Re: [PATCH v1] mm/huge_memory: improve split_huge_page_to_list_to_order() return value documentation

[John Hubbard <jhubbard@nvidia.com>]

On 4/18/24 8:18 AM, David Hildenbrand wrote:
> The documentation is wrong and relying on it almost resulted in BUGs
> in new callers: we return -EAGAIN on unexpected folio references, not
> -EBUSY.
> 
> Let's fix that and also document which other return values we can
> currently see and why they could happen.
> 
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   mm/huge_memory.c | 13 ++++++++++---
>   1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index ee12726291f1b..824eff9211db8 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2956,7 +2956,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)
>    *
>    * 3) The folio must not be pinned. Any unexpected folio references, including
>    *    GUP pins, will result in the folio not getting split; instead, the caller
> - *    will receive an -EBUSY.
> + *    will receive an -EAGAIN.
>    *
>    * 4) @new_order > 1, usually. Splitting to order-1 anonymous folios is not
>    *    supported for non-file-backed folios, because folio->_deferred_list, which
> @@ -2975,8 +2975,15 @@ bool can_split_folio(struct folio *folio, int *pextra_pins)

As an aside, the use of unconditional local_irq_disable() / local_irq_enable()
calls in this routine almost makes me believe that we should have:

5) Local IRQs should be enabled. Because this routine may enable them.

...but I can't imagine a way to end up calling this with interrupts
disabled, so it seems like documentation overkill. Just thought I'd mention
it, though.


>    *
>    * Returns 0 if the huge page was split successfully.
>    *
> - * Returns -EBUSY if @page's folio is pinned, or if the anon_vma disappeared
> - * from under us.
> + * Returns -EAGAIN if the folio has unexpected reference (e.g., GUP).

...or if the folio was removed from the page cache before this routine
got a chance to lock it, right? (See the "fail:" path.)

> + *
> + * Returns -EBUSY when trying to split the huge zeropage, if the folio is
> + * under writeback, if fs-specific folio metadata cannot currently be
> + * released, or if some unexpected race happened (e.g., anon VMA disappeared,
> + * truncation).
> + *
> + * Returns -EINVAL when trying to split to an order that is incompatible
> + * with the folio. Splitting to order 0 is compatible with all folios.
>    */
>   int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>   				     unsigned int new_order)

Otherwise, looks good.


thanks,
-- 
John Hubbard
NVIDIA