linux-kernel - Re: [PATCH v4 3/6] mm: rmap: introduce pfn_mkclean

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Date:   Wed, 9 Mar 2022 16:40:25 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        Yang Shi <shy828301@...il.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Hugh Dickins <hughd@...gle.com>, xiyuyang19@...an.edu.cn,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ross Zwisler <zwisler@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, duanxiongchun@...edance.com,
        Muchun Song <smuchun@...il.com>
Subject: Re: [PATCH v4 3/6] mm: rmap: introduce pfn_mkclean_range() to cleans PTEs

On Wed, Mar 9, 2022 at 4:26 PM Dan Williams <dan.j.williams@...el.com> wrote:
>
> On Wed, Mar 2, 2022 at 12:29 AM Muchun Song <songmuchun@...edance.com> wrote:
> >
> > The page_mkclean_one() is supposed to be used with the pfn that has a
> > associated struct page, but not all the pfns (e.g. DAX) have a struct
> > page. Introduce a new function pfn_mkclean_range() to cleans the PTEs
> > (including PMDs) mapped with range of pfns which has no struct page
> > associated with them. This helper will be used by DAX device in the
> > next patch to make pfns clean.
>
> This seems unfortunate given the desire to kill off
> CONFIG_FS_DAX_LIMITED which is the only way to get DAX without 'struct
> page'.
>
> I would special case these helpers behind CONFIG_FS_DAX_LIMITED such
> that they can be deleted when that support is finally removed.

...unless this support is to be used for other PFN_MAP scenarios where
a 'struct page' is not available? If so then the "(e.g. DAX)" should
be clarified to those other cases.