Re: [PATCH 04/13] mm/migrate_device: allocate THP deferred split heads

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] mm/migrate_device: duplicate folio_memcg_alloc_deferred() call
  and dead store
--

commit fe6e0150a0f9ab0dcf43e8c18734d603b5d63191
Author: Josef Bacik <josef@toxicpanda.com>

mm/migrate_device: allocate THP deferred split heads

This commit calls folio_memcg_alloc_deferred() after charging the folio in
migrate_vma_insert_huge_pmd_page() and falls back if the allocation fails.

> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 7919e04abc1c5..edcc13edbf6af 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -840,6 +840,13 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>  		goto abort;
>  	}
>  
> +	if (folio_memcg_alloc_deferred(folio)) {
> +		count_vm_event(THP_FAULT_FALLBACK);
> +		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
> +		ret = -ENOMEM;
> +		goto abort;
> +	}
> +
>  	__folio_mark_uptodate(folio);
>  
>  	pgtable = pte_alloc_one(vma->vm_mm);

[Severity: Low]
Does this duplicate the folio_memcg_alloc_deferred() check that commit
ccb36031e6c7 ("mm/migrate_device: allocate deferred split sublists for new
huge folios") already added immediately above?

mm/migrate_device.c:migrate_vma_insert_huge_pmd_page() {
    ...
	/* The charged memcg needs its own sublist before deferred splitting. */
	if (folio_memcg_alloc_deferred(folio)) {
		count_vm_event(THP_FAULT_FALLBACK);
		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
		goto abort;
	}

	if (folio_memcg_alloc_deferred(folio)) {
    ...
}

This calls folio_memcg_alloc_deferred() twice in a row.  In addition,
setting ret = -ENOMEM before jumping to abort is a dead store because the
abort label clears MIGRATE_PFN_MIGRATE and returns 0 without reading ret.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261111-mason-migrate-et-al-fixes-v1-0-3d2454362e8d@kernel.org?part=4