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SubjectRe: [PATCH mm-unstable] mm: clarify folio_set_compound_order() zero support
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On 12/9/22 06:27, Muchun Song wrote:
> From you advise, I think we can remove VM_BUG_ON and handle non-zero
> order page, something like:

Yes, and thanks for summarizing all the individual feedback into a
proposed solution.

If we go this route, then I'd suggest a little note above the function,
such as:

/*
* For non-large folios, this will have no effect, other than possibly
* generating a warning, if the caller attempts to set a non-zero folio order
* for a non-large folio.
*/

> static inline void folio_set_order(struct folio *folio,
> unsigned int order)
> {
> if (!folio_test_large(folio)) {
> WARN_ON(order);

Better make that a WARN_ON_ONCE(), to avoid taking the machine down
with excessive warnings in the log.

> return;
> }
>
> folio->_folio_order = order;
> #ifdef CONFIG_64BIT
> folio->_folio_nr_pages = order ? 1U << order : 0;
> #endif
> }
>
> In this case,
>
> 1) we can handle both non-zero and zero (folio_order() works as well
> for this case) order page.
> 2) it can prevent OOB for non-large folio and warn unexpected users.
> 3) Do not BUG.
> 4) No need to rename folio_set_order.
>
> What do you think?

If the new behavior is OK with everyone, it seems good to me.

thanks,
--
John Hubbard
NVIDIA

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