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authorWaiman Long <longman@redhat.com>2020-01-04 13:00:15 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2020-01-04 13:55:09 -0800
commitc77c0a8ac4c522638a8242fcb9de9496e3cdbb2d (patch)
tree419ea1a19437517b4ef3e9810a0ac55beaae16a3
parenta7c46c0c0e3d62f2764cd08b90934cd2aaaf8545 (diff)
downloadlinux-c77c0a8ac4c522638a8242fcb9de9496e3cdbb2d.tar.bz2
mm/hugetlb: defer freeing of huge pages if in non-task context
The following lockdep splat was observed when a certain hugetlbfs test was run: ================================ WARNING: inconsistent lock state 4.18.0-159.el8.x86_64+debug #1 Tainted: G W --------- - - -------------------------------- inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage. swapper/30/0 [HC0[0]:SC1[1]:HE1:SE0] takes: ffffffff9acdc038 (hugetlb_lock){+.?.}, at: free_huge_page+0x36f/0xaa0 {SOFTIRQ-ON-W} state was registered at: lock_acquire+0x14f/0x3b0 _raw_spin_lock+0x30/0x70 __nr_hugepages_store_common+0x11b/0xb30 hugetlb_sysctl_handler_common+0x209/0x2d0 proc_sys_call_handler+0x37f/0x450 vfs_write+0x157/0x460 ksys_write+0xb8/0x170 do_syscall_64+0xa5/0x4d0 entry_SYSCALL_64_after_hwframe+0x6a/0xdf irq event stamp: 691296 hardirqs last enabled at (691296): [<ffffffff99bb034b>] _raw_spin_unlock_irqrestore+0x4b/0x60 hardirqs last disabled at (691295): [<ffffffff99bb0ad2>] _raw_spin_lock_irqsave+0x22/0x81 softirqs last enabled at (691284): [<ffffffff97ff0c63>] irq_enter+0xc3/0xe0 softirqs last disabled at (691285): [<ffffffff97ff0ebe>] irq_exit+0x23e/0x2b0 other info that might help us debug this: Possible unsafe locking scenario: CPU0 ---- lock(hugetlb_lock); <Interrupt> lock(hugetlb_lock); *** DEADLOCK *** : Call Trace: <IRQ> __lock_acquire+0x146b/0x48c0 lock_acquire+0x14f/0x3b0 _raw_spin_lock+0x30/0x70 free_huge_page+0x36f/0xaa0 bio_check_pages_dirty+0x2fc/0x5c0 clone_endio+0x17f/0x670 [dm_mod] blk_update_request+0x276/0xe50 scsi_end_request+0x7b/0x6a0 scsi_io_completion+0x1c6/0x1570 blk_done_softirq+0x22e/0x350 __do_softirq+0x23d/0xad8 irq_exit+0x23e/0x2b0 do_IRQ+0x11a/0x200 common_interrupt+0xf/0xf </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. Link: http://lkml.kernel.org/r/20191217170331.30893-1-longman@redhat.com Signed-off-by: Waiman Long <longman@redhat.com> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com> Acked-by: Davidlohr Bueso <dbueso@suse.de> Acked-by: Michal Hocko <mhocko@suse.com> Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Andi Kleen <ak@linux.intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/hugetlb.c51
1 files changed, 50 insertions, 1 deletions
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);