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Date:   Thu, 20 Jan 2022 23:16:21 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Shiyang Ruan <ruansy.fnst@fujitsu.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org,
        nvdimm@lists.linux.dev, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, djwong@kernel.org,
        dan.j.williams@intel.com, david@fromorbit.com, jane.chu@oracle.com
Subject: Re: [PATCH v9 10/10] fsdax: set a CoW flag when associate reflink
 mappings
Message-ID: <YepdxZ+XrAZYv1dX@infradead.org>
References: <20211226143439.3985960-1-ruansy.fnst@fujitsu.com>
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 <YekkYAJ+QegoDKCJ@infradead.org>
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On Fri, Jan 21, 2022 at 10:33:58AM +0800, Shiyang Ruan wrote:
> > 
> > But different question, how does this not conflict with:
> > 
> > #define PAGE_MAPPING_ANON       0x1
> > 
> > in page-flags.h?
> 
> Now we are treating dax pages, so I think its flags should be different from
> normal page.  In another word, PAGE_MAPPING_ANON is a flag of rmap mechanism
> for normal page, it doesn't work for dax page.  And now, we have dax rmap
> for dax page.  So, I think this two kinds of flags are supposed to be used
> in different mechanisms and won't conflect.

It just needs someone to use folio_test_anon in a place where a DAX
folio can be passed.  This probably should not happen, but we need to
clearly document that.

> > Either way I think this flag should move to page-flags.h and be
> > integrated with the PAGE_MAPPING_FLAGS infrastucture.
> 
> And that's why I keep them in this dax.c file.

But that does not integrate it with the infrastructure.  For people
to debug things it needs to be next to PAGE_MAPPING_ANON and have
documentation explaining why they are exclusive.