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Date:   Wed, 23 Jun 2021 22:26:23 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Omar Sandoval <osandov@osandov.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Chinner <david@fromorbit.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
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        Linux API <linux-api@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>,
        Dave Chinner <dchinner@redhat.com>
Subject: Re: [PATCH RESEND x3 v9 1/9] iov_iter: add copy_struct_from_iter()
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On Wed, Jun 23, 2021 at 02:58:32PM -0700, Omar Sandoval wrote:

> Ah, I was stuck on thinking about this calling convention:
> 
> 	struct encoded_iov encoded_iov;
> 	char compressed_data[...];
> 	struct iovec iov[] = {
> 		{ &encoded_iov, sizeof(encoded_iov) },
> 		{ compressed_data, sizeof(compressed_data) },
> 	};
> 	preadv2(fd, iov, 2, -1, RWF_ENCODED);
> 
> But what you described would look more like:
> 
> 	// Needs to be large enough for maximum returned header + data.
> 	char buffer[...];
> 	struct iovec iov[] = {
> 		{ buffer, sizeof(buffer) },
> 	};
> 	preadv2(fd, iov, 2, -1, RWF_ENCODED);
> 	// We should probably align the buffer.
> 	struct encoded_iov *encoded_iov = (void *)buffer;
> 	char *data = buffer + encoded_iov->size;
> 
> That's a little uglier, but it should work, and allows for arbitrary
> extensions. So, among these three alternatives (fixed size structure
> with reserved space, variable size structure like above, or ioctl),
> which would you prefer?

Variable-sized structure would seem to be the easiest from the kernel
POV and the interface is the easiest to describe - "you read the
encoded data preceded by the header"...