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Date: Thu, 11 Jun 2026 07:57:44 +0200
From: Christoph Hellwig <hch@lst.de>
To: cem@kernel.org
Cc: brauner@vger.kernel.org, linux-block@vger.kernel.devel,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-xfs@vger.kernel.org, Keith Busch <kbusch@kernel.org>,
	Hannes Reinecke <hare@suse.de>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH] iomap: enforce DIO alignment check in iomap
Message-ID: <20260611055744.GA18538@lst.de>
References: <20260610145218.141369-1-cem@kernel.org>
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On Wed, Jun 10, 2026 at 04:52:11PM +0200, cem@kernel.org wrote:
> From: Carlos Maiolino <cem@kernel.org>
> 
> The DIO alignment check has been lifted from iomap layer to rely on the
> block layer to enforce proper alignment when issuing direct IO
> operations. This though, depending on the IO size and buffer address
> passed to the IO operation may lead to user-visible behavior change.
> 
> This has been caught initially by LTP test diotest4 running on
> PPC architecture, where the test fails because a read() operation
> with a supposedly misaligned buffer succeeds instead of an expected
> -EINVAL.
> This has no direct relationship with PPC, but seems to do with the
> IO size crossing page borders or not.

I don't understand the problem here.  Why do we want to insist on a
failure when we can support it?  I think the test is just broken.

> The problematic behavior is reproducible on x86 by reducing the IO size
> to something < PAGE_SIZE, so the misaligned read()s will also be accepted
> by the block layer.

What do you mean with misaligned here?  For a long time the kernel
supports basically arbitrary low memory alignment for diret I/O,
just bounded by the device capabilities (typical 4 byte alignment).

The supported memory alignment is reported in the statx
dio_mem_align.  What does that say compared to the alignment
expectations in this test?