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SubjectRe: [RFC] new ->perform_write fop
On Mon, May 24, 2010 at 01:09:43PM +1000, Nick Piggin wrote:
> On Sat, May 22, 2010 at 06:37:03PM +1000, Dave Chinner wrote:
> > On Sat, May 22, 2010 at 12:31:02PM +1000, Nick Piggin wrote:
> > > On Fri, May 21, 2010 at 11:15:18AM -0400, Christoph Hellwig wrote:
> > > > Nick, what exactly is the problem with the reserve + allocate design?
> > > >
> > > > In a delalloc filesystem (which is all those that will care about high
> > > > performance large writes) the write path fundamentally consists of those
> > > > two operations. Getting rid of the get_blocks mess and replacing it
> > > > with a dedicated operations vector will simplify things a lot.
> > >
> > > Nothing wrong with it, I think it's a fine idea (although you may still
> > > need a per-bh call to connect the fs metadata to each page).
> > >
> > > I just much prefer to have operations after the copy not able to fail,
> > > otherwise you get into all those pagecache corner cases.
> > >
> > > BTW. when you say reserve + allocate, this is in the page-dirty path,
> > > right? I thought delalloc filesystems tend to do the actual allocation
> > > in the page-cleaning path? Or am I confused?
> >
> > See my reply to Jan - delayed allocate has two parts to it - space
> > reservation (accounting for ENOSPC) and recording of the delalloc extents
> > (allocate). This is separate to the writeback path where we convert
> > delalloc extents to real extents....
>
> Yes I saw that. I'm sure we'll want clearer terminology in the core
> code. But I don't quite know why you need to do it in 2 parts
> (reserve, then "allocate").

Because reserve/allocate are the two steps that allocation is
generally broken down into, even in filesystems that don't do
delayed allocation. That's because....

> Surely even reservation failures are
> very rare

... ENOSPC and EDQUOT are not at all rare, and they are generated
during the reservation stage. i.e. before any real allocation or
state changes are made. Just about every filesystem does this
because failing half way through an allocation not being able to
allocate a block due to ENOSPC or EDQUOT is pretty much impossible
to undo reliably in most filesystems.

> , and obviously the error handling is required, so why not
> just do a single call?

Because if we fail after the allocation then ensuring we handle the
error *correctly* and *without further failures* is *fucking hard*.

IMO, the fundamental issue with using hole punching or direct IO
from the zero page to handle errors is that they are complex enough
that there is *no guarantee that they will succeed*. e.g. Both can
get ENOSPC/EDQUOT because they may end up with metadata allocation
requirements above and beyond what was originally reserved. If the
error handling fails to handle the error, then where do we go from
there?

In comparison, undoing a reservation is simple - maybe incrementing
a couple of counters - and is effectively guaranteed never to fail.
This is a good characteristic to have in an error handling
function...

> > > > Punching holes is a rather problematic operation, and as mentioned not
> > > > actually implemented for most filesystems - just decrementing counters
> > > > on errors increases the chances that our error handling will actually
> > > > work massively.
> > >
> > > It's just harder for the pagecache. Invalidating and throwing out old
> > > pagecache and splicing in new pages seems a bit of a hack.
> >
> > Hardly a hack - it turns a buffered write into an operation that
> > does not expose transient page state and hence prevents torn writes.
> > That will allow us to use DIF enabled storage paths for buffered
> > filesystem IO(*), perhaps even allow us to generate checksums during
> > copy-in to do end-to-end checksum protection of data....
>
> It is a hack. Invalidating is inherently racy and isn't guaranteed
> to succeed.
>
> You do not need to invalidate the pagecache to do this (which as I said
> is racy). You need to lock the page to prevent writes, and then unmap
> user mappings.

Which is the major part of invalidating a page. The other part of
invalidation is removing the page from the page cache, so if
invalidation is inherently too racy to use safely here, then I fail
to see why the above isn't also too racy to use safely....

> You'd also need to have done some magic so writable mmaps
> don't leak into get_user_pages.

Which means what, and why don't we have to do any special magic now
to prevent it?

> But this should be a different discussion anyway. Don't forget, your
> approach is forced into the invalidation requirement because of
> downsides in its error handling sequence.

I wouldn't say forced into it, Nick - it's a deliberate design
choice to make the overall stack simpler and more likely to function
correctly.

Besides, all it takes to avoid the requirement of invalidation is to
provide the guarantee that the allocation after reservation will
either succeed or the filesystem shuts down in a corrupted state.
If we provide that guarantee then the fact that transient page cache
data might appear on allocation error is irrelevant, because it
will never get written to disk and applications will error out
pretty quickly.

I'm quite happy with that requirement, because of two things.
Firstly, after the reservation nothing but a corruption or IO error
should prevent the allocation from succeeding. In that case, the
filesystem should already be in a shutdown state by the time the
failed allocation returns. Secondly, filesystems using delayed
allocation are already making this promise successfully from
get_blocks to ->writepage, hence I don't see any issues with
encoding it into an allocation interface....

> That cannot be construed as
> positive, because you are forced into it, wheras other approaches
> *could* use it, but do not have to.

Except for the fact the other alternatives have much, much worse
downsides. Yes, they could also use such a write path, but that
doesn't reduce the complexity of those solutions or prevent any of
the problems they have.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com


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