Re: [PATCH v3] mm/hugetlb: Defer freeing of huge pages if in non-task context

[Kirill Tkhai]

On 17.12.2019 20:03, Waiman Long wrote:
> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>

Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>

One more note below.

> ---
>  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))

Just for track. This llist_add() may assign to page->mapping only
a pointer to another_page->mapping, which must be properly aligned.

Thus, we never may set, say, PAGE_MAPPING_KSM bits in page->mapping,
which would be changed the page type, in this function.
So, everything is OK.

> +			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);