When a grant entry is still in use by the remote domain, Linux must put it on a deferred list. Normally, this list is very short, because the PV network and block protocols expect the backend to unmap the grant first. However, Qubes OS's GUI protocol is subject to the constraints of the X Window System, and as such winds up with the frontend unmapping the window first. As a result, the list can grow very large, resulting in a massive memory leak and eventual VM freeze.
Fix this problem by bumping the number of entries that the VM will attempt to free at each iteration to 10000. This is an ugly hack that may well make a denial of service easier, but for Qubes OS that is less bad than the problem Qubes OS users are facing today. There really needs to be a way for a frontend to be notified when the backend has unmapped the grants. Additionally, a module parameter is provided to allow tuning the reclaim speed.
The code previously used printk(KERN_DEBUG) whenever it had to defer reclaiming a page because the grant was still mapped. This resulted in a large volume of log messages that bothered users. Use pr_debug instead, which suppresses the messages by default. Developers can enable them using the dynamic debug mechanism.
Fixes: QubesOS/qubes-issues#7410 (memory leak) Fixes: QubesOS/qubes-issues#7359 (excessive logging) Fixes: 569ca5b3f94c ("xen/gnttab: add deferred freeing logic") Cc: stable@vger.kernel.org Signed-off-by: Demi Marie Obenour demi@invisiblethingslab.com --- Anyone have suggestions for improving the grant mechanism? Argo isn't a good option, as in the GUI protocol there are substantial performance wins to be had by using true shared memory. Resending as I forgot the Signed-off-by on the first submission. Sorry about that.
drivers/xen/grant-table.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c index 5c83d41..2c2faa7 100644 --- a/drivers/xen/grant-table.c +++ b/drivers/xen/grant-table.c @@ -355,14 +355,20 @@ static void gnttab_handle_deferred(struct timer_list *); static DEFINE_TIMER(deferred_timer, gnttab_handle_deferred);
+static atomic64_t deferred_count; +static atomic64_t leaked_count; +static unsigned int free_per_iteration = 10000; + static void gnttab_handle_deferred(struct timer_list *unused) { - unsigned int nr = 10; + unsigned int nr = READ_ONCE(free_per_iteration); + const bool ignore_limit = nr == 0; struct deferred_entry *first = NULL; unsigned long flags; + size_t freed = 0;
spin_lock_irqsave(&gnttab_list_lock, flags); - while (nr--) { + while ((ignore_limit || nr--) && !list_empty(&deferred_list)) { struct deferred_entry *entry = list_first_entry(&deferred_list, struct deferred_entry, list); @@ -372,10 +378,13 @@ list_del(&entry->list); spin_unlock_irqrestore(&gnttab_list_lock, flags); if (_gnttab_end_foreign_access_ref(entry->ref)) { + uint64_t ret = atomic64_sub_return(1, &deferred_count); put_free_entry(entry->ref); - pr_debug("freeing g.e. %#x (pfn %#lx)\n", - entry->ref, page_to_pfn(entry->page)); + pr_debug("freeing g.e. %#x (pfn %#lx), %llu remaining\n", + entry->ref, page_to_pfn(entry->page), + (unsigned long long)ret); put_page(entry->page); + freed++; kfree(entry); entry = NULL; } else { @@ -387,14 +396,15 @@ 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)) { + if (list_empty(&deferred_list)) + WARN_ON(atomic64_read(&deferred_count)); + else if (!timer_pending(&deferred_timer)) { deferred_timer.expires = jiffies + HZ; add_timer(&deferred_timer); } spin_unlock_irqrestore(&gnttab_list_lock, flags); + pr_debug("Freed %zu references", freed); }
static void gnttab_add_deferred(grant_ref_t ref, struct page *page) @@ -402,7 +412,7 @@ { struct deferred_entry *entry; gfp_t gfp = (in_atomic() || irqs_disabled()) ? GFP_ATOMIC : GFP_KERNEL; - const char *what = KERN_WARNING "leaking"; + uint64_t leaked, deferred;
entry = kmalloc(sizeof(*entry), gfp); if (!page) { @@ -426,12 +436,20 @@ add_timer(&deferred_timer); } spin_unlock_irqrestore(&gnttab_list_lock, flags); - what = KERN_DEBUG "deferring"; + deferred = atomic64_add_return(1, &deferred_count); + leaked = atomic64_read(&leaked_count); + pr_debug("deferring g.e. %#x (pfn %#lx) (total deferred %llu, total leaked %llu)\n", + ref, page ? page_to_pfn(page) : -1, deferred, leaked); + } else { + deferred = atomic64_read(&deferred_count); + leaked = atomic64_add_return(1, &leaked_count); + pr_warn("leaking g.e. %#x (pfn %#lx) (total deferred %llu, total leaked %llu)\n", + ref, page ? page_to_pfn(page) : -1, deferred, leaked); } - printk("%s g.e. %#x (pfn %#lx)\n", - what, ref, page ? page_to_pfn(page) : -1); }
+module_param(free_per_iteration, uint, 0600); + int gnttab_try_end_foreign_access(grant_ref_t ref) { int ret = _gnttab_end_foreign_access_ref(ref);