free_work); > iput(pool->inode); > } > @@ -2401,6 +2460,8 @@ struct zs_pool *zs_create_pool(const char *name) > if (!pool->name) > goto err; > > + init_waitqueue_head(&pool->migration_wait); > + > if (create_cache(pool)) > goto err; > > @@ -2466,6 +2527,7 @@ struct zs_pool *zs_create_pool(const char *name) > class->index = i; > class->pages_per_zspage = pages_per_zspage; > class->objs_per_zspage = objs_per_zspage; > + class->isolated = 0; > spin_lock_init(&class->lock); > pool->size_class[i] = class; > for (fullness = ZS_EMPTY; fullness < NR_ZS_FULLNESS; > -- > 2.22.0.770.g0f2c4a37fd-goog >