Re: Fwd: Re: VM_XIP Request for comments

[Carsten Otte <cotte@de.ibm.com>]

> I can't find CONFIG_XIP.  But I assume you are talking about
> filemap_xip.c and Documentation/filesystems/xip.txt.
Actually the thing consists of three parts:
- a block device that does implement the direct_access method. so far
  the only driver that does that is drivers/s390/block/dcssblk.c. We
  are aware that this one needs cleanup ;-).
- extension to good old ext2 filesystem that does implement get_xip_page
  address space operation. Uses direct_access block device operation.
- the stuff in mm/filemap_xip.c which actually does the job (read,write,
  mmap etc.) by calling get_xip_page address space operation.

> I don't know. The code and discussions about it looked very big-iron
> DSCC specific but now on second pass it was meant to more generic.  If
> I can learn this infrastructure then maybe this will work.
The only part that is architecture specific is the block device driver.
Both the ext2 extensions and filemap_xip are architecture independent.

> So I'm supposed to create a function in the target fs that gets
> plugged into get_xip_page().  Then I call that function to create an
> proper XIP'ed page in my mmap() and fread() calls.  I could use the
> first arg of get_xip_page() to pass in the start address of the cramfs
> volume and the second the offset of the page in the file I want to
> map.
> 
> Is that about right?
The first step would be to write a block device driver that allows to
mount your memory backed storage thing [flash chip?]. The block device
driver needs to implement the direct_access method. Now you can mount
ext2 filesystems with -o xip.

Ext2 does not support compression, and all files are xip once you
select -o xip. Would be interresting to have a filesystem that can do
both xip and compression on a per-file basis, but as far as I can tell
the basic layering should also work fine with such filesystem: should
work with any block device, file operations in filemap_xip.c can be
used for those files that are xip [and not compressed].
-- 

Carsten Otte
IBM Linux technology center
ARCH=s390

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