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SubjectRe: [PATCH v3] xen: speed up grant-table reclaim
On 24.06.23 22:56, Demi Marie Obenour wrote:
> 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).

Using Kconfig has been dropped.

>
> 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);

Use atomic64_dec_return()?

Please add an empty line here.

> 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);

Use atomic64_inc_return() (same below)?

> + 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);
> +

As said for v2 already: please move this closer to the related variable
definition.

> int gnttab_try_end_foreign_access(grant_ref_t ref)
> {
> int ret = _gnttab_end_foreign_access_ref(ref);


Juergen
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