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Subject: Re: [f2fs-dev] [PATCH] f2fs: initialize page->private when using for
 our internal use
To:     Matthew Wilcox <willy@infradead.org>,
        Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, linux-mm@kvack.org
References: <20210705052216.831989-1-jaegeuk@kernel.org>
 <c32642d6-6de2-eb2d-5771-c7cefa62fab5@kernel.org>
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 <e2fdf628-f25c-7495-cfd1-952899f7ff9a@kernel.org>
 <YOLxZAnaKSwBIlK9@casper.infradead.org>
From:   Chao Yu <chao@kernel.org>
Message-ID: <5ab8d01a-8fac-60b2-9c2c-a32c5a81b394@kernel.org>
Date:   Tue, 6 Jul 2021 00:09:18 +0800
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On 2021/7/5 19:47, Matthew Wilcox wrote:
> 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.

Well, that's strange.

Jaegeuk, let's add a BUGON in f2fs to track the call path where newpage
has non-zero private value? if this issue is reproducible.

Thanks,

>