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charset="utf-8" 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 --- drivers/xen/grant-table.c | 40 ++++++++++++++++++++++++++++----------- 1 file changed, 29 insertions(+), 11 deletions(-) Changes since v2: - use atomic_inc_return(x) and atomic_dec_return(x) instead of atomic_add_return(1, x) and atomic_sub_return(1, x) respectively. - move module_param macro closer to the definition of free_per_iteration. - add blank line between declarations and statements. Changes since v1: - drop setting default via Kconfig diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c index e1ec725c2819d4d5dede063eb00d86a6d52944c0..f13c3b76ad1eb7110e2a2981e9f= a4e504174e431 100644 --- a/drivers/xen/grant-table.c +++ b/drivers/xen/grant-table.c @@ -498,14 +498,21 @@ static LIST_HEAD(deferred_list); static void gnttab_handle_deferred(struct timer_list *); static DEFINE_TIMER(deferred_timer, gnttab_handle_deferred); =20 +static atomic64_t deferred_count; +static atomic64_t leaked_count; +static unsigned int free_per_iteration =3D 10; +module_param(free_per_iteration, uint, 0600); + static void gnttab_handle_deferred(struct timer_list *unused) { - unsigned int nr =3D 10; + unsigned int nr =3D READ_ONCE(free_per_iteration); + const bool ignore_limit =3D nr =3D=3D 0; struct deferred_entry *first =3D NULL; unsigned long flags; + size_t freed =3D 0; =20 spin_lock_irqsave(&gnttab_list_lock, flags); - while (nr--) { + while ((ignore_limit || nr--) && !list_empty(&deferred_list)) { struct deferred_entry *entry =3D list_first_entry(&deferred_list, struct deferred_entry, list); @@ -515,10 +522,14 @@ 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 =3D atomic64_dec_return(&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 =3D NULL; } else { @@ -530,21 +541,22 @@ 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 =3D jiffies + HZ; add_timer(&deferred_timer); } spin_unlock_irqrestore(&gnttab_list_lock, flags); + pr_debug("Freed %zu references", freed); } =20 static void gnttab_add_deferred(grant_ref_t ref, struct page *page) { struct deferred_entry *entry; gfp_t gfp =3D (in_atomic() || irqs_disabled()) ? GFP_ATOMIC : GFP_KERNEL; - const char *what =3D KERN_WARNING "leaking"; + uint64_t leaked, deferred; =20 entry =3D kmalloc(sizeof(*entry), gfp); if (!page) { @@ -567,10 +579,16 @@ static void gnttab_add_deferred(grant_ref_t ref, stru= ct page *page) add_timer(&deferred_timer); } spin_unlock_irqrestore(&gnttab_list_lock, flags); - what =3D KERN_DEBUG "deferring"; + deferred =3D atomic64_inc_return(&deferred_count); + leaked =3D atomic64_read(&leaked_count); + pr_debug("deferring g.e. %#x (pfn %#lx) (total deferred %llu, total leak= ed %llu)\n", + ref, page ? page_to_pfn(page) : -1, deferred, leaked); + } else { + deferred =3D atomic64_read(&deferred_count); + leaked =3D atomic64_inc_return(&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); } =20 int gnttab_try_end_foreign_access(grant_ref_t ref) --=20 Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab