Re: [RFC] new ->perform_write fop

From: Nick Piggin
Date: Tue May 18 2010 - 06:44:25 EST

On Tue, May 18, 2010 at 06:05:03PM +1000, Dave Chinner wrote:
> On Tue, May 18, 2010 at 04:36:47PM +1000, Nick Piggin wrote:
> > Well you could do a large span block allocation at the beginning,
> > and then dirty the pagecache one by one like we do right now.
> The problem is that if we fail to allocate a page (e.g. ENOMEM) or
> fail the copy (EFAULT) after the block allocation, we have to undo
> the allocation we have already completed. If we don't, we leave
> uninitialisaed allocations on disk that will expose stale data.
> In the second case (EFAULT) we might be able to zero the pages to
> avoid punching out blocks, but the first case where pages can't be
> allocated to cover the block allocated range makes it very
> difficult without being able to punch holes in allocated block
> ranges.
> AFAIK, only XFS and OCFS2 currently support punching out arbitrary
> ranges of allocated blocks from an inode - there is not VFS method
> for it, just an ioctl (XFS_IOC_UNRESVSP).
> Hence the way to avoid needing hole punching is to allocate and lock
> down all the pages into the page cache fÑrst, then do the copy so
> they fail before the allocation is done if they are going to fail.
> That makes it much, much easier to handle failures....

So it is just a matter of what is exposed as a vfs interface?
I would much prefer to make it a requirement to support hole
punching in the block allocator if the filesystem wishes to do
such writes.

If you have an ->allocate(inode, RESERVE/ALLOCATE, interval) API
then it makes sense to have a DEALLOCATE operation there too.

That seems like it should be cleaner than to work around it if
we're talking about adding new APIs anyway.

> Remember, we still don't do the "truncate blocks past eof on failure"
> correctly in block_write_begin (still calls vmtruncate, not
> ->setattr), so we can't claim that the current way of doing things
> is a model of perfection that we ca't improve on....

Right, but the API (write_begin/write_end) is sufficient now to
make it work correctly. What should really happen is the
filesystem detects and handles these error cases.

In the truncate patchset (that deprecates ->truncate and vmtruncate
entirely), that is exactly what happens.

> > The only reason to do operations on multiple pages at once is if
> > we need to lock them all.
> Well, to avoid the refaulting of pages we just unmapped we'd need to
> do that...

Well, the lock/unmap/copy/unlock could be done on a per-page

> > Now the fs might well have that requirement
> > (if it is not using i_mutex for block (de)allocation
> > serialisation), but I don't think generic code needs to be doing
> > that.
> XFS already falls into the category of a filesystem using the
> generic code that does not use i_mutex for allocation serialisation.
> I'm sure it isn't the only filesystem that this is true for, so it
> seems sensible for the generic code to handle this case.

Well, does it need page lock? All pages locked concurrently in
a range under which block allocation is happening? I would much
prefer an allocation API that supports allocation/freeing
without requiring any pagecache at all.

> > Basically, once pagecache is marked uptodate, I don't think we should
> > ever put maybe-invalid data into it -- the way to do it is to invalidate
> > that page and put a *new* page in there.
> Ok, so lets do that...
> > Why? Because user mappings are just one problem, but once you had a
> > user mapping, you can have been subject to get_user_pages, so it could
> > be in the middle of a DMA operation or something.
> ... because we already know this behaviour causes problems for
> high end enterprise level features like hardware checksumming IO
> paths.
> Hence it seems that a multipage write needs to:
> 1. allocate new pages
> 2. attach bufferheads/mapping structures to pages (if required)
> 3. copy data into pages
> 4. allocate space
> 5. for each old page in the range:
> lock page
> invalidate mappings
> clear page uptodate flag
> remove page from page cache
> 6. for each new page:
> map new page to allocated space
> lock new page
> insert new page into pagecache
> update new page state (write_end equivalent)
> unlock new page
> 7. free old pages
> Steps 1-4 can all fail, and can all be backed out from without
> changing the current state. Steps 5-7 can't fail AFAICT, so we
> should be able to run this safely after the allocation without
> needing significant error unwinding...
> Thoughts?

Possibly. The importance of hot cache is reduced, because we are
doing full-page copies, and bulk copies, by definition. But it
could still be an issue. The allocations and deallocations could
cost a little as well.

Compared to having a nice API to just do bulk allocate/free block
operations and then just doing simple per-page copies, I think it
doesn't look so nice.

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