linux-ext4 - Re: [PATCH v2 13/18] dax: Prep mapping helpers for compound pages

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Date:   Wed, 21 Sep 2022 08:19:11 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>
CC:     <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>,
        "Jan Kara" <jack@...e.cz>, "Darrick J. Wong" <djwong@...nel.org>,
        "Christoph Hellwig" <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        <linux-fsdevel@...r.kernel.org>, <nvdimm@...ts.linux.dev>,
        <linux-xfs@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v2 13/18] dax: Prep mapping helpers for compound pages

Jason Gunthorpe wrote:
> On Thu, Sep 15, 2022 at 08:36:25PM -0700, Dan Williams wrote:
> > In preparation for device-dax to use the same mapping machinery as
> > fsdax, add support for device-dax compound pages.
> > 
> > Presently this is handled by dax_set_mapping() which is careful to only
> > update page->mapping for head pages. However, it does that by looking at
> > properties in the 'struct dev_dax' instance associated with the page.
> > Switch to just checking PageHead() directly in the functions that
> > iterate over pages in a large mapping.
> > 
> > Cc: Matthew Wilcox <willy@...radead.org>
> > Cc: Jan Kara <jack@...e.cz>
> > Cc: "Darrick J. Wong" <djwong@...nel.org>
> > Cc: Jason Gunthorpe <jgg@...dia.com>
> > Cc: Christoph Hellwig <hch@....de>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> > ---
> >  drivers/dax/Kconfig   |    1 +
> >  drivers/dax/mapping.c |   16 ++++++++++++++++
> >  2 files changed, 17 insertions(+)
> > 
> > diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
> > index 205e9dda8928..2eddd32c51f4 100644
> > --- a/drivers/dax/Kconfig
> > +++ b/drivers/dax/Kconfig
> > @@ -9,6 +9,7 @@ if DAX
> >  config DEV_DAX
> >  	tristate "Device DAX: direct access mapping device"
> >  	depends on TRANSPARENT_HUGEPAGE
> > +	depends on !FS_DAX_LIMITED
> >  	help
> >  	  Support raw access to differentiated (persistence, bandwidth,
> >  	  latency...) memory via an mmap(2) capable character
> > diff --git a/drivers/dax/mapping.c b/drivers/dax/mapping.c
> > index 70576aa02148..5d4b9601f183 100644
> > --- a/drivers/dax/mapping.c
> > +++ b/drivers/dax/mapping.c
> > @@ -345,6 +345,8 @@ static vm_fault_t dax_associate_entry(void *entry,
> >  	for_each_mapped_pfn(entry, pfn) {
> >  		struct page *page = pfn_to_page(pfn);
> >  
> > +		page = compound_head(page);
> 
> I feel like the word folio is need here.. pfn_to_folio() or something?
> 
> At the very least we should have a struct folio after doing the
> compound_head, right?

True, I can move this to the folio helpers.