Re: Loop devices under NTFS

["Adam J. Richter" <adam@yggdrasil.com>]

At 05:48 27/08/02, Andre Bonin wrote:
>"mount -o loop -r -t iso9660 /mnt/win_d/Source/Iso/Red\ Hat\ 
>7.3/valhalla-i386-disc1.iso /mnt/rh7.3/cd1"
>
>It gives me an error
>"ioctl: LOOP_SET_FD: Invalid argument"

	I believe that NTFS does not provide
address_space_operations->{prepare,commit}_write for plain files, so
the version of loop.c in stock 2.5.31 will not work.

	I posted an update for loop.c that I posted on August 15th:
( http://marc.theaimsgroup.com/?l=linux-kernel&m=102941520919910&w=2 ).
That update included a hack (originally conceived by Andrew Morton and
originally implemented Jari Ruusu) to use file_operations->{read,write}
for mounting files on file systems that do not provide
{prepare,commit}_write, although this involves an extra data copy if
a transformation (such as encryption) is selected.

	I submitted the loop.c update to Linus at least a week ago, but
I have not seen it appear in
ftp://ftp.kernel.org/pub/linux/kernel/people/dwmw2/bk-2.5.

Side note:

	There are only a few file systems that provide writable files
without aops->{prepare,commit}_write.  I think they are just tmpfs,
ntfs and intermezzo.  If all file systems that provided writable files
could be expected to provide {prepare,commit}_write, I could eliminate
the file_ops->{read,write} code from loop.c.  I wish I understood if
there really is a need for a file system to be able to provide a
writable random access file (as opposed to /dev/tty) that does not
have aops->{prepare,commit}_write.  I would be interesting in knowing
if there is anything preventing implementation of
{prepare,commit}_wriet in ntfs.

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