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Date:   Wed, 7 Oct 2020 14:59:59 -0700
From:   Dan Williams <>
To:     Jan Kara <>
Cc:     Ralph Campbell <>,
        Linux MM <>,
        linux-xfs <>,
        linux-fsdevel <>,
        linux-nvdimm <>,
        Linux Kernel Mailing List <>,
        linux-ext4 <>,
        Matthew Wilcox <>,
        Alexander Viro <>,
        "Theodore Ts'o" <>, Christoph Hellwig <>,
        Andreas Dilger <>,
        "Darrick J. Wong" <>,
        Andrew Morton <>
Subject: Re: [PATCH] ext4/xfs: add page refcount helper

On Wed, Oct 7, 2020 at 1:25 AM Jan Kara <> wrote:
> On Tue 06-10-20 16:09:30, Ralph Campbell wrote:
> > There are several places where ZONE_DEVICE struct pages assume a reference
> > count == 1 means the page is idle and free. Instead of open coding this,
> > add a helper function to hide this detail.
> >
> > Signed-off-by: Ralph Campbell <>
> > Reviewed-by: Christoph Hellwig <>
> Looks as sane direction but if we are going to abstract checks when
> ZONE_DEVICE page is idle, we should also update e.g.
> mm/swap.c:put_devmap_managed_page() or
> mm/gup.c:__unpin_devmap_managed_user_page() (there may be more places like
> this but I found at least these two...). Maybe Dan has more thoughts about
> this.

Yes, but I think that cleanup comes once the idle page count is
unified to be 0 across typical and ZONE_DEVICE pages. Then
free_devmap_managed_page() can be moved internal to __put_page(). For
this patch it's just hiding the "idle == 1" assumption from

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