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SubjectRe: [PATCH] mm/page_alloc: avoid deadlocks for &pagesets.lock
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On 7/7/21 8:25 pm, Mel Gorman wrote:
> On Wed, Jul 07, 2021 at 07:12:45PM +0800, Desmond Cheong Zhi Xi wrote:
>> Syzbot reports a number of potential deadlocks for &pagesets.lock. It
>> seems that this new lock is being used as both an inner and outer
>> lock, which makes it prone to creating circular dependencies.
>>
>> For example, one such call trace goes as follows:
>> __alloc_pages_bulk()
>> local_lock_irqsave(&pagesets.lock, flags) <---- outer lock here
>> prep_new_page():
>> post_alloc_hook():
>> set_page_owner():
>> __set_page_owner():
>> save_stack():
>> stack_depot_save():
>> alloc_pages():
>> alloc_page_interleave():
>> __alloc_pages():
>> get_page_from_freelist():
>> rm_queue():
>> rm_queue_pcplist():
>> local_lock_irqsave(&pagesets.lock, flags);
>> *** DEADLOCK ***
>>
>> The common culprit for the lockdep splats seems to be the call to
>> local_lock_irqsave(&pagesets.lock, flags) inside
>> __alloc_pages_bulk(). &pagesets.lock becomes an outer lock if it's
>> held during the call to prep_new_page().
>>
>> As the local lock is used to protect the PCP structure, we adjust the
>> locking in __alloc_pages_bulk so that only the necessary structures
>> are protected.
>>
>> Fixes: dbbee9d5cd83 ("mm/page_alloc: convert per-cpu list protection to local_lock")
>> Reported-and-tested-by: syzbot+127fd7828d6eeb611703@syzkaller.appspotmail.com
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
>
> Hi Desmond,
>
> Thanks for the report. Unfortunately, this patch incurs a performance
> penalty for the bulk allocator even if PAGE_OWNER is disabled. Can you
> try the following as an alternative please? It passed a build and boot
> test but I didn't try triggering the actual bug.
>

Hi Mel,

Thanks for the feedback, I hadn't thought of the performance penalty. I
think you're right that if the recursive call to __set_page_owner is
avoided, then that also avoids creating the circular lock hierarchy.

Your proposed patch passed the Syzbot repro test:

Reported-and-tested-by:
syzbot+127fd7828d6eeb611703@syzkaller.appspotmail.com

Best wishes,
Desmond

> --8<--
> mm/page_alloc: Avoid page allocator recursion with pagesets.lock held
>
> Syzbot is reporting potential deadlocks due to pagesets.lock when
> PAGE_OWNER is enabled. One example from Desmond Cheong Zhi Xi is
> as follows
>
> __alloc_pages_bulk()
> local_lock_irqsave(&pagesets.lock, flags) <---- outer lock here
> prep_new_page():
> post_alloc_hook():
> set_page_owner():
> __set_page_owner():
> save_stack():
> stack_depot_save():
> alloc_pages():
> alloc_page_interleave():
> __alloc_pages():
> get_page_from_freelist():
> rm_queue():
> rm_queue_pcplist():
> local_lock_irqsave(&pagesets.lock, flags);
> *** DEADLOCK ***
>
> Zhang, Qiang also reported
>
> BUG: sleeping function called from invalid context at mm/page_alloc.c:5179
> in_atomic(): 0, irqs_disabled(): 1, non_block: 0, pid: 1, name: swapper/0
> .....
> __dump_stack lib/dump_stack.c:79 [inline]
> dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:96
> ___might_sleep.cold+0x1f1/0x237 kernel/sched/core.c:9153
> prepare_alloc_pages+0x3da/0x580 mm/page_alloc.c:5179
> __alloc_pages+0x12f/0x500 mm/page_alloc.c:5375
> alloc_page_interleave+0x1e/0x200 mm/mempolicy.c:2147
> alloc_pages+0x238/0x2a0 mm/mempolicy.c:2270
> stack_depot_save+0x39d/0x4e0 lib/stackdepot.c:303
> save_stack+0x15e/0x1e0 mm/page_owner.c:120
> __set_page_owner+0x50/0x290 mm/page_owner.c:181
> prep_new_page mm/page_alloc.c:2445 [inline]
> __alloc_pages_bulk+0x8b9/0x1870 mm/page_alloc.c:5313
> alloc_pages_bulk_array_node include/linux/gfp.h:557 [inline]
> vm_area_alloc_pages mm/vmalloc.c:2775 [inline]
> __vmalloc_area_node mm/vmalloc.c:2845 [inline]
> __vmalloc_node_range+0x39d/0x960 mm/vmalloc.c:2947
> __vmalloc_node mm/vmalloc.c:2996 [inline]
> vzalloc+0x67/0x80 mm/vmalloc.c:3066
>
> There are a number of ways it could be fixed. The page owner code could
> be audited to strip GFP flags that allow sleeping but it'll impair the
> functionality of PAGE_OWNER if allocations fail. The bulk allocator
> could add a special case to release/reacquire the lock for prep_new_page
> and lookup PCP after the lock is reacquired at the cost of performance.
> Both options are relatively complex and the second one still incurs a
> performance penalty when PAGE_OWNER is active so this patch takes the
> simple approach -- disable bulk allocation of PAGE_OWNER is active. The
> caller will be forced to allocate one page at a time incurring a
> performance penalty but PAGE_OWNER is already a performance penalty.
>
> Fixes: dbbee9d5cd83 ("mm/page_alloc: convert per-cpu list protection to local_lock")
> Reported-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> Reported-by: "Zhang, Qiang" <Qiang.Zhang@windriver.com>
> Reported-by: syzbot+127fd7828d6eeb611703@syzkaller.appspotmail.com
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
> mm/page_alloc.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3b97e17806be..6ef86f338151 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5239,6 +5239,18 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> if (nr_pages - nr_populated == 1)
> goto failed;
>
> +#ifdef CONFIG_PAGE_OWNER
> + /*
> + * PAGE_OWNER may recurse into the allocator to allocate space to
> + * save the stack with pagesets.lock held. Releasing/reacquiring
> + * removes much of the performance benefit of bulk allocation so
> + * force the caller to allocate one page at a time as it'll have
> + * similar performance to added complexity to the bulk allocator.
> + */
> + if (static_branch_unlikely(&page_owner_inited))
> + goto failed;
> +#endif
> +
> /* May set ALLOC_NOFRAGMENT, fragmentation will return 1 page. */
> gfp &= gfp_allowed_mask;
> alloc_gfp = gfp;
>

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