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Date: Fri, 7 Jun 2024 20:30:26 +0000
From: "Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>
To: Zi Yan <ziy@nvidia.com>
Cc: david@fromorbit.com, djwong@kernel.org, chandan.babu@oracle.com,
	brauner@kernel.org, akpm@linux-foundation.org, willy@infradead.org,
	mcgrof@kernel.org, linux-mm@kvack.org, hare@suse.de,
	linux-kernel@vger.kernel.org, yang@os.amperecomputing.com,
	linux-xfs@vger.kernel.org, p.raghav@samsung.com,
	linux-fsdevel@vger.kernel.org, hch@lst.de, gost.dev@samsung.com,
	cl@os.amperecomputing.com, john.g.garry@oracle.com
Subject: Re: [PATCH v7 05/11] mm: split a folio in minimum folio order chunks
Message-ID: <20240607203026.zj3akxdjeykchnnf@quentin>
References: <20240607145902.1137853-1-kernel@pankajraghav.com>
 <20240607145902.1137853-6-kernel@pankajraghav.com>
 <75CCE180-EC90-4BDC-B5D8-0ED1B710BE49@nvidia.com>
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In-Reply-To: <75CCE180-EC90-4BDC-B5D8-0ED1B710BE49@nvidia.com>

On Fri, Jun 07, 2024 at 12:58:33PM -0400, Zi Yan wrote:
> Hi Pankaj,
> 
> Can you use ziy@nvidia.com instead of zi.yan@sent.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@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>
> > ---
> >  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