Re: [PATCH 2/2] drm/nouveau/dmem: fix callocated underflow on large folio split

[lyude@redhat.com]

This patch looks suspiciously human written (or at the very least, the
commit message sure does). FWIW: if the code wasn't itself generated by
an LLM, it's fine by me to avoid adding the tag. Most of us are
concerned about generated LLM code that gets submitted, not human
written code written to fix a legitimate issue that an LLM managed to
help find. If we were concerned about the latter, I think anything
sashiko touched would end up having an assisted-by tag on it :).

Either way, this whole patch series is:

Reviewed-by: Lyude Paul <lyude@redhat.com>

Issue looks quite legitimate, fix seems fine to me.

On Tue, 2026-08-11 at 22:28 +0800, Zhenhao Wan wrote:
> nouveau_dmem_folio_free() drops chunk->callocated once per freed
> folio,
> while a large (compound) device-private folio is only counted once
> when
> it is allocated.  When such a folio is split, the mm core invokes
> ->folio_split() (nouveau_dmem_folio_split()) once for each new
> sub-folio, but the hook only fixes up the sub-folio metadata and
> leaves
> chunk->callocated unchanged.
> 
> Each resulting sub-folio is later freed separately, so after a split
> the single allocation (+1) is met by N frees (-N), leaving
> chunk->callocated short by N-1.  On the first split/free cycle it
> underflows: WARN_ON(!chunk->callocated) fires, the unsigned counter
> wraps and never returns to zero, so the chunk can no longer be
> reclaimed (nouveau_dmem_fini() also warns on the leaked count).
> 
> Account for the new sub-folio in the split hook, under the same lock
> as
> nouveau_dmem_folio_free(), so the count stays balanced.
> 
> Fixes: c32287471077 ("gpu/drm/nouveau: enable THP support for GPU
> memory migration")
> Reported-by: Yuhao Jiang <danisjiang@gmail.com>
> Assisted-by: Claude:claude-opus-5
> Cc: stable@vger.kernel.org
> Signed-off-by: Zhenhao Wan <whi4ed0g@gmail.com>
> ---
>  drivers/gpu/drm/nouveau/nouveau_dmem.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c
> b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> index d2abee3efb9a..ad4570c50be7 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> @@ -279,11 +279,25 @@ static vm_fault_t
> nouveau_dmem_migrate_to_ram(struct vm_fault *vmf)
>  
>  static void nouveau_dmem_folio_split(struct folio *head, struct
> folio *tail)
>  {
> +	struct nouveau_dmem_chunk *chunk;
> +	struct nouveau_dmem *dmem;
> +
>  	if (tail == NULL)
>  		return;
>  	tail->pgmap = head->pgmap;
>  	tail->mapping = head->mapping;
>  	folio_set_zone_device_data(tail,
> folio_zone_device_data(head));
> +
> +	/*
> +	 * The split hands out a new independently-freeable folio
> that will
> +	 * later be released via nouveau_dmem_folio_free(); account
> for it so
> +	 * chunk->callocated stays balanced.
> +	 */
> +	chunk = nouveau_page_to_chunk(&head->page);
> +	dmem = chunk->drm->dmem;
> +	spin_lock(&dmem->lock);
> +	chunk->callocated++;
> +	spin_unlock(&dmem->lock);
>  }
>  
>  static const struct dev_pagemap_ops nouveau_dmem_pagemap_ops = {