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

Uladzislau Rezki
2020-08-13T09:58:46Z

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