Re: ubifs: fix page_count in ->ubifs_migrate_page()

[Richard Weinberger]

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@xxxxxxxxx>

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