netdev - Re: [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm

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Date:   Mon, 21 Sep 2020 16:29:37 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Laight <David.Laight@...lab.com>
Cc:     Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-aio@...ck.org" <linux-aio@...ck.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 02/11] mm: call import_iovec() instead of
 rw_copy_check_uvector() in process_vm_rw()

On Mon, Sep 21, 2020 at 03:21:35PM +0000, David Laight wrote:

> You really don't want to be looping through the array twice.

Profiles, please.

> I think the 'length' check can be optimised to do something like:
> 	for (...) {
> 		ssize_t len = (ssize_t)iov[seg].iov_len;
> 		ret += len;
> 		len_hi += (unsigned long)len >> 20;
> 	}
> 	if (len_hi) {
> 		/* Something potentially odd in the lengths.
> 		 * Might just be a very long fragment.
> 		 * Check the individial values. */
> 		Add the exiting loop here.
> 	}

Far too ugly to live.