Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area

[Jan Kara <jack@suse.cz>]

On Thu 31-07-14 08:54:40, Gioh Kim wrote:
> 2014-07-30 i??i?? 7:11, Jan Kara i?' e,?:
> >On Wed 30-07-14 16:44:24, Gioh Kim wrote:
> >>2014-07-22 i??i?? 6:38, Jan Kara i?' e,?:
> >>>On Tue 22-07-14 09:30:05, Peter Zijlstra wrote:
> >>>>On Tue, Jul 22, 2014 at 02:18:47PM +0900, Gioh Kim wrote:
> >>>>>Hello,
> >>>>>
> >>>>>This patch try to solve problem that a long-lasting page cache of
> >>>>>ext4 superblock disturbs page migration.
> >>>>>
> >>>>>I've been testing CMA feature on my ARM-based platform
> >>>>>and found some pages for page caches cannot be migrated.
> >>>>>Some of them are page caches of superblock of ext4 filesystem.
> >>>>>
> >>>>>Current ext4 reads superblock with sb_bread(). sb_bread() allocates page
> >>>>>from movable area. But the problem is that ext4 hold the page until
> >>>>>it is unmounted. If root filesystem is ext4 the page cannot be migrated forever.
> >>>>>
> >>>>>I introduce a new API for allocating page from non-movable area.
> >>>>>It is useful for ext4 and others that want to hold page cache for a long time.
> >>>>
> >>>>There's no word on why you can't teach ext4 to still migrate that page.
> >>>>For all I know it might be impossible, but at least mention why.
> >>
> >>I am very sorry for lacking of details.
> >>
> >>In ext4_fill_super() the buffer-head of superblock is stored in sbi->s_sbh.
> >>The page belongs to the buffer-head is allocated from movable area.
> >>To migrate the page the buffer-head should be released via brelse().
> >>But brelse() is not called until unmount.
> >   Hum, I don't see where in the code do we check buffer_head use count. Can
> >you please point me? Thanks.
> 
> Filesystem code does not check buffer_head use count.  sb_bread() returns
> the buffer_head that is included in bh_lru and has non-zero use count.
> You can see the bh_lru code in buffer.c: __find_get_clock() and
> lookup_bh_lru().  bh_lru_install() inserts the buffer_head into the
> bh_lru().  It first calls get_bh() to increase the use count and insert
> bh into the lru array.
> 
> The buffer_head use count is non-zero until brelse() is called.
  So I probably didn't phrase the question precisely enough. What I was
asking about is where exactly *migration* code checks buffer use count?
Because as I'm looking at buffer_migrate_page() we lock the buffers on a
migrated page but we don't look at buffer use counts... So it seems to me
that migration of a page with buffers should succeed even if buffer head
has an elevated use count. Now I think that it *should* check the buffer
use counts (it is dangerous to migrate buffers someone holds reference to)
but I just cannot find that place. Or does CMA use some other migration
function for buffer pages than buffer_migrate_page()?

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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