Re: [Intel-gfx] [PATCH] i915: Limit the length of an sg list to the requested length

[Andrzej Hajda <andrzej.hajda@intel.com>]

On 19.09.2023 21:48, Matthew Wilcox (Oracle) wrote:
> The folio conversion changed the behaviour of shmem_sg_alloc_table() to
> put the entire length of the last folio into the sg list, even if the sg
> list should have been shorter.  gen8_ggtt_insert_entries() relied on the
> list being the right langth and would overrun the end of the page tables.

s/langth/length/, I can fix it on applying.

> Other functions may also have been affected.
> 
> Clamp the length of the last entry in the sg list to be the expected
> length.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Fixes: 0b62af28f249 ("i915: convert shmem_sg_free_table() to use a folio_batch")
> Cc: stable@vger.kernel.org # 6.5.x
> Link: https://gitlab.freedesktop.org/drm/intel/-/issues/9256
> Link: https://lore.kernel.org/lkml/6287208.lOV4Wx5bFT@natalenko.name/
> Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>

Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>

Regards
Andrzej

> ---
>   drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> index 8f1633c3fb93..73a4a4eb29e0 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> @@ -100,6 +100,7 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
>   	st->nents = 0;
>   	for (i = 0; i < page_count; i++) {
>   		struct folio *folio;
> +		unsigned long nr_pages;
>   		const unsigned int shrink[] = {
>   			I915_SHRINK_BOUND | I915_SHRINK_UNBOUND,
>   			0,
> @@ -150,6 +151,8 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
>   			}
>   		} while (1);
>   
> +		nr_pages = min_t(unsigned long,
> +				folio_nr_pages(folio), page_count - i);
>   		if (!i ||
>   		    sg->length >= max_segment ||
>   		    folio_pfn(folio) != next_pfn) {
> @@ -157,13 +160,13 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
>   				sg = sg_next(sg);
>   
>   			st->nents++;
> -			sg_set_folio(sg, folio, folio_size(folio), 0);
> +			sg_set_folio(sg, folio, nr_pages * PAGE_SIZE, 0);
>   		} else {
>   			/* XXX: could overflow? */
> -			sg->length += folio_size(folio);
> +			sg->length += nr_pages * PAGE_SIZE;
>   		}
> -		next_pfn = folio_pfn(folio) + folio_nr_pages(folio);
> -		i += folio_nr_pages(folio) - 1;
> +		next_pfn = folio_pfn(folio) + nr_pages;
> +		i += nr_pages - 1;
>   
>   		/* Check that the i965g/gm workaround works. */
>   		GEM_BUG_ON(gfp & __GFP_DMA32 && next_pfn >= 0x00100000UL);