Re: [PATCH RFC v4 2/2] mm: support large folios swapin as a whole for zRAM-like swapfile

[Yosry Ahmed <yosryahmed@google.com>]

[..]
> +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;
> +