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 20:30:26 +0000
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Zi Yan <ziy@...dia.com>
Cc: david@...morbit.com, djwong@...nel.org, chandan.babu@...cle.com,
	brauner@...nel.org, akpm@...ux-foundation.org, willy@...radead.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:
> Hi Pankaj,
> 
> Can you use ziy@...dia.com instead of zi.yan@...t.com? Since I just use the latter
> to send patches. Thanks.

Got it!

> 
> On 7 Jun 2024, at 10:58, Pankaj Raghav (Samsung) wrote:
> 
> > From: Luis Chamberlain <mcgrof@...nel.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@...nel.org>
> > Signed-off-by: Pankaj Raghav <p.raghav@...sung.com>
> > ---
> >  include/linux/huge_mm.h | 14 ++++++++---
> >  mm/huge_memory.c        | 55 ++++++++++++++++++++++++++++++++++++++---
> >  2 files changed, 61 insertions(+), 8 deletions(-)
> >
> 
> <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);
> 
> 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.
> 
Hmm, but we don't have mTHP support for non-anonymous memory right? In
that case it won't be applicable for file backed memory? 

I am not an expert there so correct me if I am wrong.

--
Regards,
Pankaj