linux-kernel - Re: [PATCH v9 10/10] fsdax: set a CoW flag when associate reflink mappings

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Date:   Thu, 20 Jan 2022 23:16:21 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        nvdimm@...ts.linux.dev, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, djwong@...nel.org,
        dan.j.williams@...el.com, david@...morbit.com, jane.chu@...cle.com
Subject: Re: [PATCH v9 10/10] fsdax: set a CoW flag when associate reflink
 mappings

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.