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Date:   Mon, 5 Jul 2021 12:47:48 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Chao Yu <chao@kernel.org>
Cc:     Jaegeuk Kim <jaegeuk@kernel.org>, linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, linux-mm@kvack.org
Subject: Re: [f2fs-dev] [PATCH] f2fs: initialize page->private when using for
 our internal use
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On Mon, Jul 05, 2021 at 07:33:35PM +0800, Chao Yu wrote:
> On 2021/7/5 16:56, Jaegeuk Kim wrote:
> > On 07/05, Chao Yu wrote:
> > > On 2021/7/5 13:22, Jaegeuk Kim wrote:
> > > > We need to guarantee it's initially zero. Otherwise, it'll hurt entire flag
> > > > operations.
> > > 
> > > Oops, I didn't get the point, shouldn't .private be zero after page was
> > > just allocated by filesystem? What's the case we will encounter stall
> > > private data left in page?
> > 
> > I'm seeing f2fs_migrate_page() has the newpage with some value without Private
> > flag. That causes a kernel panic later due to wrong private flag used in f2fs.
> 
> I'm not familiar with that part of codes, so Cc mm mailing list for help.
> 
> My question is newpage in .migrate_page() may contain non-zero value in .private
> field but w/o setting PagePrivate flag, is it a normal case?

I think freshly allocated pages have a page->private of 0.  ie this
code in mm/page_alloc.c:

                page = rmqueue(ac->preferred_zoneref->zone, zone, order,
                                gfp_mask, alloc_flags, ac->migratetype);
                if (page) {
                        prep_new_page(page, order, gfp_mask, alloc_flags);

where prep_new_page() calls post_alloc_hook() which contains:
        set_page_private(page, 0);

Now, I do see in __buffer_migrate_page() (mm/migrate.c):

        attach_page_private(newpage, detach_page_private(page));

but as far as I can tell, f2fs doesn't call any of the
buffer_migrate_page() paths.  So I'm not sure why you're seeing
a non-zero page->private.