Re: [RFC PATCH 2/2] mm, mempool: do not throttle PF_LESS_THROTTLE tasks

[Mikulas Patocka]

On Mon, 18 Jul 2016, Michal Hocko wrote:

> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> Mikulas has reported that a swap backed by dm-crypt doesn't work
> properly because the swapout cannot make a sufficient forward progress
> as the writeout path depends on dm_crypt worker which has to allocate
> memory to perform the encryption. In order to guarantee a forward
> progress it relies on the mempool allocator. mempool_alloc(), however,
> prefers to use the underlying (usually page) allocator before it grabs
> objects from the pool. Such an allocation can dive into the memory
> reclaim and consequently to throttle_vm_writeout. If there are too many
> dirty or pages under writeback it will get throttled even though it is
> in fact a flusher to clear pending pages.
> 
> [  345.352536] kworker/u4:0    D ffff88003df7f438 10488     6      2 0x00000000
> [  345.352536] Workqueue: kcryptd kcryptd_crypt [dm_crypt]
> [  345.352536]  ffff88003df7f438 ffff88003e5d0380 ffff88003e5d0380 ffff88003e5d8e80
> [  345.352536]  ffff88003dfb3240 ffff88003df73240 ffff88003df80000 ffff88003df7f470
> [  345.352536]  ffff88003e5d0380 ffff88003e5d0380 ffff88003df7f828 ffff88003df7f450
> [  345.352536] Call Trace:
> [  345.352536]  [<ffffffff818d466c>] schedule+0x3c/0x90
> [  345.352536]  [<ffffffff818d96a8>] schedule_timeout+0x1d8/0x360
> [  345.352536]  [<ffffffff81135e40>] ? detach_if_pending+0x1c0/0x1c0
> [  345.352536]  [<ffffffff811407c3>] ? ktime_get+0xb3/0x150
> [  345.352536]  [<ffffffff811958cf>] ? __delayacct_blkio_start+0x1f/0x30
> [  345.352536]  [<ffffffff818d39e4>] io_schedule_timeout+0xa4/0x110
> [  345.352536]  [<ffffffff8121d886>] congestion_wait+0x86/0x1f0
> [  345.352536]  [<ffffffff810fdf40>] ? prepare_to_wait_event+0xf0/0xf0
> [  345.352536]  [<ffffffff812061d4>] throttle_vm_writeout+0x44/0xd0
> [  345.352536]  [<ffffffff81211533>] shrink_zone_memcg+0x613/0x720
> [  345.352536]  [<ffffffff81211720>] shrink_zone+0xe0/0x300
> [  345.352536]  [<ffffffff81211aed>] do_try_to_free_pages+0x1ad/0x450
> [  345.352536]  [<ffffffff81211e7f>] try_to_free_pages+0xef/0x300
> [  345.352536]  [<ffffffff811fef19>] __alloc_pages_nodemask+0x879/0x1210
> [  345.352536]  [<ffffffff810e8080>] ? sched_clock_cpu+0x90/0xc0
> [  345.352536]  [<ffffffff8125a8d1>] alloc_pages_current+0xa1/0x1f0
> [  345.352536]  [<ffffffff81265ef5>] ? new_slab+0x3f5/0x6a0
> [  345.352536]  [<ffffffff81265dd7>] new_slab+0x2d7/0x6a0
> [  345.352536]  [<ffffffff810e7f87>] ? sched_clock_local+0x17/0x80
> [  345.352536]  [<ffffffff812678cb>] ___slab_alloc+0x3fb/0x5c0
> [  345.352536]  [<ffffffff811f71bd>] ? mempool_alloc_slab+0x1d/0x30
> [  345.352536]  [<ffffffff810e7f87>] ? sched_clock_local+0x17/0x80
> [  345.352536]  [<ffffffff811f71bd>] ? mempool_alloc_slab+0x1d/0x30
> [  345.352536]  [<ffffffff81267ae1>] __slab_alloc+0x51/0x90
> [  345.352536]  [<ffffffff811f71bd>] ? mempool_alloc_slab+0x1d/0x30
> [  345.352536]  [<ffffffff81267d9b>] kmem_cache_alloc+0x27b/0x310
> [  345.352536]  [<ffffffff811f71bd>] mempool_alloc_slab+0x1d/0x30
> [  345.352536]  [<ffffffff811f6f11>] mempool_alloc+0x91/0x230
> [  345.352536]  [<ffffffff8141a02d>] bio_alloc_bioset+0xbd/0x260
> [  345.352536]  [<ffffffffc02f1a54>] kcryptd_crypt+0x114/0x3b0 [dm_crypt]
> 
> Memory pools are usually used for the writeback paths and it doesn't
> really make much sense to throttle them just because there are too many
> dirty/writeback pages. The main purpose of throttle_vm_writeout is to
> make sure that the pageout path doesn't generate too much dirty data.
> Considering that we are in mempool path which performs __GFP_NORETRY
> requests the risk shouldn't be really high.
> 
> Fix this by ensuring that mempool users will get PF_LESS_THROTTLE and
> that such processes are not throttled in throttle_vm_writeout. They can
> still get throttled due to current_may_throttle() sleeps but that should
> happen when the backing device itself is congested which sounds like a
> proper reaction.
> 
> Please note that the bonus given by domain_dirty_limits() alone is not
> sufficient because at least dm-crypt has to double buffer each page
> under writeback so this won't be sufficient to prevent from being
> throttled.
> 
> There are other users of the flag but they are in the writeout path so
> this looks like a proper thing for them as well.
> 
> Reported-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Reviewed-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Tested-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

