Hi Krzysztof,
This version finally looks good to me, however ...
On Tue, 2026-10-06 at 09:58 +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: [email protected]
> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/15816
> Assisted-by: LLM
> Signed-off-by: Krzysztof Karas <[email protected]>
> ---
> v7:
> * Derive the page offset within the current folio from
> next_pfn. (Janusz)
> * Update next_pfn using that offset and clarify its role in
> merging contiguous physical pages. (Andi)
>
> drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 108 +++++++++++++---------
> 1 file changed, 63 insertions(+), 45 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..645da248e25d 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> @@ -70,7 +70,9 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915,
> struct sg_table *st,
> unsigned int page_count; /* restricted by sg_alloc_table */
> unsigned long i;
> struct scatterlist *sg;
> - unsigned long next_pfn = 0; /* suppress gcc warning */
> + /* suppress gcc warnings */
> + unsigned long next_pfn = 0;
Why plural? Why moved to a separate line? Do we really need such
comments? I'm not asking for dropping it, only not touching as not
related, unless no longer applicable after your changes.
> + struct folio *folio;
However, if initialization of next_pfn is needed because its lack can still
trigger a gcc warning, shouldn't we now also initialize folio before first
use?
Thanks,
Janusz
> gfp_t noreclaim;
> int ret;
>
> @@ -104,7 +106,7 @@ 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 +114,61 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915,
> struct sg_table *st,
> }, *s = shrink;
> gfp_t gfp = noreclaim;
>
> - do {
> - cond_resched();
> - folio = shmem_read_folio_gfp(mapping, i, gfp);
> - if (!IS_ERR(folio))
> - break;
> -
> - if (!*s) {
> - ret = PTR_ERR(folio);
> - goto err_sg;
> - }
> -
> - i915_gem_shrink(NULL, i915, 2 * page_count, NULL, *s++);
> -
> - /*
> - * 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);
> + 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;
> +
> + if (!*s) {
> + ret = PTR_ERR(folio);
> + goto err_sg;
> + }
> +
> + 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 +178,19 @@ 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. */
