Re: [PATCH 2/2] mm/zsmalloc.c: Fix race condition in zs_destroy_pool

[Minchan Kim <minchan@kernel.org>]

On Mon, Aug 05, 2019 at 10:34:41AM -0700, Henry Burns wrote:
> On Sun, Aug 4, 2019 at 9:28 PM Minchan Kim <minchan@kernel.org> wrote:
> >
> > 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?
> 
> What I am describing is a call to zsmalloc_aops->migratepage() by
> kcompactd (which can call schedule work in either
> zs_page_migrate() or zs_page_putback should the zspage become empty).
> 
> While we are migrating a page, we remove it from the class. Suppose
> zs_free() loses a race with migration. We would schedule
> async_free_zspage() to handle freeing that zspage, however we have no
> guarantee that migration has finished
> by the time we finish flush_work(&pool->work). In that case we then
> call iput(inode), and now we have a page
> pointing to a non-existent inode. (At which point something like
> kcompactd would potentially BUG() if it tries to get a page
> (from the inode) that doesn't exist anymore)
> 

True.
I totally got mixed up internal migration and external migration. :-/

> 
> >
> > 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.
> 
> We could have a pool level counter of isolated pages, and wait for
> that to finish before starting flush_work(&pool->work); However,
> that would require an atomic_long in zs_pool, and we would have to eat
> the cost of any contention over that lock. Still, it might be
> preferable to a per-class granularity.

That would be better for performance-wise but how it's significant?
Migration is not already hot path so adding a atomic variable in that path
wouldn't make noticible slow.

Rather than performance, my worry is maintainance so prefer simple and
not fragile.

> 
> >
> > 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 <henryburns@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
> > >