linux-ext4 - Re: [PATCH v3 00/12] Enable per-file/directory DAX operations V3

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Message-ID: <20200220163024.GV9506@magnolia>
Date:   Thu, 20 Feb 2020 08:30:24 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Jeff Moyer <jmoyer@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3 00/12] Enable per-file/directory DAX operations V3

On Thu, Feb 20, 2020 at 08:20:28AM -0800, Ira Weiny wrote:
> On Tue, Feb 18, 2020 at 03:54:30PM -0800, 'Ira Weiny' wrote:
> > On Tue, Feb 18, 2020 at 09:22:58AM -0500, Jeff Moyer wrote:
> > > Ira Weiny <ira.weiny@...el.com> writes:
> > > > If my disassembly of read_pages is correct it looks like readpage is null which
> > > > makes sense because all files should be IS_DAX() == true due to the mount option...
> > > >
> > > > But tracing code indicates that the patch:
> > > >
> > > > 	fs: remove unneeded IS_DAX() check
> > > >
> > > > ... may be the culprit and the following fix may work...
> > > >
> > > > diff --git a/mm/filemap.c b/mm/filemap.c
> > > > index 3a7863ba51b9..7eaf74a2a39b 100644
> > > > --- a/mm/filemap.c
> > > > +++ b/mm/filemap.c
> > > > @@ -2257,7 +2257,7 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
> > > >         if (!count)
> > > >                 goto out; /* skip atime */
> > > >  
> > > > -       if (iocb->ki_flags & IOCB_DIRECT) {
> > > > +       if (iocb->ki_flags & IOCB_DIRECT || IS_DAX(inode)) {
> > > >                 struct file *file = iocb->ki_filp;
> > > >                 struct address_space *mapping = file->f_mapping;
> > > >                 struct inode *inode = mapping->host;
> > > 
> > > Well, you'll have to up-level the inode variable instantiation,
> > > obviously.  That solves this particular issue.
> > 
> > Well...  This seems to be a random issue.  I've had BMC issues with
> > my server most of the day...  But even with this patch I still get the failure
> > in read_pages().  :-/
> > 
> > And I have gotten it to both succeed and fail with qemu...  :-/
> 
> ... here is the fix.  I made the change in xfs_diflags_to_linux() early on with
> out factoring in the flag logic changes we have agreed upon...
> 
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 62d9f622bad1..d592949ad396 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1123,11 +1123,11 @@ xfs_diflags_to_linux(
>                 inode->i_flags |= S_NOATIME;
>         else
>                 inode->i_flags &= ~S_NOATIME;
> -       if (xflags & FS_XFLAG_DAX)
> +
> +       if (xfs_inode_enable_dax(ip))
>                 inode->i_flags |= S_DAX;
>         else
>                 inode->i_flags &= ~S_DAX;
> -
>  }
> 
> But the one thing which tripped me up, and concerns me, is we have 2 functions
> which set the inode flags.
> 
> xfs_diflags_to_iflags()
> xfs_diflags_to_linux()
> 
> xfs_diflags_to_iflags() is geared toward initialization but logically they do
> the same thing.  I see no reason to keep them separate.  Does anyone?
> 
> Based on this find, the discussion on behavior in this thread, and the comments
> from Dave I'm reworking the series because the flag check/set functions have
> all changed and I really want to be as clear as possible with both the patches
> and the resulting code.[*]  So v4 should be out today including attempting to
> document what we have discussed here and being as clear as possible on the
> behavior.  :-D
> 
> Thanks so much for testing this!
> 
> Ira
> 
> [*] I will probably throw in a patch to remove xfs_diflags_to_iflags() as I
> really don't see a reason to keep it.
> 

I prefer you keep the one in xfs_iops.c since ioctls are a higher level
function than general inode operations.

--D