On 07/26/2011 05:48 AM, Yongqiang Yang wrote:
On Tue, Jul 26, 2011 at 8:12 PM, Jan Kara<jack@xxxxxxx> wrote:
Hi Yongqiang,
On Tue 26-07-11 09:20:28, Yongqiang Yang wrote:
I have been thinking if we can handle fiemap much simpler for a while.
Current code is very ugly due to page cache look up. I have a
thought on simplifying these code. The reason leading us to looking
up page cache is that delayed extents are not in extents tree. I
think we can add an in-memory delayed extents list in inode, and we
can delete entries in the list after we allocate blocks for them.
There is no limit on length of extents in the list, this way can an
entry contain as many blocks as they are contiguous logically.
What's your opinion?
Yes, that should be doable and shouldn't have too big overhead. It's just
stupid we'll do all this stuff only for fiemap call which is relatively
rare.
I guess there are other places where delayed extents should be handled
by looking up page cache.
SEEK_HOLE and SEEK_DATA also need to lookup page cache to handle
delayed extents.
Hi Allison,
If a delayed extents list added in the inode, could punch hole code be simpler?
Yongqiang.
Hi there,
Well, I think we may be able to make it more efficient if we had the
delayed extent list.
The earlier versions of punch hole were complex because of the different
mechanisms needed to identify when extents were mapped, delayed or a
hole. Later we decided that this was too complex, and the pages that
covered the hole need to be sync'd anyway, which eliminated the need to
detect the delayed extents, but it is a wasteful operation if the
extents in the hole were just unwritten. If we had the delayed extent
list, I think we may just be able to sync extents as needed instead of
syncing the entire hole.
Allison Henderson
Honza
On Mon, Jul 25, 2011 at 11:58 PM, Jan Kara<jack@xxxxxxx> wrote:
Hello,
I just had a look at the code checking delayed allocated buffers in
ext4_ext_fiemap_cb(). I believe the checks there could use some elimiation
of common patterns but that's just a minor thing. The main problem is that
the code can easily crash the kernel when it races with page reclaim. You
just cannot access most of the page contents (and for buffers it is
especially true) without locking the page. Getting a reference via
find_get_pages_tag() guarantees you the structure cannot go away but mm is
still free to detach the page from the mapping at any moment. So you must
always lock a page and check that it still belongs to the desired mapping
before you check 'page_has_buffers()'.
Honza
--
Jan Kara<jack@xxxxxxx>
SUSE Labs, CR
--
Best Wishes
Yongqiang Yang
--
Jan Kara<jack@xxxxxxx>
SUSE Labs, CR
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