Re: [PATCH] drm/i915: Fix page cleanup on DMA remap failure

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Jan 16, 2025 at 08:56:36AM -0500, Brian Geffon wrote:
> When converting to folios the cleanup path of shmem_get_pages() was
> missed. When a DMA remap fails and the max segment size is greater than
> PAGE_SIZE it will attempt to retry the remap with a PAGE_SIZEd segment
> size. The cleanup code isn't properly using the folio apis and as a
> result isn't handling compound pages correctly.
> 
> Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/13487
> Fixes: 0b62af28f249 ("i915: convert shmem_sg_free_table() to use a folio_batch")
> Signed-off-by: Brian Geffon <bgeffon@google.com>
> Suggested-by: Tomasz Figa <tfiga@google.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> index fe69f2c8527d..02ddab5bf5c0 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> @@ -37,8 +37,6 @@ void shmem_sg_free_table(struct sg_table *st, struct address_space *mapping,
>  	struct folio *last = NULL;
>  	struct page *page;
>  
> -	mapping_clear_unevictable(mapping);
> -

The assymmetry here between the alloc vs. free is a bit annoying.
Maybe we can just keep this here? 

Or if avoiding the ping-pong actually mattes in the gtt prepare
error case, then maybe we should rename this guy into
__shmem_sg_free_table() without the mapping_clear_unevictable()
and wrap it in a higher level shmem_sg_free_table() that does
everything?

>  	folio_batch_init(&fbatch);
>  	for_each_sgt_page(page, sgt_iter, st) {
>  		struct folio *folio = page_folio(page);
> @@ -180,10 +178,10 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
>  	return 0;
>  err_sg:
>  	sg_mark_end(sg);
> +	mapping_clear_unevictable(mapping);
>  	if (sg != st->sgl) {
>  		shmem_sg_free_table(st, mapping, false, false);
>  	} else {
> -		mapping_clear_unevictable(mapping);
>  		sg_free_table(st);
>  	}
>  
> @@ -209,8 +207,6 @@ static int shmem_get_pages(struct drm_i915_gem_object *obj)
>  	struct address_space *mapping = obj->base.filp->f_mapping;
>  	unsigned int max_segment = i915_sg_segment_size(i915->drm.dev);
>  	struct sg_table *st;
> -	struct sgt_iter sgt_iter;
> -	struct page *page;
>  	int ret;
>  
>  	/*
> @@ -239,9 +235,8 @@ static int shmem_get_pages(struct drm_i915_gem_object *obj)
>  		 * for PAGE_SIZE chunks instead may be helpful.
>  		 */
>  		if (max_segment > PAGE_SIZE) {
> -			for_each_sgt_page(page, sgt_iter, st)
> -				put_page(page);
> -			sg_free_table(st);
> +			/* Leave the mapping unevictable while we retry */
> +			shmem_sg_free_table(st, mapping, false, false);
>  			kfree(st);
>  
>  			max_segment = PAGE_SIZE;
> @@ -265,6 +260,7 @@ static int shmem_get_pages(struct drm_i915_gem_object *obj)
>  	return 0;
>  
>  err_pages:
> +	mapping_clear_unevictable(mapping);
>  	shmem_sg_free_table(st, mapping, false, false);
>  	/*
>  	 * shmemfs first checks if there is enough memory to allocate the page
> @@ -402,6 +398,7 @@ void i915_gem_object_put_pages_shmem(struct drm_i915_gem_object *obj, struct sg_
>  	if (i915_gem_object_needs_bit17_swizzle(obj))
>  		i915_gem_object_save_bit_17_swizzle(obj, pages);
>  
> +	mapping_clear_unevictable(file_inode(obj->base.filp)->i_mapping);
>  	shmem_sg_free_table(pages, file_inode(obj->base.filp)->i_mapping,
>  			    obj->mm.dirty, obj->mm.madv == I915_MADV_WILLNEED);
>  	kfree(pages);
> -- 
> 2.48.0.rc2.279.g1de40edade-goog

-- 
Ville Syrjälä
Intel