linux-kernel - Re: [PATCH v7 8/8] fsdax: add exception for reflinked files

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Date:   Thu, 14 Oct 2021 12:24:50 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        nvdimm@...ts.linux.dev, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, dan.j.williams@...el.com,
        david@...morbit.com, hch@...radead.org, jane.chu@...cle.com
Subject: Re: [PATCH v7 8/8] fsdax: add exception for reflinked files

On Fri, Sep 24, 2021 at 09:09:59PM +0800, Shiyang Ruan wrote:
> For reflinked files, one dax page may be associated more than once with
> different fime mapping and index.  It will report warning.  Now, since
> we have introduced dax-RMAP for this case and also have to keep its
> functionality for other filesystems who are not support rmap, I add this
> exception here.
> 
> Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>
> ---
>  fs/dax.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 2536c105ec7f..1a57211b1bc9 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -352,9 +352,10 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
>  	for_each_mapped_pfn(entry, pfn) {
>  		struct page *page = pfn_to_page(pfn);
>  
> -		WARN_ON_ONCE(page->mapping);
> -		page->mapping = mapping;
> -		page->index = index + i++;
> +		if (!page->mapping) {
> +			page->mapping = mapping;
> +			page->index = index + i++;

It feels a little dangerous to have page->mapping for shared storage
point to an actual address_space when there are really multiple
potential address_spaces out there.  If the mm or dax folks are ok with
doing this this way then I'll live with it, but it seems like you'd want
to leave /some/ kind of marker once you know that the page has multiple
owners and therefore regular mm rmap via page->mapping won't work.

--D

> +		}
>  	}
>  }
>  
> @@ -370,9 +371,10 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
>  		struct page *page = pfn_to_page(pfn);
>  
>  		WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> -		WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> -		page->mapping = NULL;
> -		page->index = 0;
> +		if (page->mapping == mapping) {
> +			page->mapping = NULL;
> +			page->index = 0;
> +		}
>  	}
>  }
>  
> -- 
> 2.33.0
> 
> 
>