linux-kernel - RE: [PATCH v3 2/2] iov_iter: Don't deal with iter->copy_mc in memcpy_from_iter

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Message-ID: <d0232378a64a46659507e5c00d0c6599@AcuMS.aculab.com>
Date:   Thu, 17 Aug 2023 08:41:56 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>
CC:     Al Viro <viro@...iv.linux.org.uk>, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...t.de>,
        Christian Brauner <christian@...uner.io>,
        Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 2/2] iov_iter: Don't deal with iter->copy_mc in
 memcpy_from_iter_mc()

> This patch only does that for the 'user_backed' thing, which was a similar case.

Yes, having two fields that have to be set to match is a recipe
for disaster.

Although I'm not sure the bit-fields really help.
There are 8 bytes at the start of the structure, might as well
use them :-)
OTOH the 'nofault' and 'copy_mc' flags could be put into much
higher bits of a 32bit value.

Alternatively put both/all the USER values first.
Then an ordered compare could be used.

If everything is actually inlined could the 'iter' be passed
through to the step() functions?
Although is might be better to pass a cached copy of iter->iter_type
(although that might just end up spilling to stack.)

	David

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