Re: O_DIRECT fails in some kernel and FS

[Chris Mason <mason@suse.com>]

On Saturday, February 02, 2002 08:54:38 PM +0100 Andrea Arcangeli <andrea@suse.de> wrote:

>> Chris and I had initially decided to unpack the tails on file open
>> if O_DIRECT is used, but it seems cleaner to add a 
>> reiserfs_get_block_direct_io, and have it return -EINVAL if a read
>> went to a tail.  writes that happen to a tail will trigger tail
>> conversion.
> 
> This is a safe approch (no risk of corruption etc..). However to provide
> the same semantics of the other filesystems it would be even better if
> we could unpack the tail within reiserfs_get_block_direct_io rather than
> returning -EINVAL, but ok, most apps should work fine anyways (and as
> worse people can workaround the magic by remounting reiserfs with notail
> before writing the data that will need to be handled later via
> O_DIRECT).

In the normal case, O_DIRECT can't be done on a file with a tail. 

The way I read generic_file_direct_IO, O_DIRECT is only done in 
units that start block aligned, and continue for a block aligned 
length.  So, this can never include a packed file tail.  

We should only need to worry if i_size on the file is wrong, and allows a 
read/write to a block aligned chunk on a file with a tail, which should
only be legal in the expanding truncate case from older kernels.  The
-EINVAL return should only happen in this (very unlikely) case.

-chris