Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

[Uladzislau Rezki]

On Thu, Aug 13, 2020 at 09:50:27AM +0200, Michal Hocko wrote:
> On Wed 12-08-20 02:13:25, Thomas Gleixner wrote:
> [...]
> > I can understand your rationale and what you are trying to solve. So, if
> > we can actually have a distinct GFP variant:
> > 
> >   GFP_I_ABSOLUTELY_HAVE_TO_DO_THAT_AND_I_KNOW_IT_CAN_FAIL_EARLY
> 
> Even if we cannot make the zone->lock raw I would prefer to not
> introduce a new gfp flag. Well we can do an alias for easier grepping
> #define GFP_RT_SAFE	0
> 
> that would imply nowait semantic and would exclude waking up kswapd as
> well. If we can make wake up safe under RT then the alias would reflect
> that without any code changes.
> 
> The second, and the more important part, would be to bail out anytime
> the page allocator is to take a lock which is not allowed in the current
> RT context. Something like 
> 	
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 67a0774e080b..3ef3ac82d110 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -237,6 +237,9 @@ struct vm_area_struct;
>   * that subsystems start with one of these combinations and then set/clear
>   * %__GFP_FOO flags as necessary.
>   *
> + * %GFP_RT_SAFE users can not sleep and they are running under RT atomic context
> + * e.g. under raw_spin_lock. Failure of an allocation is to be expected.
> + *
>   * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
>   * watermark is applied to allow access to "atomic reserves"
>   *
> @@ -293,6 +296,7 @@ struct vm_area_struct;
>   * version does not attempt reclaim/compaction at all and is by default used
>   * in page fault path, while the non-light is used by khugepaged.
>   */
> +#define GFP_RT_SAFE	0
>  #define GFP_ATOMIC	(__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM)
>  #define GFP_KERNEL	(__GFP_RECLAIM | __GFP_IO | __GFP_FS)
>  #define GFP_KERNEL_ACCOUNT (GFP_KERNEL | __GFP_ACCOUNT)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e028b87ce294..268ae872cc2a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2824,6 +2824,13 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
>  {
>  	int i, alloced = 0;
>  
> +	/*
> +	 * Hard atomic contexts are not supported by the allocator for
> +	 * anything but pcp requests
> +	 */
> +	if (!preemtable())
> +		return 0;
> +
>  	spin_lock(&zone->lock);
>  	for (i = 0; i < count; ++i) {
>  		struct page *page = __rmqueue(zone, order, migratetype,
> @@ -3371,6 +3378,13 @@ struct page *rmqueue(struct zone *preferred_zone,
>  		goto out;
>  	}
>  
> +	/*
> +	 * Hard atomic contexts are not supported by the allocator for high
> +	 * order requests
> +	 */
> +	if (WARN_ON_ONCE(!preemtable()))
> +		reurn NULL;
> +
>  	/*
>  	 * We most definitely don't want callers attempting to
>  	 * allocate greater than order-1 page units with __GFP_NOFAIL.
> @@ -4523,6 +4537,12 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  				(__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
>  		gfp_mask &= ~__GFP_ATOMIC;
>  
> +	/* Hard atomic contexts support is very limited to the fast path */
> +	if (!preemtable()) {
> +		WARN_ON_ONCE(gfp_mask != GFP_RT_SAFE);
> +		return NULL;
> +	}
> +
>  retry_cpuset:
>  	compaction_retries = 0;
>  	no_progress_loops = 0;
> 
> What do you think?
>  
> > which is easy to grep for then having the page allocator go down to the
> > point where zone lock gets involved is not the end of the world for
> > RT in theory - unless that damned reality tells otherwise. :)
> > 
> > The page allocator allocations should also have a limit on the number of
> > pages and eventually also page order (need to stare at the code or let
> > Michal educate me that the order does not matter).
> 
> In practice anything but order 0 is out of question because we need
> zone->lock for that currently. Maybe we can introduce pcp lists for
> higher orders in the future - I have a vague recollection Mel was
> playing with that some time ago.
> 
> > To make it consistent the same GFP_ variant should allow the slab
> > allocator go to the point where the slab cache is exhausted.
> > 
> > Having a distinct and clearly defined GFP_ variant is really key to
> > chase down offenders and to make reviewers double check upfront why this
> > is absolutely required.
> 
> Having a high level and recognizable gfp mask is OK but I would really
> like not to introduce a dedicated flag. The page allocator should be
> able to recognize the context which cannot be handled. 
>
Sorry for jumping in. We can rely on preemptable() for sure, if CONFIG_PREEMPT_RT
is enabled, something like below:

if (IS_ENABLED_RT && preemptebale())

Also i have a question about pcp-lists. Can we introduce and use all allowed 
MIGRATE_PCPTYPES? If called with GFP_RT_SAFE? If not please elaborate.
According to my tests it really helps when it comes to: succeed(return the page) or NULL.
Because on of the list of below types:
 MIGRATE_UNMOVABLE,
 MIGRATE_MOVABLE,
 MIGRATE_RECLAIMABLE,

can have a page making allocation succeed.

Thanks!

--
Vlad Rezki