Re: [RFC PATCH 1/2] mempool: do not consume memory reserves from the reclaim path

[Mikulas Patocka <mpatocka@redhat.com>]

On Mon, 18 Jul 2016, Michal Hocko wrote:

> From: Michal Hocko <mhocko@suse.com>
> 
> There has been a report about OOM killer invoked when swapping out to
> a dm-crypt device. The primary reason seems to be that the swapout
> out IO managed to completely deplete memory reserves. Mikulas was
> able to bisect and explained the issue by pointing to f9054c70d28b
> ("mm, mempool: only set __GFP_NOMEMALLOC if there are free elements").
> 
> The reason is that the swapout path is not throttled properly because
> the md-raid layer needs to allocate from the generic_make_request path
> which means it allocates from the PF_MEMALLOC context. dm layer uses
> mempool_alloc in order to guarantee a forward progress which used to
> inhibit access to memory reserves when using page allocator. This has
> changed by f9054c70d28b ("mm, mempool: only set __GFP_NOMEMALLOC if
> there are free elements") which has dropped the __GFP_NOMEMALLOC
> protection when the memory pool is depleted.
> 
> If we are running out of memory and the only way forward to free memory
> is to perform swapout we just keep consuming memory reserves rather than
> throttling the mempool allocations and allowing the pending IO to
> complete up to a moment when the memory is depleted completely and there
> is no way forward but invoking the OOM killer. This is less than
> optimal.
> 
> The original intention of f9054c70d28b was to help with the OOM
> situations where the oom victim depends on mempool allocation to make a
> forward progress. We can handle that case in a different way, though. We
> can check whether the current task has access to memory reserves ad an
> OOM victim (TIF_MEMDIE) and drop __GFP_NOMEMALLOC protection if the pool
> is empty.
> 
> David Rientjes was objecting that such an approach wouldn't help if the
> oom victim was blocked on a lock held by process doing mempool_alloc. This
> is very similar to other oom deadlock situations and we have oom_reaper
> to deal with them so it is reasonable to rely on the same mechanism
> rather inventing a different one which has negative side effects.
> 
> Fixes: f9054c70d28b ("mm, mempool: only set __GFP_NOMEMALLOC if there are free elements")
> Bisected-by: Mikulas Patocka <mpatocka@redhat.com>

Bisect was done by Ondrej Kozina.

> Signed-off-by: Michal Hocko <mhocko@suse.com>

Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Mikulas Patocka <mpatocka@redhat.com>

> ---
>  mm/mempool.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/mempool.c b/mm/mempool.c
> index 8f65464da5de..ea26d75c8adf 100644
> --- a/mm/mempool.c
> +++ b/mm/mempool.c
> @@ -322,20 +322,20 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>  
>  	might_sleep_if(gfp_mask & __GFP_DIRECT_RECLAIM);
>  
> +	gfp_mask |= __GFP_NOMEMALLOC;   /* don't allocate emergency reserves */
>  	gfp_mask |= __GFP_NORETRY;	/* don't loop in __alloc_pages */
>  	gfp_mask |= __GFP_NOWARN;	/* failures are OK */
>  
>  	gfp_temp = gfp_mask & ~(__GFP_DIRECT_RECLAIM|__GFP_IO);
>  
>  repeat_alloc:
> -	if (likely(pool->curr_nr)) {
> -		/*
> -		 * Don't allocate from emergency reserves if there are
> -		 * elements available.  This check is racy, but it will
> -		 * be rechecked each loop.
> -		 */
> -		gfp_temp |= __GFP_NOMEMALLOC;
> -	}
> +	/*
> +	 * Make sure that the OOM victim will get access to memory reserves
> +	 * properly if there are no objects in the pool to prevent from
> +	 * livelocks.
> +	 */
> +	if (!likely(pool->curr_nr) && test_thread_flag(TIF_MEMDIE))
> +		gfp_temp &= ~__GFP_NOMEMALLOC;
>  
>  	element = pool->alloc(gfp_temp, pool->pool_data);
>  	if (likely(element != NULL))
> @@ -359,7 +359,7 @@ void *mempool_alloc(mempool_t *pool, gfp_t gfp_mask)
>  	 * We use gfp mask w/o direct reclaim or IO for the first round.  If
>  	 * alloc failed with that and @pool was empty, retry immediately.
>  	 */
> -	if ((gfp_temp & ~__GFP_NOMEMALLOC) != gfp_mask) {
> +	if ((gfp_temp & __GFP_DIRECT_RECLAIM) != (gfp_mask & __GFP_DIRECT_RECLAIM)) {
>  		spin_unlock_irqrestore(&pool->lock, flags);
>  		gfp_temp = gfp_mask;
>  		goto repeat_alloc;
> -- 
> 2.8.1
>