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Subject: Re: [f2fs-dev] [PATCH] f2fs: initialize page->private when using for
 our internal use
To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     Matthew Wilcox <willy@infradead.org>, linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, linux-mm@kvack.org
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From:   Chao Yu <chao@kernel.org>
Message-ID: <5700f9ec-20e9-7de9-7f8e-c11ec7279c20@kernel.org>
Date:   Tue, 6 Jul 2021 08:16:11 +0800
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On 2021/7/6 2:06, Jaegeuk Kim wrote:
> On 07/06, Chao Yu wrote:
>> 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.
> 
> We can debug anything tho, this issue is blocking the production, and I'd
> like to get this in this merge windows. Could you please check the patch
> has any holes?

The code looks good to me,

Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,

> 
>>
>> Thanks,
>>
>>>