> ---
>  mm/mempool.c        | 19 +++++++++++++++----
>  mm/page-writeback.c |  3 +++
>  2 files changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/mempool.c b/mm/mempool.c
> index ea26d75c8adf..916e95c4192c 100644
> --- a/mm/mempool.c
> +++ b/mm/mempool.c
> @@ -310,7 +310,8 @@ EXPORT_SYMBOL(mempool_resize);
>   */
>  void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>  {
> -	void *element;
> +	unsigned int pflags = current->flags;
> +	void *element = NULL;
>  	unsigned long flags;
>  	wait_queue_t wait;
>  	gfp_t gfp_temp;
> @@ -328,6 +329,12 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>  
>  	gfp_temp = gfp_mask & ~(__GFP_DIRECT_RECLAIM|__GFP_IO);
>  
> +	/*
> +	 * Make sure that the allocation doesn't get throttled during the
> +	 * reclaim
> +	 */
> +	if (gfpflags_allow_blocking(gfp_mask))
> +		current->flags |= PF_LESS_THROTTLE;
>  repeat_alloc:
>  	/*
>  	 * Make sure that the OOM victim will get access to memory reserves
> @@ -339,7 +346,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>  
>  	element = pool->alloc(gfp_temp, pool->pool_data);
>  	if (likely(element != NULL))
> -		return element;
> +		goto out;
>  
>  	spin_lock_irqsave(&pool->lock, flags);
>  	if (likely(pool->curr_nr)) {
> @@ -352,7 +359,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>  		 * for debugging.
>  		 */
>  		kmemleak_update_trace(element);
> -		return element;
> +		goto out;
>  	}
>  
>  	/*
> @@ -369,7 +376,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>  	/* We must not sleep if !__GFP_DIRECT_RECLAIM */
>  	if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
>  		spin_unlock_irqrestore(&pool->lock, flags);
> -		return NULL;
> +		goto out;
>  	}
>  
>  	/* Let's wait for someone else to return an element to @pool */
> @@ -386,6 +393,10 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>  
>  	finish_wait(&pool->wait, &wait);
>  	goto repeat_alloc;
> +out:
> +	if (gfpflags_allow_blocking(gfp_mask))
> +		tsk_restore_flags(current, pflags, PF_LESS_THROTTLE);
> +	return element;
>  }
>  EXPORT_SYMBOL(mempool_alloc);
>  
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 7fbb2d008078..a37661f1a11b 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1971,6 +1971,9 @@ void throttle_vm_writeout(gfp_t gfp_mask)
>  	unsigned long background_thresh;
>  	unsigned long dirty_thresh;
>  
> +	if (current->flags & PF_LESS_THROTTLE)
> +		return;
> +
>          for ( ; ; ) {
>  		global_dirty_limits(&background_thresh, &dirty_thresh);
>  		dirty_thresh = hard_dirty_limit(&global_wb_domain, dirty_thresh);
> -- 
> 2.8.1
>