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

[Gioh Kim]

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?

> 								Honza
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