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

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

On Tue, Jun 25, 2024 at 10:45:09AM -0400, Zi Yan wrote:
> On Tue Jun 25, 2024 at 7:44 AM EDT, Pankaj Raghav (Samsung) wrote:
> > From: Luis Chamberlain <mcgrof@kernel.org>
> >
> > 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@kernel.org>
> > Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> > Reviewed-by: Hannes Reinecke <hare@suse.de>
> > ---
> > 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")

You are right, I will change that. I didn't notice this commit. 

> 	
> > +			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
>