linux-kernel - Re: [PATCH] ext4/xfs: add page refcount helper

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Date:   Wed, 7 Oct 2020 14:59:59 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ralph Campbell <rcampbell@...dia.com>,
        Linux MM <linux-mm@...ck.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Theodore Ts'o" <tytso@....edu>, Christoph Hellwig <hch@....de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] ext4/xfs: add page refcount helper

On Wed, Oct 7, 2020 at 1:25 AM Jan Kara <jack@...e.cz> 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 <rcampbell@...dia.com>
> > Reviewed-by: Christoph Hellwig <hch@....de>
>
> 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
dax-filesystems.