linux-kernel - Re: [RFC PATCH-tip v4 02/10] locking/rwsem: Stop active read lock ASAP

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Date:   Sat, 15 Oct 2016 22:57:31 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <Waiman.Long@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-alpha@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        Jason Low <jason.low2@...com>,
        Jonathan Corbet <corbet@....net>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v4 02/10] locking/rwsem: Stop active read lock
 ASAP

On Wed, Oct 12, 2016 at 08:06:40AM +1100, Dave Chinner wrote:
> Um, I seem to have completely missed that change - when did that
> happen and why?
> 
> Oh, it was part of the misguided "enable DAX on block devices"
> changes -

o, that commit just switched it to use ->f_mapping:

-       return (filp->f_flags & O_DIRECT) || IS_DAX(file_inode(filp));
+       return (filp->f_flags & O_DIRECT) || IS_DAX(filp->f_mapping->host);

The original version of it goes all the way back to introducing the
current-day DAX code in d475c6346 ("dax,ext2: replace XIP read and write
with DAX I/O");

> Hence I'd suggest that DAX check in io_is_direct() should be removed
> ASAP; the filesystems don't need it as they check the inode DAX
> state directly, and the code it "fixed" is no longer in the tree.

As long as ext4 still uses the overloaded direct_IO we need the
checks for DAX in the filemap.c generic read/write code.  It seems
like that's only two spots anyway, but I'd feel much safer once ext4
is switched over to the iomap version of the dax code.