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Date:   Mon, 5 Jul 2021 19:45:26 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     Chao Yu <chao@kernel.org>, linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, linux-mm@kvack.org,
        Mel Gorman <mgorman@techsingularity.net>
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 11:04:21AM -0700, Jaegeuk Kim wrote:
> On 07/05, Matthew Wilcox wrote:
> > 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);
> 
> Hmm, I can see it in 4.14 and 5.10 kernel.
> 
> The trace is on:
> 
>  30875 [ 1065.118750] c3     87  f2fs_migrate_page+0x354/0x45c
>  30876 [ 1065.123872] c3     87  move_to_new_page+0x70/0x30c
>  30877 [ 1065.128813] c3     87  migrate_pages+0x3a0/0x964
>  30878 [ 1065.133583] c3     87  compact_zone+0x608/0xb04
>  30879 [ 1065.138257] c3     87  kcompactd+0x378/0x4ec
>  30880 [ 1065.142664] c3     87  kthread+0x11c/0x12c
>  30881 [ 1065.146897] c3     87  ret_from_fork+0x10/0x18
> 
>  It seems compaction_alloc() gets a free page which doesn't reset the fields?

I'm not really familiar with the compaction code.  Mel, I see a call
to post_alloc_hook() in split_map_pages().  Are there other ways of
getting the compaction code to allocate a page which don't go through
split_map_pages()?