linux-ext4 - Re: [PATCH V3 0/8] Cleancache: overview

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Message-ID: <4C49468B.40307@vflare.org>
Date:	Fri, 23 Jul 2010 13:06:43 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	chris.mason@...cle.com, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, adilger@....com, tytso@....edu,
	mfasheh@...e.com, joel.becker@...cle.com, matthew@....cx,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	ocfs2-devel@....oracle.com, linux-mm@...ck.org, jeremy@...p.org,
	JBeulich@...ell.com, kurt.hackel@...cle.com, npiggin@...e.de,
	dave.mccracken@...cle.com, riel@...hat.com, avi@...hat.com,
	konrad.wilk@...cle.com
Subject: Re: [PATCH V3 0/8] Cleancache: overview

On 06/22/2010 04:48 AM, Dan Magenheimer wrote:
> [PATCH V3 0/8] Cleancache: overview
> 
<snip>
> 
>  Documentation/ABI/testing/sysfs-kernel-mm-cleancache |   11 +
>  Documentation/vm/cleancache.txt                      |  194 +++++++++++++++++++
>  fs/btrfs/extent_io.c                                 |    9 
>  fs/btrfs/super.c                                     |    2 
>  fs/buffer.c                                          |    5 
>  fs/ext3/super.c                                      |    2 
>  fs/ext4/super.c                                      |    2 
>  fs/mpage.c                                           |    7 
>  fs/ocfs2/super.c                                     |    3 
>  fs/super.c                                           |    7 
>  include/linux/cleancache.h                           |   88 ++++++++
>  include/linux/fs.h                                   |    5 
>  mm/Kconfig                                           |   22 ++
>  mm/Makefile                                          |    1 
>  mm/cleancache.c                                      |  169 ++++++++++++++++
>  mm/filemap.c                                         |   11 +
>  mm/truncate.c                                        |   10 
>  17 files changed, 548 insertions(+)
> 
> (following is a copy of Documentation/vm/cleancache.txt)
> 
> MOTIVATION
> 
> Cleancache can be thought of as a page-granularity victim cache for clean
> pages that the kernel's pageframe replacement algorithm (PFRA) would like
> to keep around, but can't since there isn't enough memory.  So when the
> PFRA "evicts" a page, it first attempts to put it into a synchronous
> concurrency-safe page-oriented "pseudo-RAM" device (such as Xen's Transcendent
> Memory, aka "tmem", or in-kernel compressed memory, aka "zmem", or other
> RAM-like devices) which is not directly accessible or addressable by the
> kernel and is of unknown and possibly time-varying size.  And when a
> cleancache-enabled filesystem wishes to access a page in a file on disk,
> it first checks cleancache to see if it already contains it; if it does,
> the page is copied into the kernel and a disk access is avoided.
> 


Since zcache is now one of its use cases, I think the major objection that
remains against cleancache is its intrusiveness -- in particular, need to
change individual filesystems (even though one liners). Changes below should
help avoid these per-fs changes and make it more self contained. I haven't
tested these changes myself, so there might be missed cases or other mysterious
problems:

1. Cleancache requires filesystem specific changes primarily to make a call to
cleancache init and store (per-fs instance) pool_id. I think we can get rid of
these by directly passing 'struct super_block' pointer which is also
sufficient to identify FS instance a page belongs to. This should then be used
as a 'handle' by cleancache_ops provider to find corresponding memory pool or
create a new pool when a new handle is encountered.

This leaves out case of ocfs2 for which cleancache needs 'uuid' to decide if a
shared pool should be created. IMHO, this case (and cleancache.init_shared_fs)
should be removed from cleancache_ops since it is applicable only for Xen's
cleancache_ops provider.

2. I think change in btrfs can be avoided by moving cleancache_get_page()
from do_mpage_reapage() to filemap_fault() and this should work for all
filesystems. See:

handle_pte_fault() -> do_(non)linear_fault() -> __do_fault()
						-> vma->vm_ops->fault()

which is defined as filemap_fault() for all filesystems. If some future
filesystem uses its own custom function (why?) then it will have to arrange for
call to cleancache_get_page(), if it wants this feature.

With above changes, cleancache will be fairly self-contained:
 - cleancache_put_page() when page is removed from page-cache
 - cleacacache_get_page() when PF occurs (and after page-cache is searched)
 - cleancache_flush_*() on truncate_*()

Thanks,
Nitin
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