Re: [PATCH v8 04/10] mm: split a folio in minimum folio order chunks

[Zi Yan]

On Tue Jun 25, 2024 at 7:44 AM EDT, Pankaj Raghav (Samsung) wrote:
> From: Luis Chamberlain <mcgrof@xxxxxxxxxx>
>
> split_folio() and split_folio_to_list() assume order 0, to support
> minorder for non-anonymous folios, we must expand these to check the
> folio mapping order and use that.
>
> Set new_order to be at least minimum folio order if it is set in
> split_huge_page_to_list() so that we can maintain minimum folio order
> requirement in the page cache.
>
> Update the debugfs write files used for testing to ensure the order
> is respected as well. We simply enforce the min order when a file
> mapping is used.
>
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> Reviewed-by: Hannes Reinecke <hare@xxxxxxx>
> ---
> There was a discussion about whether we need to consider truncation of
> folio to be considered a split failure or not [1]. The new code has
> retained the existing behaviour of returning a failure if the folio was
> truncated. I think we need to have a separate discussion whethere or not
> to consider it as a failure.

<snip>

>
> +int split_folio_to_list(struct folio *folio, struct list_head *list)
> +{
> +	unsigned int min_order = 0;
> +
> +	if (!folio_test_anon(folio)) {
> +		if (!folio->mapping) {
> +			count_vm_event(THP_SPLIT_PAGE_FAILED);

Regardless this folio split is from a truncation or not, you should not
count every folio split as a THP_SPLIT_PAGE_FAILED. Since not every
folio is a THP. You need to do:

if (folio_test_pmd_mappable(folio))
	count_vm_event(THP_SPLIT_PAGE_FAILED);

See commit 835c3a25aa37 ("mm: huge_memory: add the missing
folio_test_pmd_mappable() for THP split statistics")
	
> +			return -EBUSY;
> +		}
> +		min_order = mapping_min_folio_order(folio->mapping);
> +	}
> +
> +	return split_huge_page_to_list_to_order(&folio->page, list, min_order);
> +}
> +

-- 
Best Regards,
Yan, Zi

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