Re: [PATCH] fix page_count in ->iomap_migrate_page()
From: Richard Weinberger
Date: Fri Dec 14 2018 - 06:25:57 EST
[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
> 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@xxxxxxxxx>
> 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
But page migration does that. Including parts of the write back code.