Re: [RFC] iomap: use huge zero folio in iomap_dio_zero

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On Wed, May 15, 2024 at 03:59:43PM +0000, Pankaj Raghav (Samsung) wrote:
>  static int __init iomap_init(void)
>  {
> +       void            *addr = kzalloc(16 * PAGE_SIZE, GFP_KERNEL);

Don't use XFS coding style outside XFS.

kzalloc() does not guarantee page alignment much less alignment to
a folio.  It happens to work today, but that is an implementation
artefact.

> +
> +       if (!addr)
> +               return -ENOMEM;
> +
> +       zero_fsb_folio = virt_to_folio(addr);

We also don't guarantee that calling kzalloc() gives you a virtual
address that can be converted to a folio.  You need to allocate a folio
to be sure that you get a folio.

Of course, you don't actually need a folio.  You don't need any of the
folio metadata and can just use raw pages.

> +       /*
> +        * The zero folio used is 64k.
> +        */
> +       WARN_ON_ONCE(len > (16 * PAGE_SIZE));

PAGE_SIZE is not necessarily 4KiB.

> +       bio = iomap_dio_alloc_bio(iter, dio, BIO_MAX_VECS,
> +                                 REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);

The point was that we now only need one biovec, not MAX.





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