From: Demi Marie Obenour demi@invisiblethingslab.com
commit c04e9894846c663f3278a414f34416e6e45bbe68 upstream.
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 via a module parameter.
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 Reviewed-by: Juergen Gross jgross@suse.com Link: https://lore.kernel.org/r/20230726165354.1252-1-demi@invisiblethingslab.com Signed-off-by: Juergen Gross jgross@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- Documentation/ABI/testing/sysfs-module | 11 +++++++++ drivers/xen/grant-table.c | 40 +++++++++++++++++++++++---------- 2 files changed, 40 insertions(+), 11 deletions(-)
--- a/Documentation/ABI/testing/sysfs-module +++ b/Documentation/ABI/testing/sysfs-module @@ -60,3 +60,14 @@ Description: Module taint flags: C staging driver module E unsigned module == ===================== + +What: /sys/module/grant_table/parameters/free_per_iteration +Date: July 2023 +KernelVersion: 6.5 but backported to all supported stable branches +Contact: Xen developer discussion xen-devel@lists.xenproject.org +Description: Read and write number of grant entries to attempt to free per iteration. + + Note: Future versions of Xen and Linux may provide a better + interface for controlling the rate of deferred grant reclaim + or may not need it at all. +Users: Qubes OS (https://www.qubes-os.org) --- 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);
+static atomic64_t deferred_count; +static atomic64_t leaked_count; +static unsigned int free_per_iteration = 10; +module_param(free_per_iteration, uint, 0600); + 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 +522,14 @@ static void gnttab_handle_deferred(struc list_del(&entry->list); spin_unlock_irqrestore(&gnttab_list_lock, flags); if (_gnttab_end_foreign_access_ref(entry->ref)) { + uint64_t ret = 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 = NULL; } else { @@ -530,21 +541,22 @@ static void gnttab_handle_deferred(struc 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) { @@ -567,10 +579,16 @@ static void gnttab_add_deferred(grant_re add_timer(&deferred_timer); } spin_unlock_irqrestore(&gnttab_list_lock, flags); - what = KERN_DEBUG "deferring"; + deferred = atomic64_inc_return(&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_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); }
int gnttab_try_end_foreign_access(grant_ref_t ref)