Re: [PATCH 3/3] pnfsblock: bail out unaligned DIO

[Peng Tao <tao.peng@emc.com>]

On Tue, May 29, 2012 at 12:33 AM, Myklebust, Trond
<Trond.Myklebust@netapp.com> wrote:
> On Mon, 2012-05-28 at 00:26 -0400, tao.peng@emc.com wrote:
>> > -----Original Message-----
>> > From: Myklebust, Trond [mailto:Trond.Myklebust@netapp.com]
>> > Sent: Monday, May 28, 2012 11:45 AM
>> > To: Peng, Tao
>> > Cc: bergwolf@gmail.com; linux-nfs@vger.kernel.org
>> > Subject: RE: [PATCH 3/3] pnfsblock: bail out unaligned DIO
>> >
>> > On Sun, 2012-05-27 at 22:30 -0400, tao.peng@emc.com wrote:
>> > > As explain in the other mail, it is necessary to have pnfs_dio_begin/end, so I prefer to do the test
>> > as early as possible, which is in pnfs_dio_begin.
>> >
>> > There is no pnfs_dio_begin/end.
>> OK. I will put DIO alignment tests inside pg_init.
>>
>> And any comments on for stable patch in the thread ([PATCH] pnfsblock: bail out page unaligned IO)? If you agree, I will base DIO changes on top of it to avoid conflicts.
>> There are reasons to test alignment at different place for buffer IO and DIO. For buffer IO, pg_init isn't the right place because we don't have nfs page there. For DIO, pg_test isn't the right place because we need to check cross page boundary.
>
> Since all pages in the struct nfs_pageio_descriptor are guaranteed to be
> contiguous, you really only need to check the first and last page in the
> series for alignment.
>
> pg_init() does take the first nfs_page request as its argument and so it
> should be possible to check the alignment of the first page in the
> series there.
> You can then check the alignment of the last page in your
> ->write_pagelist() and return PNFS_NOT_ATTEMPTED if appropriate.
>
Thanks. Will do it as you suggested.

> Note that all applications that use O_DIRECT are expected to use aligned
> memory, since that's what the open() manpage implies is the safe option
> for all filesystems. Unaligned memory is therefore not something that we
> need to optimise for.
>
do_blockdev_direct_IO() is checking memory alignment for direct IO. Is
there any difference between NFS and other FS here?

Cheers,
Tao