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

[Jan Kara <jack@suse.cz>]

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.

> >   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.
  OK, but why do you care about a single page (of at most handful if you
have more filesystems) which isn't movable? That shouldn't make a big
difference to compaction...

> 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.
  We don't copy some data from the superblock to other structure so from
time to time we need to look e.g. at feature bits within superblock buffer.
Historically we were updating numbers of free blocks and inodes in the
superblock with each allocation but we don't do that anymore because it
scales poorly. So there is no fundamental reason for keeping sb buffer
pinned anymore. Just someone would have to rewrite the code to copy some
pieces of data from the buffer to some other structure and use it there.

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

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