Re: [PATCH v3 0/3] Add XIP support to ext4

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Dec 19, 2013 at 11:17:28AM -0500, Theodore Ts'o wrote:
> On Thu, Dec 19, 2013 at 08:20:49AM -0700, Matthew Wilcox wrote:
> > We could have ext4_get_xip_mem() check buffer_unwritten(); if it's set,
> > zero the blocks and call ext4_convert_unwritten_extents().  Would that
> > work?
> 
> That would work, but we'd be doing a double write to each data block
> --- first writing all zero's, and then later, writing the actual data.
> So this would halve our write bandwidth, and double the write wear on
> the device.

Once this patch is done that I'm working on in the other thread, the
only place we'll be calling get_xip_mem() is in the page fault path,
which always has to zero no matter whether it's a load or a store.

As I said in my last reply to Dave:

  At this point, I'm thinking a
  custom ->fault handler that looks something like this:

  static int ext4_xip_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
  {
          return xip_fault(vma, vmf, ext4_get_block_write, ext4_end_io_dio);
  }

... I think it'll actually be ext4_get_block_fault, not _write, and it
will include code to zero the returned blocks if they're uninitialised.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."