Hi Krzysztof,
While I'm finally more or less OK with your commit description :-), I
still have some comments to code changes you propose.
On Mon, 2026-08-17 at 09:56 +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.
>
> 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")
> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/15816
> Signed-off-by: Krzysztof Karas <[email protected]>
> ---
> v5:
> * Moved max_segment value validation before allocating shmem table
> (Sebastian).
> * Aligned comments (Sebastian).
>
> drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 121 ++++++++++++++--------
> 1 file changed, 76 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 06543ae60706..f338dc39fad1 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> @@ -68,10 +68,13 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915,
> struct sg_table *st,
> unsigned int max_segment)
> {
> unsigned int page_count; /* restricted by sg_alloc_table */
> - unsigned long i;
> + unsigned long next_pfn = 0; /* suppress gcc warning */
> + unsigned long folio_start = 0;
> + unsigned long folio_end = 0;
> + struct folio *folio = NULL;
> struct scatterlist *sg;
> - unsigned long next_pfn = 0; /* suppress gcc warning */
> gfp_t noreclaim;
> + unsigned long i;
> int ret;
>
> if (overflows_type(size / PAGE_SIZE, page_count))
> @@ -85,6 +88,9 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915,
> struct sg_table *st,
> if (size > resource_size(&mr->region))
> return -ENOMEM;
>
> + if (max_segment < PAGE_SIZE)
> + return -EINVAL;
> +
> if (sg_alloc_table(st, page_count, GFP_KERNEL | __GFP_NOWARN))
> return -ENOMEM;
>
> @@ -101,7 +107,7 @@ 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 = 0;
> unsigned long nr_pages;
> const unsigned int shrink[] = {
> I915_SHRINK_BOUND | I915_SHRINK_UNBOUND,
> @@ -109,71 +115,95 @@ 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;
> + /* Grab the next folio if we exhausted the current one. */
I think that even with folio_start < i < folio_end,
shmem_read_folio_gfp(..., i, ...) should always return the right folio, so
we shouldn't worry about getting the same folio again in a relatively rare
case of the folio size exceeding a scatterlist capacity, we only need to
take care of tracking an offset within that folio. Then, ...
> + if (!i || i > folio_end) {
> + 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;
> - }
> + 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);
> + 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);
> +
> + folio_start = folio_pgoff(folio);
> + folio_end = folio_start + folio_nr_pages(folio) - 1;
> + }
> +
> + folio_page_index = i - folio_start;
> + if (WARN_ON_ONCE(folio_page_index >= folio_nr_pages(folio))) {
> + ret = -EINVAL;
> + folio_put(folio);
> + goto err_sg;
> + }
... the existing code above, including the existing 'do' loop, may be left
untouched, I believe, an your folio_page_index can easily be calculated
from an already maintained next_pfn as:
+ folio_page_index = next_pfn - folio_pfn(folio);
+ if (folio_page_index < 0 || folio_page_index >=
folio_nr_pages(folio))
+ folio_page_index = 0;
IOW, we need to calculate and apply an offset within the folio only if
next_pfn is still within the folio's PFN range, otherwise that must be a
new folio and the offset we apply must be 0.
Then, unless I'm missing something, I believe the patch could be much more
compact while still correct with my approach. However, if other reviewers
are more OK with your proposed changes rather than what I suggest then I
won't oppose.
>
> nr_pages = min_array(((unsigned long[]) {
> - folio_nr_pages(folio),
> + folio_nr_pages(folio) -
> folio_page_index,
> page_count - i,
> max_segment / PAGE_SIZE,
> }), 3);
>
> if (!i ||
> sg->length >= max_segment ||
> - folio_pfn(folio) != next_pfn) {
> + folio_pfn(folio) + folio_page_index != next_pfn) {
I think the existing condition was correct, and still applicable even when
tracking potential offsets within folios: when there is still some room in
the current scatterlist, enter the else part that fills up the scatterlist
with more data only if we've got a new folio that starts exactly where
the previous one ended to have contiguity of pages preserved, otherwise
switch to a new scatterlist.
> if (i)
> 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 {
> + /*
> + * If our prediction about folio placement is true and
> + * scatterlist still has space left for more pages,
> + * then we land here.
> + */
> nr_pages = min_t(unsigned long, nr_pages,
> (max_segment - sg->length) /
> PAGE_SIZE);
>
> sg->length += nr_pages * PAGE_SIZE;
> }
> - next_pfn = folio_pfn(folio) + nr_pages;
> +
> + /*
> + * We assume folios are placed one after the other in memory
> + * and predict where the next folio begins.
> + */
> + next_pfn = folio_pfn(folio) + folio_page_index + nr_pages;
> i += nr_pages - 1;
>
> /* Check that the i965g/gm workaround works. */
> @@ -186,6 +216,7 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915,
> struct sg_table *st,
> i915_sg_trim(st);
>
> return 0;
> +
Not related, I wouldn't mix it in, unless you have a good justification.
Thanks,
Janusz
> err_sg:
> sg_mark_end(sg);
> if (sg != st->sgl) {
