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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Mon, 11 Jun 2018 10:08:30 +0200
Message-ID: <CAJfpegteZatDXOV+6FQKOE9pOA=_FErGRn1Jj1fHUYm_QLb4ew@mail.gmail.com>
Subject: Re: [PATCH 23/39] ovl: add O_DIRECT support
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Miklos Szeredi <mszeredi@redhat.com>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
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On Sun, Jun 10, 2018 at 7:31 AM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Tue, May 29, 2018 at 04:43:23PM +0200, Miklos Szeredi wrote:
>> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
>> ---
>>  fs/overlayfs/file.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
>> index 3f610a5b38e4..e5e7ccaaf9ec 100644
>> --- a/fs/overlayfs/file.c
>> +++ b/fs/overlayfs/file.c
>> @@ -110,6 +110,9 @@ static int ovl_open(struct inode *inode, struct file *file)
>>       if (IS_ERR(realfile))
>>               return PTR_ERR(realfile);
>>
>> +     /* For O_DIRECT dentry_open() checks f_mapping->a_ops->direct_IO */
>> +     file->f_mapping = realfile->f_mapping;
>
> Umm...  What happens if upper layer doesn't allow O_DIRECT, while the lower one does?

Will get EINVAL on read(2) after copy up.  Not sure if it can be
called a regression, since it's a corner case of a corner case.

I think proper solution is to support O_DIRECT unconditionally on
upper (and for the likes of shmfs, just fall back to "cached" I/O).

Thanks,
Miklos