[..]
> +static struct folio *alloc_swap_folio(struct vm_fault *vmf)
> +{
> + struct vm_area_struct *vma = vmf->vma;
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> + unsigned long orders;
> + struct folio *folio;
> + unsigned long addr;
> + spinlock_t *ptl;
> + pte_t *pte;
> + gfp_t gfp;
> + int order;
> +
> + /*
> + * If uffd is active for the vma we need per-page fault fidelity to
> + * maintain the uffd semantics.
> + */
> + if (unlikely(userfaultfd_armed(vma)))
> + goto fallback;
> +
> + /*
> + * a large folio being swapped-in could be partially in
> + * zswap and partially in swap devices, zswap doesn't
> + * support large folios yet, we might get corrupted
> + * zero-filled data by reading all subpages from swap
> + * devices while some of them are actually in zswap
> + */
If we read all subpages from swap devices while some of them are
actually in zswap, the corrupted data won't be zero-filled AFAICT, it
could be anything (old swapped out data). There are also more ways
this can go wrong: if the first page is in zswap, we will only fill
the first page and leave the rest of the folio uninitialized.
How about a more generic comment? Perhaps something like:
A large swapped out folio could be partially or fully in zswap. We
lack handling for such cases, so fallback to swapping in order-0
folio.
> + if (!zswap_never_enabled())
> + goto fallback;
> +
