Re: [PATCH v8 2/6] drm/i915/gem: Count mapped pages in a folio

[Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>]

Hi Krzysztof,

On Mon, 2026-11-09 at 11:37 +0000, Krzysztof Karas wrote:
> Before addition of commit 029ae067431a
> ("drm/i915: Fix potential overflow of shmem scatterlist length")
> and after folios were introduced complete folios were always
> allocated, possibly overloading the scatterlist capacity which
> was never truly limited to PAGE_SIZE when requested via
> max_segment. The above commit addressed scatterlist overloading,
> but unintentionally disabled PAGE_SIZE as a valid max_segment
> value and failed to take care of remaining pages from folios
> above max_segment boundary.
> This created a state, where multitude of scatterlists were used
> for the same folio, but never counting enough of its pages to
> jump to the next folio.
> 
> Furthermore, current do-while loop goes over the same folio
> multiple times, increasing the refcount each time, and breaks
> get/put symmetry: many shmem_read_folio_gfp() gets and only one
> put in i915_gem_object_put_pages_shmem() per folio.
> 
> Track how many pages have already been counted in a folio and
> use that number as an offset on consecutive allocations from the
> same folio to ensure it is fully covered before reading next
> folio.
> 
> Fixes: 029ae067431a ("drm/i915: Fix potential overflow of shmem scatterlist length")
> Cc: stable@vger.kernel.org
> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/15816
> Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 101 +++++++++++++---------
>  1 file changed, 59 insertions(+), 42 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> index e35ee4bd456f..d57b5c67d47b 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> @@ -71,6 +71,7 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
>  	unsigned long i;
>  	struct scatterlist *sg;
>  	unsigned long next_pfn = 0;	/* suppress gcc warning */
> +	struct folio *folio = NULL;
>  	gfp_t noreclaim;
>  	int ret;
>  
> @@ -104,7 +105,7 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
>  	sg = st->sgl;
>  	st->nents = 0;
>  	for (i = 0; i < page_count; i++) {
> -		struct folio *folio;
> +		unsigned long folio_page_index;
>  		unsigned long nr_pages;
>  		const unsigned int shrink[] = {
>  			I915_SHRINK_BOUND | I915_SHRINK_UNBOUND,
> @@ -112,51 +113,61 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
>  		}, *s = shrink;
>  		gfp_t gfp = noreclaim;
>  

I was looking for a way to make this patch still more easily readable, but 
all I invented would look like below, so not a standard syntax, then:

Reviewed-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>

> -		do {
+		if (i && (next_pfn < folio_pfn(folio) + folio_nr_pages(folio))) {
+			/*
+			 * We are still inside the same folio,
+			 * so just update its page index.
+			 */
+			folio_page_index = next_pfn - folio_pfn(folio);
+		} else do {
+			/* Grab the next folio if we exhausted the current one. */

> -			cond_resched();
> -			folio = shmem_read_folio_gfp(mapping, i, gfp);
> -			if (!IS_ERR(folio))
> -				break;
> -
> -			if (!*s) {
> -				ret = PTR_ERR(folio);
> -				goto err_sg;
> -			}
> +		if (!i || (next_pfn >= folio_pfn(folio) + folio_nr_pages(folio))) {
> +			/* Grab the next folio if we exhausted the current one. */
> +			do {
> +				cond_resched();
> +				folio = shmem_read_folio_gfp(mapping, i, gfp);
> +				folio_page_index = 0;
> +				if (!IS_ERR(folio))
> +					break;
>  
> -			i915_gem_shrink(NULL, i915, 2 * page_count, NULL, *s++);
> +				if (!*s) {
> +					ret = PTR_ERR(folio);
> +					goto err_sg;
> +				}
>  
> -			/*
> -			 * We've tried hard to allocate the memory by reaping
> -			 * our own buffer, now let the real VM do its job and
> -			 * go down in flames if truly OOM.
> -			 *
> -			 * However, since graphics tend to be disposable,
> -			 * defer the oom here by reporting the ENOMEM back
> -			 * to userspace.
> -			 */
> -			if (!*s) {
> -				/* reclaim and warn, but no oom */
> -				gfp = mapping_gfp_mask(mapping);
> +				i915_gem_shrink(NULL, i915, 2 * page_count, NULL, *s++);
>  
>  				/*
> -				 * Our bo are always dirty and so we require
> -				 * kswapd to reclaim our pages (direct reclaim
> -				 * does not effectively begin pageout of our
> -				 * buffers on its own). However, direct reclaim
> -				 * only waits for kswapd when under allocation
> -				 * congestion. So as a result __GFP_RECLAIM is
> -				 * unreliable and fails to actually reclaim our
> -				 * dirty pages -- unless you try over and over
> -				 * again with !__GFP_NORETRY. However, we still
> -				 * want to fail this allocation rather than
> -				 * trigger the out-of-memory killer and for
> -				 * this we want __GFP_RETRY_MAYFAIL.
> +				 * We've tried hard to allocate the memory by reaping
> +				 * our own buffer, now let the real VM do its job and
> +				 * go down in flames if truly OOM.
> +				 *
> +				 * However, since graphics tend to be disposable,
> +				 * defer the oom here by reporting the ENOMEM back
> +				 * to userspace.
>  				 */
> -				gfp |= __GFP_RETRY_MAYFAIL | __GFP_NOWARN;
> -			}
> -		} while (1);
> +				if (!*s) {
> +					/* reclaim and warn, but no oom */
> +					gfp = mapping_gfp_mask(mapping);
> +
> +					/*
> +					 * Our bo are always dirty and so we require
> +					 * kswapd to reclaim our pages (direct reclaim
> +					 * does not effectively begin pageout of our
> +					 * buffers on its own). However, direct reclaim
> +					 * only waits for kswapd when under allocation
> +					 * congestion. So as a result __GFP_RECLAIM is
> +					 * unreliable and fails to actually reclaim our
> +					 * dirty pages -- unless you try over and over
> +					 * again with !__GFP_NORETRY. However, we still
> +					 * want to fail this allocation rather than
> +					 * trigger the out-of-memory killer and for
> +					 * this we want __GFP_RETRY_MAYFAIL.
> +					 */
> +					gfp |= __GFP_RETRY_MAYFAIL | __GFP_NOWARN;
> +				}
> +			} while (1);
> +		} else {
> +			/*
> +			 * Otherwise, we are still inside the same folio,
> +			 * so just update its page index.
> +			 */
> +			folio_page_index = next_pfn - folio_pfn(folio);
> +		}
>  
> -		nr_pages = min3(folio_nr_pages(folio),
> +		nr_pages = min3(folio_nr_pages(folio) - folio_page_index,
>  				page_count - i, max_segment / PAGE_SIZE);
>  
>  		if (!i ||
> @@ -166,13 +177,19 @@ 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, nr_pages * PAGE_SIZE, 0);
> +			sg_set_page(sg, folio_page(folio, folio_page_index),
> +				    nr_pages * PAGE_SIZE, 0);
>  		} else {
>  			nr_pages = min(nr_pages, (max_segment - sg->length) / PAGE_SIZE);
>  
>  			sg->length += nr_pages * PAGE_SIZE;
>  		}
> -		next_pfn = folio_pfn(folio) + nr_pages;
> +
> +		/*
> +		 * Store the next PFN, so contiguous physical pages
> +		 * can be merged into the same SG entry.
> +		 */
> +		next_pfn = folio_pfn(folio) + folio_page_index + nr_pages;
>  		i += nr_pages - 1;
>  
>  		/* Check that the i965g/gm workaround works. */