[REGRESSION] [BISECTED] Panic in gen8_ggtt_insert_entries() with v6.5

[Oleksandr Natalenko]

Hello.

On úterý 19. září 2023 17:43:40 CEST Matthew Wilcox wrote:
> On Tue, Sep 19, 2023 at 10:26:42AM +0200, Oleksandr Natalenko wrote:
> > Andrzej asked me to try to revert commits 0b62af28f249, e0b72c14d8dc and 1e0877d58b1e, and reverting those fixed the i915 crash for me. The e0b72c14d8dc and 1e0877d58b1e commits look like just prerequisites, so I assume 0b62af28f249 ("i915: convert shmem_sg_free_table() to use a folio_batch") is the culprit here.
> > 
> > Could you please check this?
> > 
> > Our conversation with Andrzej is available at drm-intel GitLab [1].
> > 
> > Thanks.
> > 
> > [1] https://gitlab.freedesktop.org/drm/intel/-/issues/9256
> 
> Wow, that is some great debugging.  Thanks for all the time & effort
> you and others have invested.  Sorry for breaking your system.
> 
> You're almost right about the "prerequisites", but it's in the other
> direction; 0b62af28f249 is a prerequisite for the later two cleanups,
> so reverting all three is necessary to test 0b62af28f249.
> 
> It seems to me that you've isolated the problem to constructing overly
> long sg lists.  I didn't realise that was going to be a problem, so
> that's my fault.
> 
> Could I ask you to try this patch?  I'll follow up with another patch
> later because I think I made another assumption that may not be valid.

I can confirm this one fixes the issue for me on T460s laptop. Thank you!

Should you submit it, please add:

Fixes: 0b62af28f2 ("i915: convert shmem_sg_free_table() to use a folio_batch")
Cc: stable at 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 at natalenko.name>
Tested-by: Oleksandr Natalenko <oleksandr at natalenko.name>

> 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);

-- 
Oleksandr Natalenko (post-factum)
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