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Date:   Wed, 21 Sep 2022 11:06:03 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     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

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?

Jason

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