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

[zhangjun]

On 2018/12/13 äå10:23, 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@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
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?