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SubjectRe: Loop devices under NTFS
On Tue, 27 Aug 2002, Christoph Hellwig:
>Yes. Anything but the filesystem itself and the generic read/write path
>is not supposed to use address space operations directly.

Why?

According to linux-2.5.31/Documentation/Locking,
"->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
may be called from the request handler (/dev/loop)."

Using the page cache in loop.c saves a copy when there is a
data transformation (such as encryption) involved, and that can be
important for reducing the cost of privacy.

>> >Note that there is a more severe bug in loop.c: it's abuse of
>> >do_generic_file_read.
>>
>> Could you please elaborate on this and give an example where
>> it return incorrect data, deadlock, generate a kernel oops, etc.?

>Depending on the filesystem implementation _anything_ may happen.
>With current intree filesystems the only real life problem is that
>it doesn't work on certain filesystems.

Sorry for repeating myself here: If you're referring to the
stock loop.c not working with tmpfs because tmpfs lacks
{prepare,commit}_write which my patch works around (based on Jari's
patch before mine, and a patch by Andrew Morton as well). I have yet
to hear a clear reason why any writable plain file on any given file
system could not have {prepare,commit}_write operations available.

>I think at least the network
>filesystems might be oopsable with some preparation.

Please come up with a clear example. I'm not asking you for a
test case that can produce it, just some narrative of the problem
occurring.

I am aware that you can get races if someone mounts a loop
device while accessing the underlying file by some other mechanism,
but I believe that the only case where that would be done in practice
is to change the encryption of a device, and, because of the read and
write patterns involved in that, it should not be a problem.

Adam J. Richter __ ______________ 575 Oroville Road
adam@yggdrasil.com \ / Milpitas, California 95035
+1 408 309-6081 | g g d r a s i l United States of America
"Free Software For The Rest Of Us."

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