On Wed, Jun 27, 2007 at 07:50:56AM -0400, Chris Mason wrote:
> On Wed, Jun 27, 2007 at 07:32:45AM +0200, Nick Piggin wrote:
> > On Tue, Jun 26, 2007 at 08:34:49AM -0400, Chris Mason wrote:
> > > On Tue, Jun 26, 2007 at 07:23:09PM +1000, David Chinner wrote:
> > > > On Tue, Jun 26, 2007 at 01:55:11PM +1000, Nick Piggin wrote:
> > >
> > > [ ... fsblocks vs extent range mapping ]
> > >
> > > > iomaps can double as range locks simply because iomaps are
> > > > expressions of ranges within the file. Seeing as you can only
> > > > access a given range exclusively to modify it, inserting an empty
> > > > mapping into the tree as a range lock gives an effective method of
> > > > allowing safe parallel reads, writes and allocation into the file.
> > > >
> > > > The fsblocks and the vm page cache interface cannot be used to
> > > > facilitate this because a radix tree is the wrong type of tree to
> > > > store this information in. A sparse, range based tree (e.g. btree)
> > > > is the right way to do this and it matches very well with
> > > > a range based API.
> > >
> > > I'm really not against the extent based page cache idea, but I kind of
> > > assumed it would be too big a change for this kind of generic setup. At
> > > any rate, if we'd like to do it, it may be best to ditch the idea of
> > > "attach mapping information to a page", and switch to "lookup mapping
> > > information and range locking for a page".
> >
> > Well the get_block equivalent API is extent based one now, and I'll
> > look at what is required in making map_fsblock a more generic call
> > that could be used for an extent-based scheme.
> >
> > An extent based thing IMO really isn't appropriate as the main generic
> > layer here though. If it is really useful and popular, then it could
> > be turned into generic code and sit along side fsblock or underneath
> > fsblock...
>
> Lets look at a typical example of how IO actually gets done today,
> starting with sys_write():
>
> sys_write(file, buffer, 1MB)
> for each page:
> prepare_write()
> allocate contiguous chunks of disk
> attach buffers
> copy_from_user()
> commit_write()
> dirty buffers
>
> pdflush:
> writepages()
> find pages with contiguous chunks of disk
> build and submit large bios
>
> So, we replace prepare_write and commit_write with an extent based api,
> but we keep the dirty each buffer part. writepages has to turn that
> back into extents (bio sized), and the result is completely full of dark
> dark corner cases.
Yup - I've been on the painful end of those dark corner cases several
times in the last few months.
It's also worth pointing out that mpage_readpages() already works on
an extent basis - it overloads bufferheads to provide a "map_bh" that
can point to a range of blocks in the same state. The code then iterates
the map_bh range a page at a time building bios (i.e. not even using
buffer heads) from that map......
> I do think fsblocks is a nice cleanup on its own, but Dave has a good
> point that it makes sense to look for ways generalize things even more.
*nod*
Cheers,
Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
