i915: Limit the length of an sg list to the requested length

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 length and would overrun the end of the page tables.
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) <[e-mail]>
Fixes: 0b62af28f249 ("i915: convert shmem_sg_free_table() to use a folio_batch")
Cc: [e-mail] # 6.5.x
Link: https://gitlab.freedesktop.org/drm/intel/-/issues/9256
Link: https://lore.kernel.org/lkml/[e-mail]/
Reported-by: Oleksandr Natalenko <[e-mail]>
Tested-by: Oleksandr Natalenko <[e-mail]>
Reviewed-by: Andrzej Hajda <[e-mail]>
Signed-off-by: Andrzej Hajda <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
(cherry picked from commit 26a8e32e6d77900819c0c730fbfb393692dbbeea)
Signed-off-by: Rodrigo Vivi <[e-mail]>

drivers/gpu/drm/i915/gem/i915_gem_shmem.c

···
 	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,
···
 			}
 		} 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) {
···
 				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);