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Date:   Fri, 8 Jan 2021 11:13:37 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        "David S. Miller" <davem@...emloft.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: RE: [PATCH 5/9 next] scsi: Use iovec_import() instead of
 import_iovec().

From: Christoph Hellwig
> Sent: 21 September 2020 15:22
> 
> So looking at the various callers I'm not sure this API is the
> best.  If we want to do something fancy I'd hide the struct iovec
> instances entirely with something like:
> 
> struct iov_storage {
> 	struct iovec stack[UIO_FASTIOV], *vec;
> }
> 
> int iov_iter_import_iovec(struct iov_iter *iter, struct iov_storage *s,
> 		const struct iovec __user *vec, unsigned long nr_segs,
> 		int type);
> 
> and then add a new helper to free the thing if needed:
> 
> void iov_iter_release_iovec(struct iov_storage *s)
> {
> 	if (s->vec != s->stack)
> 		kfree(s->vec);
> }

I've been looking at this code again now most of the pending changes
are in Linus's tree.

I was actually looking at going one stage further.
The 'iov_iter' is always allocated with the 'iov_storage' *above).
Usually both are on the callers stack - possibly in different functions.

So add:
struct iovec_iter {
	struct iov_iter iter;
	struct iovec to_free;
	struct iovec stack[UIO_FASTIOV];
};

int __iovec_import(struct iovec_iter *, const struct iovec __user *vec,
	unsigned long nr_segs, int type, bool compat);

And a 'clean' function to do kfree(iovec->to_free);

This reduces the complexity of most of the callers.

I started doing the changes, but got in a mess in io_uring.c (as usual).
I think I've got a patch pending (in my brain) to simplify the io_uring code.

The plan is to add:
	if (iter->iov != xxx->to_free)
		iter->iov = xxx->stack;
Prior to every use of the iter.
This fixes up anything that got broken by a memcpy() of the fields.
The tidyup code is then always kfree(xxx->to_free).

	David

	

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