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SubjectRe: [PATCH v2] mm: might_sleep warning
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On 03/06/2018 11:40 PM, Pavel Tatashin wrote:
> Robot reported this issue:
> https://lkml.org/lkml/2018/2/27/851
>
> That is introduced by:
> mm: initialize pages on demand during boot
>
> The problem is caused by changing static branch value within spin lock.
> Spin lock disables preemption, and changing static branch value takes
> mutex lock in its path, and thus may sleep.
>
> The fix is to add another boolean variable to avoid the need to change
> static branch within spinlock.
>
> Also, as noticed by Andrew, change spin_lock to spin_lock_irq, in order
> to disable interrupts and avoid possible deadlock with
> deferred_grow_zone().
>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
> ---
> mm/page_alloc.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b337a026007c..5df1ca40a2ff 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1579,6 +1579,7 @@ static int __init deferred_init_memmap(void *data)
> * page_alloc_init_late() soon after smp_init() is complete.
> */
> static __initdata DEFINE_SPINLOCK(deferred_zone_grow_lock);
> +static bool deferred_zone_grow __initdata = true;
> static DEFINE_STATIC_KEY_TRUE(deferred_pages);
>
> /*
> @@ -1616,7 +1617,7 @@ deferred_grow_zone(struct zone *zone, unsigned int order)
> * Bail if we raced with another thread that disabled on demand
> * initialization.
> */
> - if (!static_branch_unlikely(&deferred_pages)) {
> + if (!static_branch_unlikely(&deferred_pages) || !deferred_zone_grow) {
> spin_unlock_irqrestore(&deferred_zone_grow_lock, flags);
> return false;
> }
> @@ -1683,10 +1684,15 @@ void __init page_alloc_init_late(void)
> /*
> * We are about to initialize the rest of deferred pages, permanently
> * disable on-demand struct page initialization.

Hi,

I've noticed that this function first disables the on-demand
initialization, and then runs the kthreads. Doesn't that leave a window
where allocations can fail? The chances are probably small, but I think
it would be better to avoid it completely, rare failures suck.

Fixing that probably means rethinking the whole synchronization more
dramatically though :/

Vlastimil

> + *
> + * Note: it is prohibited to modify static branches in non-preemptible
> + * context. Since, spin_lock() disables preemption, we must use an
> + * extra boolean deferred_zone_grow.
> */
> - spin_lock(&deferred_zone_grow_lock);
> + spin_lock_irq(&deferred_zone_grow_lock);
> + deferred_zone_grow = false;
> + spin_unlock_irq(&deferred_zone_grow_lock);
> static_branch_disable(&deferred_pages);
> - spin_unlock(&deferred_zone_grow_lock);
>
> /* There will be num_node_state(N_MEMORY) threads */
> atomic_set(&pgdat_init_n_undone, num_node_state(N_MEMORY));
>

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