[Xen-devel] [PATCH] xen/gnttab: add deferred freeing logic

["Jan Beulich" <JBeulich@xxxxxxxx>]

Rather than just leaking pages that can't be freed at the point where
access permission for the backend domain gets revoked, put them on a
list and run a timer to (infrequently) retry freeing them. (This can
particularly happen when unloading a frontend driver when devices are
still present, and the backend still has them in non-closed state or
hasn't finished closing them yet.)

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

---
 drivers/xen/grant-table.c |  106 +++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 96 insertions(+), 10 deletions(-)

--- 3.4-rc1/drivers/xen/grant-table.c
+++ 3.4-rc1-xen-gnttab-deferred-end-access/drivers/xen/grant-table.c
@@ -426,10 +426,8 @@ static int gnttab_end_foreign_access_ref
        nflags = *pflags;
        do {
                flags = nflags;
-               if (flags & (GTF_reading|GTF_writing)) {
-                       printk(KERN_ALERT "WARNING: g.e. still in use!\n");
+               if (flags & (GTF_reading|GTF_writing))
                        return 0;
-               }
        } while ((nflags = sync_cmpxchg(pflags, flags, 0)) != flags);
 
        return 1;
@@ -458,12 +456,103 @@ static int gnttab_end_foreign_access_ref
        return 1;
 }
 
-int gnttab_end_foreign_access_ref(grant_ref_t ref, int readonly)
+static inline int _gnttab_end_foreign_access_ref(grant_ref_t ref, int readonly)
 {
        return gnttab_interface->end_foreign_access_ref(ref, readonly);
 }
+
+int gnttab_end_foreign_access_ref(grant_ref_t ref, int readonly)
+{
+       if (_gnttab_end_foreign_access_ref(ref, readonly))
+               return 1;
+       pr_warn("WARNING: g.e. %#x still in use!\n", ref);
+       return 0;
+}
 EXPORT_SYMBOL_GPL(gnttab_end_foreign_access_ref);
 
+struct deferred_entry {
+       struct list_head list;
+       grant_ref_t ref;
+       bool ro;
+       uint16_t warn_delay;
+       struct page *page;
+};
+static LIST_HEAD(deferred_list);
+static void gnttab_handle_deferred(unsigned long);
+static DEFINE_TIMER(deferred_timer, gnttab_handle_deferred, 0, 0);
+
+static void gnttab_handle_deferred(unsigned long unused)
+{
+       unsigned int nr = 10;
+       struct deferred_entry *first = NULL;
+       unsigned long flags;
+
+       spin_lock_irqsave(&gnttab_list_lock, flags);
+       while (nr--) {
+               struct deferred_entry *entry
+                       = list_first_entry(&deferred_list,
+                                          struct deferred_entry, list);
+
+               if (entry == first)
+                       break;
+               list_del(&entry->list);
+               spin_unlock_irqrestore(&gnttab_list_lock, flags);
+               if (_gnttab_end_foreign_access_ref(entry->ref, entry->ro)) {
+                       put_free_entry(entry->ref);
+                       if (entry->page) {
+                               pr_debug("freeing g.e. %#x (pfn %#lx)\n",
+                                        entry->ref, page_to_pfn(entry->page));
+                               __free_page(entry->page);
+                       } else
+                               pr_info("freeing g.e. %#x\n", entry->ref);
+                       kfree(entry);
+                       entry = NULL;
+               } else {
+                       if (!--entry->warn_delay)
+                               pr_info("g.e. %#x still pending\n",
+                                       entry->ref);
+                       if (!first)
+                               first = entry;
+               }
+               spin_lock_irqsave(&gnttab_list_lock, flags);
+               if (entry)
+                       list_add_tail(&entry->list, &deferred_list);
+               else if (list_empty(&deferred_list))
+                       break;
+       }
+       if (!list_empty(&deferred_list) && !timer_pending(&deferred_timer)) {
+               deferred_timer.expires = jiffies + HZ;
+               add_timer(&deferred_timer);
+       }
+       spin_unlock_irqrestore(&gnttab_list_lock, flags);
+}
+
+static void gnttab_add_deferred(grant_ref_t ref, bool readonly,
+                               struct page *page)
+{
+       struct deferred_entry *entry = kmalloc(sizeof(*entry), GFP_ATOMIC);
+       const char *what = KERN_WARNING "leaking";
+
+       if (entry) {
+               unsigned long flags;
+
+               entry->ref = ref;
+               entry->ro = readonly;
+               entry->page = page;
+               entry->warn_delay = 60;
+               spin_lock_irqsave(&gnttab_list_lock, flags);
+               list_add_tail(&entry->list, &deferred_list);
+               if (!timer_pending(&deferred_timer)) {
+                       deferred_timer.expires = jiffies + HZ;
+                       add_timer(&deferred_timer);
+               }
+               spin_unlock_irqrestore(&gnttab_list_lock, flags);
+               what = KERN_DEBUG "deferring";
+       }
+       printk("%s g.e. %#x (pfn %#lx)\n",
+              what, ref, page ? page_to_pfn(page) : -1);
+}
+
 void gnttab_end_foreign_access(grant_ref_t ref, int readonly,
                               unsigned long page)
 {
@@ -471,12 +560,9 @@ void gnttab_end_foreign_access(grant_ref
                put_free_entry(ref);
                if (page != 0)
                        free_page(page);
-       } else {
-               /* XXX This needs to be fixed so that the ref and page are
-                  placed on a list to be freed up later. */
-               printk(KERN_WARNING
-                      "WARNING: leaking g.e. and page still in use!\n");
-       }
+       } else
+               gnttab_add_deferred(ref, readonly,
+                                   page ? virt_to_page(page) : NULL);
 }
 EXPORT_SYMBOL_GPL(gnttab_end_foreign_access);
 



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