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Date:   Fri, 22 Jul 2022 13:05:32 -0400
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, linux-xfs@vger.kernel.org,
        linux-api@vger.kernel.org, linux-fscrypt@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        Keith Busch <kbusch@kernel.org>
Subject: Re: [PATCH v4 4/9] ext4: support STATX_DIOALIGN
Message-ID: <YtrY3A/nC023v+/V@mit.edu>
References: <20220722071228.146690-1-ebiggers@kernel.org>
 <20220722071228.146690-5-ebiggers@kernel.org>
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On Fri, Jul 22, 2022 at 12:12:23AM -0700, Eric Biggers wrote:
> -static bool ext4_dio_supported(struct kiocb *iocb, struct iov_iter *iter)
> +/*
> + * Returns %true if the given DIO request should be attempted with DIO, or
> + * %false if it should fall back to buffered I/O.
> + *
> + * DIO isn't well specified; when it's unsupported (either due to the request
> + * being misaligned, or due to the file not supporting DIO at all), filesystems
> + * either fall back to buffered I/O or return EINVAL.  For files that don't use
> + * any special features like encryption or verity, ext4 has traditionally
> + * returned EINVAL for misaligned DIO.  iomap_dio_rw() uses this convention too.
> + * In this case, we should attempt the DIO, *not* fall back to buffered I/O.
> + *
> + * In contrast, in cases where DIO is unsupported due to ext4 features, ext4
> + * traditionally falls back to buffered I/O.
> + *
> + * This function implements the traditional ext4 behavior in all these cases.

Heh.  I had been under the impression that misaligned I/O fell back to
buffered I/O for ext4, since that's what a lot of historical Unix
systems did.  Obviously, it's not something I've tested since "you
should never do that".

There's actually some interesting discussion about what Linux *should*
be doing in the futre in this discussion:

https://patchwork.ozlabs.org/project/linux-ext4/patch/1461472078-20104-1-git-send-email-tytso@mit.edu/

Including the following from Christoph Hellwig:

https://patchwork.ozlabs.org/project/linux-ext4/patch/1461472078-20104-1-git-send-email-tytso@mit.edu/#1335016

> I've been doing an audit of our direct I/O implementations, and most
> of them does some form of transparent fallback, including some that
> only pretend to support O_DIRECT, but do anything special for it at all,
> while at the same time we go through greast efforts to check a file
> system actualy supports direct I/O, leading to nasty no-op ->direct_IO
> implementations as we even got that abstraction wrong.
> 
> At this point I wonder if we should simply treat O_DIRECT as a hint
> and always allow it, and just let the file system optimize for it
> (skip buffering, require alignment, relaxed Posix atomicy requirements)
> if it is set.

The thread also mentioned XFS_IOC_DIOINFO and how We Really Should
have something with equivalent functionality to the VFS --- six years
ago.  :-)


Anyway, this change to ext4 looks good.

Acked-by: Theodore Ts'o <tytso@mit.edu>

							- Ted