linux-kernel - Re: [PATCH v7 05/11] mm: split a folio in minimum folio order chunks

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Date: Fri, 7 Jun 2024 18:01:56 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Zi Yan <ziy@...dia.com>
Cc: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
	david@...morbit.com, djwong@...nel.org, chandan.babu@...cle.com,
	brauner@...nel.org, akpm@...ux-foundation.org, mcgrof@...nel.org,
	linux-mm@...ck.org, hare@...e.de, linux-kernel@...r.kernel.org,
	yang@...amperecomputing.com, linux-xfs@...r.kernel.org,
	p.raghav@...sung.com, linux-fsdevel@...r.kernel.org, hch@....de,
	gost.dev@...sung.com, cl@...amperecomputing.com,
	john.g.garry@...cle.com
Subject: Re: [PATCH v7 05/11] mm: split a folio in minimum folio order chunks

On Fri, Jun 07, 2024 at 12:58:33PM -0400, Zi Yan wrote:
> > +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);
> 
> You should only increase this counter when the input folio is a THP, namely
> folio_test_pmd_mappable(folio) is true. For other large folios, we will
> need a separate counter. Something like MTHP_STAT_FILE_SPLIT_FAILED.
> See enum mthp_stat_item in include/linux/huge_mm.h.

Also, why should this count as a split failure?  If we see a NULL
mapping, the folio has been truncated and so no longer needs to be
split.  I understand we currently count it as a failure, but I
don't think we should.