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

[Barry Song]

On Tue, Jul 2, 2024 at 1:53 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> [..]
> > +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.

looks good to me, thanks!

>
> > +       if (!zswap_never_enabled())
> > +               goto fallback;
> > +