Messages in this thread | | | Date | Mon, 26 Jun 2023 15:05:12 +0200 | From | Juergen Gross <> | Subject | Re: [PATCH v3] xen: speed up grant-table reclaim |
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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 [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |