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Subject[PATCH v3] mm/hugetlb: Defer freeing of huge pages if in non-task context
Date
The following lockdep splat was observed when a certain hugetlbfs test
was run:

[ 612.388273] ================================
[ 612.411273] WARNING: inconsistent lock state
[ 612.432273] 4.18.0-159.el8.x86_64+debug #1 Tainted: G W --------- - -
[ 612.469273] --------------------------------
[ 612.489273] inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
[ 612.517273] swapper/30/0 [HC0[0]:SC1[1]:HE1:SE0] takes:
[ 612.541273] ffffffff9acdc038 (hugetlb_lock){+.?.}, at: free_huge_page+0x36f/0xaa0
[ 612.576273] {SOFTIRQ-ON-W} state was registered at:
[ 612.598273] lock_acquire+0x14f/0x3b0
[ 612.616273] _raw_spin_lock+0x30/0x70
[ 612.634273] __nr_hugepages_store_common+0x11b/0xb30
[ 612.657273] hugetlb_sysctl_handler_common+0x209/0x2d0
[ 612.681273] proc_sys_call_handler+0x37f/0x450
[ 612.703273] vfs_write+0x157/0x460
[ 612.719273] ksys_write+0xb8/0x170
[ 612.736273] do_syscall_64+0xa5/0x4d0
[ 612.753273] entry_SYSCALL_64_after_hwframe+0x6a/0xdf
[ 612.777273] irq event stamp: 691296
[ 612.794273] hardirqs last enabled at (691296): [<ffffffff99bb034b>] _raw_spin_unlock_irqrestore+0x4b/0x60
[ 612.839273] hardirqs last disabled at (691295): [<ffffffff99bb0ad2>] _raw_spin_lock_irqsave+0x22/0x81
[ 612.882273] softirqs last enabled at (691284): [<ffffffff97ff0c63>] irq_enter+0xc3/0xe0
[ 612.922273] softirqs last disabled at (691285): [<ffffffff97ff0ebe>] irq_exit+0x23e/0x2b0
[ 612.962273]
[ 612.962273] other info that might help us debug this:
[ 612.993273] Possible unsafe locking scenario:
[ 612.993273]
[ 613.020273] CPU0
[ 613.031273] ----
[ 613.042273] lock(hugetlb_lock);
[ 613.057273] <Interrupt>
[ 613.069273] lock(hugetlb_lock);
[ 613.085273]
[ 613.085273] *** DEADLOCK ***
:
[ 613.245273] Call Trace:
[ 613.256273] <IRQ>
[ 613.265273] dump_stack+0x9a/0xf0
[ 613.281273] mark_lock+0xd0c/0x12f0
[ 613.297273] ? print_shortest_lock_dependencies+0x80/0x80
[ 613.322273] ? sched_clock_cpu+0x18/0x1e0
[ 613.341273] __lock_acquire+0x146b/0x48c0
[ 613.360273] ? trace_hardirqs_on+0x10/0x10
[ 613.379273] ? trace_hardirqs_on_caller+0x27b/0x580
[ 613.401273] lock_acquire+0x14f/0x3b0
[ 613.419273] ? free_huge_page+0x36f/0xaa0
[ 613.440273] _raw_spin_lock+0x30/0x70
[ 613.458273] ? free_huge_page+0x36f/0xaa0
[ 613.477273] free_huge_page+0x36f/0xaa0
[ 613.495273] bio_check_pages_dirty+0x2fc/0x5c0
[ 613.516273] clone_endio+0x17f/0x670 [dm_mod]
[ 613.536273] ? disable_discard+0x90/0x90 [dm_mod]
[ 613.558273] ? bio_endio+0x4ba/0x930
[ 613.575273] ? blk_account_io_completion+0x400/0x530
[ 613.598273] blk_update_request+0x276/0xe50
[ 613.617273] scsi_end_request+0x7b/0x6a0
[ 613.636273] ? lock_downgrade+0x6f0/0x6f0
[ 613.654273] scsi_io_completion+0x1c6/0x1570
[ 613.674273] ? sd_completed_bytes+0x3a0/0x3a0 [sd_mod]
[ 613.698273] ? scsi_mq_requeue_cmd+0xc0/0xc0
[ 613.718273] blk_done_softirq+0x22e/0x350
[ 613.737273] ? blk_softirq_cpu_dead+0x230/0x230
[ 613.758273] __do_softirq+0x23d/0xad8
[ 613.776273] irq_exit+0x23e/0x2b0
[ 613.792273] do_IRQ+0x11a/0x200
[ 613.806273] common_interrupt+0xf/0xf
[ 613.823273] </IRQ>

Both the hugetbl_lock and the subpool lock can be acquired in
free_huge_page(). One way to solve the problem is to make both locks
irq-safe. However, Mike Kravetz had learned that the hugetlb_lock is
held for a linear scan of ALL hugetlb pages during a cgroup reparentling
operation. So it is just too long to have irq disabled unless we can
break hugetbl_lock down into finer-grained locks with shorter lock
hold times.

Another alternative is to defer the freeing to a workqueue job. This
patch implements the deferred freeing by adding a free_hpage_workfn()
work function to do the actual freeing. The free_huge_page() call in
a non-task context saves the page to be freed in the hpage_freelist
linked list in a lockless manner using the llist APIs.

The generic workqueue is used to process the work, but a dedicated
workqueue can be used instead if it is desirable to have the huge page
freed ASAP.

Thanks to Kirill Tkhai <ktkhai@virtuozzo.com> for suggesting the use
of llist APIs which simplfy the code.

[v2: Add more comment & remove unneeded racing check]
[v3: Update commit log, remove pr_debug & use llist APIs]

Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
mm/hugetlb.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ac65bb5e38ac..dd8737a94bec 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -27,6 +27,7 @@
#include <linux/swapops.h>
#include <linux/jhash.h>
#include <linux/numa.h>
+#include <linux/llist.h>

#include <asm/page.h>
#include <asm/pgtable.h>
@@ -1136,7 +1137,7 @@ static inline void ClearPageHugeTemporary(struct page *page)
page[2].mapping = NULL;
}

-void free_huge_page(struct page *page)
+static void __free_huge_page(struct page *page)
{
/*
* Can't pass hstate in here because it is called from the
@@ -1199,6 +1200,54 @@ void free_huge_page(struct page *page)
spin_unlock(&hugetlb_lock);
}

+/*
+ * As free_huge_page() can be called from a non-task context, we have
+ * to defer the actual freeing in a workqueue to prevent potential
+ * hugetlb_lock deadlock.
+ *
+ * free_hpage_workfn() locklessly retrieves the linked list of pages to
+ * be freed and frees them one-by-one. As the page->mapping pointer is
+ * going to be cleared in __free_huge_page() anyway, it is reused as the
+ * llist_node structure of a lockless linked list of huge pages to be freed.
+ */
+static LLIST_HEAD(hpage_freelist);
+
+static void free_hpage_workfn(struct work_struct *work)
+{
+ struct llist_node *node;
+ struct page *page;
+
+ node = llist_del_all(&hpage_freelist);
+
+ while (node) {
+ page = container_of((struct address_space **)node,
+ struct page, mapping);
+ node = node->next;
+ __free_huge_page(page);
+ }
+}
+static DECLARE_WORK(free_hpage_work, free_hpage_workfn);
+
+void free_huge_page(struct page *page)
+{
+ /*
+ * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
+ */
+ if (!in_task()) {
+ /*
+ * Only call schedule_work() if hpage_freelist is previously
+ * empty. Otherwise, schedule_work() had been called but the
+ * workfn hasn't retrieved the list yet.
+ */
+ if (llist_add((struct llist_node *)&page->mapping,
+ &hpage_freelist))
+ schedule_work(&free_hpage_work);
+ return;
+ }
+
+ __free_huge_page(page);
+}
+
static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
{
INIT_LIST_HEAD(&page->lru);
--
2.18.1
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