linux-kernel - Re: ubifs: fix page_count in ->ubifs_migrate

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Date:   Thu, 13 Dec 2018 23:14:37 +0800
From:   zhangjun <openzhangj@...il.com>
To:     Richard Weinberger <richard@....at>
Cc:     Artem Bityutskiy <dedekind1@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kirill.shutemov@...ux.intel.com, hch@....de,
        linux-fsdevel@...r.kernel.org
Subject: Re: ubifs: fix page_count in ->ubifs_migrate_page()


On 2018/12/13 下午10:23, Richard Weinberger wrote:
> Hello zhangjun,
>
> thanks a lot for bringing this up!
>
> Am Mittwoch, 12. Dezember 2018, 15:13:57 CET schrieb zhangjun:
>> Because the PagePrivate() in UBIFS is different meanings,
>> alloc_cma() will fail when one dirty page cache located in
>> the type of MIGRATE_CMA
>>
>> If not adjust the 'extra_count' for dirty page,
>> ubifs_migrate_page() -> migrate_page_move_mapping() will
>> always return -EAGAIN for:
>> 	expected_count += page_has_private(page)
>> This causes the migration to fail until the page cache is cleaned
>>
>> In general, PagePrivate() indicates that buff_head is already bound
>> to this page, and at the same time page_count() will also increase.
>> But UBIFS set private flag when the cache is dirty, and page_count()
>> not increase.
>> Therefore, the expected_count of UBIFS is different from the general
>> case.
> As you noted, UBIFS uses PG_private a little different.
> It uses it as marker and when set, the page counter is not incremented,
> since no private data is attached.
> The migration code assumes that PG_private indicates a counter of +1.
> So, we have to pass a extra count of -1 to migrate_page_move_mapping() if
> the flag is set.
> Just like F2FS does. Not really nice but hey...
>
>> Signed-off-by: zhangjun <openzhangj@...il.com>
> Fixes: 4ac1c17b2044 ("UBIFS: Implement ->migratepage()")
>
>> ---
>>   fs/ubifs/file.c | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
>> index 1b78f2e..2136a5c 100644
>> --- a/fs/ubifs/file.c
>> +++ b/fs/ubifs/file.c
>> @@ -1480,8 +1480,15 @@ static int ubifs_migrate_page(struct address_space *mapping,
>>   		struct page *newpage, struct page *page, enum migrate_mode mode)
>>   {
>>   	int rc;
>> +	int extra_count;
>>   
>> -	rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
>> +	/*
>> +	 * UBIFS is using PagePrivate() which can have different meanings across
>> +	 * filesystems. So here adjusting the 'extra_count' make it work.
>> +	 */
> Please rewrite that comment.
> /*
>   * UBIFS uses PG_private as marker and does not raise the page counter.
>   * migrate_page_move_mapping() expects a incremented counter if PG_private
>   * is set. Therefore pass -1 as extra_count for this case.
>   */
>
>> +	extra_count = 0 - page_has_private(page);
> if (page_has_private(page))
> 	extra_count = -1;
>
> That way this corner is much more obvious.
>
> Thanks,
> //richard
>
>
Hello Richard.

Thank you very much for your help.

1. Can i use your description for comment? like this:

/*
  * UBIFS uses PG_private a little different.
  * It uses it as marker and when set, the page counter is not incremented,
  * since no private data is attached.
  * The migration code assumes that PG_private indicates a counter of +1.
  * So, we have to pass a extra count of -1 to migrate_page_move_mapping() if
  * the flag is set.
  */

2. It's more obvious, but the branch may break the cpu pipeline?