Re: [RFC] mm,drm/i915: Mark pinned shmemfs pages as unevictable

[Michal Hocko <mhocko@suse.com>]

On Tue 06-06-17 13:04:36, Chris Wilson wrote:
> Similar in principle to the treatment of get_user_pages, pages that
> i915.ko acquires from shmemfs are not immediately reclaimable and so
> should be excluded from the mm accounting and vmscan until they have
> been returned to the system via shrink_slab/i915_gem_shrink. By moving
> the unreclaimable pages off the inactive anon lru, not only should
> vmscan be improved by avoiding walking unreclaimable pages, but the
> system should also have a better idea of how much memory it can reclaim
> at that moment in time.

That is certainly desirable. Peter has proposed a generic pin_page (or
similar) API. What happened with it? I think it would be a better
approach than (ab)using mlock API. I am also not familiar with the i915
code to be sure that using lock_page is really safe here. I think that
all we need is to simply move those pages in/out to/from unevictable LRU
list on pin/unpining.

> Note, however, the interaction with shrink_slab which will move some
> mlocked pages back to the inactive anon lru.
> 
> Suggested-by: Dave Hansen <dave.hansen@intel.com>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@suse.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 17 ++++++++++++++++-
>  mm/mlock.c                      |  2 ++
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 8cb811519db1..37a98fbc6a12 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2193,6 +2193,9 @@ void __i915_gem_object_truncate(struct drm_i915_gem_object *obj)
>  	obj->mm.pages = ERR_PTR(-EFAULT);
>  }
>  
> +extern void mlock_vma_page(struct page *page);
> +extern unsigned int munlock_vma_page(struct page *page);
> +
>  static void
>  i915_gem_object_put_pages_gtt(struct drm_i915_gem_object *obj,
>  			      struct sg_table *pages)
> @@ -2214,6 +2217,10 @@ i915_gem_object_put_pages_gtt(struct drm_i915_gem_object *obj,
>  		if (obj->mm.madv == I915_MADV_WILLNEED)
>  			mark_page_accessed(page);
>  
> +		lock_page(page);
> +		munlock_vma_page(page);
> +		unlock_page(page);
> +
>  		put_page(page);
>  	}
>  	obj->mm.dirty = false;
> @@ -2412,6 +2419,10 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
>  		}
>  		last_pfn = page_to_pfn(page);
>  
> +		lock_page(page);
> +		mlock_vma_page(page);
> +		unlock_page(page);
> +
>  		/* Check that the i965g/gm workaround works. */
>  		WARN_ON((gfp & __GFP_DMA32) && (last_pfn >= 0x00100000UL));
>  	}
> @@ -2450,8 +2461,12 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
>  err_sg:
>  	sg_mark_end(sg);
>  err_pages:
> -	for_each_sgt_page(page, sgt_iter, st)
> +	for_each_sgt_page(page, sgt_iter, st) {
> +		lock_page(page);
> +		munlock_vma_page(page);
> +		unlock_page(page);
>  		put_page(page);
> +	}
>  	sg_free_table(st);
>  	kfree(st);
>  
> diff --git a/mm/mlock.c b/mm/mlock.c
> index b562b5523a65..531d9f8fd033 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -94,6 +94,7 @@ void mlock_vma_page(struct page *page)
>  			putback_lru_page(page);
>  	}
>  }
> +EXPORT_SYMBOL_GPL(mlock_vma_page);
>  
>  /*
>   * Isolate a page from LRU with optional get_page() pin.
> @@ -211,6 +212,7 @@ unsigned int munlock_vma_page(struct page *page)
>  out:
>  	return nr_pages - 1;
>  }
> +EXPORT_SYMBOL_GPL(munlock_vma_page);
>  
>  /*
>   * convert get_user_pages() return value to posix mlock() error
> -- 
> 2.11.0
> 

-- 
Michal Hocko
SUSE Labs

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