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
