Re: [PATCH] fix page_count in ->iomap_migrate_page()

[Richard Weinberger <richard@nod.at>]

[CC'ing authors of the code plus mm folks]

Am Freitag, 14. Dezember 2018, 06:56:01 CET schrieb zhangjun:
> IOMAP uses PG_private a little different with buffer_head based
> filesystem.
> It uses it as marker and when set, the page counter is not incremented,
> migrate_page_move_mapping() 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.
> 
> Signed-off-by: zhangjun <openzhangj@gmail.com>
> ---
>  fs/iomap.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap.c b/fs/iomap.c
> index 64ce240..352e58a 100644
> --- a/fs/iomap.c
> +++ b/fs/iomap.c
> @@ -544,8 +544,17 @@ iomap_migrate_page(struct address_space *mapping, struct page *newpage,
>  		struct page *page, enum migrate_mode mode)
>  {
>  	int ret;
> +	int extra_count = 0;
>  
> -	ret = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
> +	/*
> +	 * IOMAP 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.
> +	 */
> +	if (page_has_private(page))
> +		extra_count = -1;
> +	ret = migrate_page_move_mapping(mapping, newpage, page,
> +		       NULL, mode, extra_count);
>  	if (ret != MIGRATEPAGE_SUCCESS)
>  		return ret;

This is the third place which needs this workaround.
UBIFS, F2FS, and now iomap.

I agree with Dave that nobody can assume that PG_private implies an additional
page reference.
But page migration does that. Including parts of the write back code.

Thanks,
//richard