Re: [V9fs-developer] [PATCH] [fs/9p] Add a Direct IO address_space_operation.

["Venkateswararao Jujjuri (JV)" <jvrao@linux.vnet.ibm.com>]

Eric Van Hensbergen wrote:
> Is this just adding dead-code then? (I know I'm conflicting with my no
> silent errors motto -- but I'm keen to keep our line-count down)

Not really a dead code. if the O_DIRECT flag is set, __dentry_open() checks for
f->f_mapping->a_ops->direct_IO before proceeding with open.
In the absense of this routine, open with O_DIRECT flag fails.

> By what path would we normally end up in this function?  If caches were enabled?

If cache is enabled we use generic_file_aio_read() as aio_read() operation.
This ends up calling this new routine and fails with the error. So, until we
actually
implement a directio handle in the cached mode, there won't be a direct_IO support
in the cached mode.

generic_file_aio_read()
{
...
	if (filp->f_flags & O_DIRECT) {
		...
		...
		retval = mapping->a_ops->direct_IO(READ, iocb,
                                                        iov, pos, nr_segs);

              <<<The new routine returns error here >>>
	}
}

Will that be OK to move forwarded with directio support only for non-cached mode,
and deal with cached mode later?

Thanks,
JV

> 
>            -eric
> 
> 
> On Tue, Aug 17, 2010 at 12:19 PM, Venkateswararao Jujjuri (JV)
> <jvrao@linux.vnet.ibm.com> wrote:
>> The presence of v9fs_direct_IO() in the address space ops vector means
>> the 9P client supports direct I/O.  However, we shunt off direct
>> read and write requests before the VFS gets them, so this method
>> should never be called.
>>
>> Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
>> ---
>>  fs/9p/vfs_addr.c |   24 ++++++++++++++++++++++++
>>  1 files changed, 24 insertions(+), 0 deletions(-)
>>
>> diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
>> index 90e3844..fedcf90 100644
>> --- a/fs/9p/vfs_addr.c
>> +++ b/fs/9p/vfs_addr.c
>> @@ -154,10 +154,34 @@ static int v9fs_launder_page(struct page *page)
>>        return 0;
>>  }
>>
>> +/**
>> + * v9fs_direct_IO - 9P address space operation for direct I/O
>> + * @rw: direction (read or write)
>> + * @iocb: target I/O control block
>> + * @iov: array of vectors that define I/O buffer
>> + * @pos: offset in file to begin the operation
>> + * @nr_segs: size of iovec array
>> + *
>> + * The presence of this routine in the address space ops vector means
>> + * the 9P client supports direct I/O.  However, we shunt off direct
>> + * read and write requests before the VFS gets them, so this method
>> + * should never be called.
>> + */
>> +ssize_t v9fs_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
>> +               loff_t pos, unsigned long nr_segs)
>> +{
>> +       P9_DPRINTK(P9_DEBUG_VFS, "v9fs_direct_IO: v9fs_direct_IO (%s) "
>> +                       "off/no(%lld/%lu) EINVAL\n",
>> +                       iocb->ki_filp->f_path.dentry->d_name.name,
>> +                       (long long) pos, nr_segs);
>> +
>> +       return -EINVAL;
>> +}
>>  const struct address_space_operations v9fs_addr_operations = {
>>       .readpage = v9fs_vfs_readpage,
>>       .readpages = v9fs_vfs_readpages,
>>       .releasepage = v9fs_release_page,
>>       .invalidatepage = v9fs_invalidate_page,
>>       .launder_page = v9fs_launder_page,
>> +      .direct_IO = v9fs_direct_IO,
>>  };
>> --
>> 1.6.5.2
>>
>>
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