Re: [prepatch] address_space-based writeback

[Andrew Morton <akpm@zip.com.au>]

Anton Altaparmakov wrote:
> 
> ...
> It would be great to be able to submit variable size "io entities" even
> greater than PAGE_CACHE_SIZE (by giving a list of pages, starting offset
> in first page and total request size for example) and saying write that to
> the device starting at offset xyz. That would suit ntfs perfectly. (-:
> 

Yes, I'll be implementing that.  Writeback will hit the filesystem
via a_ops->writeback_mapping(mapping, nr_pages).  The filesytem
will then call in to generic_writeback_mapping(), which will walk
the pages, assemble BIOs and send them off.

The filesystem needs to pass a little state structure into the
generic writeback function.  An important part of that is a
semaphore which is held while writeback is locking multiple
pages.  To avoid ab/ba deadlocks.

The current implementation of this bio-assembler is for no-buffer
(delalloc) fileystems only.  It need to be enhanced (or forked)
to also cope with buffer-backed pages. It will need to peek
inside the buffer-heads to detect clean buffers (maybe.  It
definitely needs to skip unmapped ones).  When such a buffer
is encountered the BIO will be sent off and a new one will be started.
The code for this is going to be quite horrendous.  I suspect
the comment/code ratio will exceed 1.0, which is a bad sign :)

One thing upon which I am undecided:  for syncalloc filesystems
like ext2, do we attach buffers at ->readpages() time, or do
we just leave the page bufferless?

That's a hard call.  It helps the common case, but in the uncommon
case (we overwrite the file after reading it), we need to run
get_block again.

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