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

From: Jan Kara
Date: Thu Jul 31 2014 - 08:21:23 EST

On Thu 31-07-14 09:37:15, Gioh Kim wrote:
> 2014-07-31 ìì 9:03, Jan Kara ì ê:
> >On Thu 31-07-14 08:54:40, Gioh Kim wrote:
> >>2014-07-30 ìí 7:11, Jan Kara ì ê:
> >>>On Wed 30-07-14 16:44:24, Gioh Kim wrote:
> >>>>2014-07-22 ìí 6:38, Jan Kara ì ê:
> >>>>>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()?
> CMA allocation function is cma_alloc().
> Function flow is alloc_contig_range() -> __alloc_contig_migrate_range() -> migrate_pages -> unmap_and_move
> -> __unmap_and_move -> try_to_free_buffers -> drop_buffers -> buffer_busy.
> The buffer_busy() is checking b_count.
> If buffer is busy buffer-cache cannot be removed.
> So the page that includes buffer_head and the page that is refered by
> buffer_head are not movable.
> Is this what you need?
Yes, this is what I was asking about. Thanks! But as I'm looking into
__unmap_and_move() it calls try_to_free_buffers() only if page->mapping ==
NULL. As the comment before that test states, this can happen only for swap
cache (not our case) or for pagecache pages that were truncated and not yet
fully cleaned up. But superblock page cannot really be truncated. So I
somewhat doubt you can hit the above path for a page holding superblock...

Jan Kara <jack@xxxxxxx>
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