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

[Gioh Kim]

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.

For example, fat_fill_super() reads superblock via sb_bread()
and release it via brelse() immediately. Therefore the page that stores superblock can be migrated.



>    It doesn't seem to be worth the effort to make that page movable to me
> (it's reasonably doable since superblock buffer isn't accessed in *that*
> many places but single movable page doesn't seem like a good tradeoff for
> the complexity).
>
> But this made me look into the migration code and it isn't completely clear
> to me what makes the migration code decide that sb buffer isn't movable? We
> seem to be locking the buffers before moving the underlying page but we
> don't do any reference or state checks on the buffers... That seems to be
> assuming that noone looks at bh->b_data without holding buffer lock. That
> is likely true for ordinary data but definitely not true for metadata
> buffers (i.e., buffers for pages from block device mappings).

The sb buffer is not movable because it is not released.
sb_bread increase the reference counter of buffer-head so that
the page of the buffer-head cannot be movable.

sb_bread allocates page from movable area but it is not movable until the
reference counter of the buffer-head becomes zero.
There is no lock for the buffer but the reference counter acts like lock.

Actually it is strange that ext4 keeps buffer-head in superblock structure until unmount (it can be long time)
I thinks the buffer-head should be released immediately like fat_fill_super() did.
I believe there is a reason to keep buffer-head so that I suggest this patch.



> Added linux-mm to CC to enlighten me a bit ;)
>
> 								Honza
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