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SubjectRe: [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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