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From:   Demi Marie Obenour <demi@invisiblethingslab.com>
To:     Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        Jan Beulich <JBeulich@suse.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc:     Demi Marie Obenour <demi@invisiblethingslab.com>,
        Xen developer discussion <xen-devel@lists.xenproject.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>, stable@vger.kernel.org
Subject: [PATCH v3] xen: speed up grant-table reclaim
Date:   Sat, 24 Jun 2023 16:56:22 -0400
Message-ID: <20230624205624.1817-1-demi@invisiblethingslab.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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.

To partially solve this problem, make the number of entries that the VM
will attempt to free at each iteration tunable.  The default is still
10, but it can be overridden at compile-time (via Kconfig), boot-time
(via a kernel command-line option), or runtime (via sysfs).

This is Cc: stable because (when combined with appropriate userspace
changes) it fixes a severe performance and stability problem for Qubes
OS users.

Cc: stable@vger.kernel.org
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
 drivers/xen/grant-table.c | 40 ++++++++++++++++++++++++++++-----------
 2 files changed, 41 insertions(+), 11 deletions(-)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index e1ec725c2819d4d5dede063eb00d86a6d52944c0..fa666aa6abc3e786dddc94f895641505ec0b23d8 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -498,14 +498,20 @@ static LIST_HEAD(deferred_list);
 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 = 10;
+
 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);
@@ -515,10 +521,13 @@ static void gnttab_handle_deferred(struct timer_list *unused)
 		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 {
@@ -530,21 +539,22 @@ static void gnttab_handle_deferred(struct timer_list *unused)
 		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)
 {
 	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) {
@@ -567,12 +577,20 @@ static void gnttab_add_deferred(grant_ref_t ref, struct page *page)
 			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);
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab