linux-kernel - About patch NFS: Fix O_DIRECT accounting of number of bytes read/written

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Message-ID: <9608C5E5429A7846A89FDA46D5296B97334E3824@G08CNEXMBPEKD03.g08.fujitsu.local>
Date:   Wed, 16 Oct 2019 01:00:30 +0000
From:   "Su, Yanjun" <suyj.fnst@...fujitsu.com>
To:     "trondmy@...il.com" <trondmy@...il.com>
CC:     "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: About patch NFS: Fix O_DIRECT accounting of number of bytes
 read/written

Hi trond,
Because My mail system cant receive nfs mail list’s mails, I reply your patch here.
I have some question for the patch.

>No. Basic O_DIRECT does not guarantee atomicity of requests, which is
>why we do not have generic locking at the VFS level when reading and
>writing. The only guarantee being offered is that O_DIRECT and buffered
>writes do not collide.
Do you mean other fs also cant guarantee atomicity of O_DIRECT request or just nfs?

>IOW: I think the basic premise for this test is just broken (as I
>commented in the patch series I sent) because it is assuming a
>behaviour that is simply not guaranteed.
So the generic/465 of xfstests can’t apply to nfs for now, am I right?

>BTW: note that buffered writes have the same property. They are ordered
>when being written into the page cache, meaning that reads on the same
>client will see no holes, however if you try to read from another
>client, then you will see the same behaviour, with temporary holes
>magically appearing in the file.
As you say, nfs buffered write also has the hole problem with multiple r/w on different clients.
I want to know if the problem exists in other local fs such as xfs,ext4?

Thanks in advance.