linux-kernel - Re: [RFC 5/9] iov_iter: Add iov_iter_fault_in

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Message-ID: <YMUrcUXS5jprfGPF@zeniv-ca.linux.org.uk>
Date:   Sat, 12 Jun 2021 21:47:29 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        cluster-devel <cluster-devel@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC 5/9] iov_iter: Add iov_iter_fault_in_writeable()

On Sat, Jun 12, 2021 at 02:33:31PM -0700, Linus Torvalds wrote:

> That said, reads are obviously much easier, and I'd probably prefer
> the model for writes to be to not necessarily pre-fault anything at
> all, but just write to user space with page faults disabled.

*nod*
I don't like that write pre-fault model at all - note that unlike read
we'll end up with atomic operations, etc. and there's a plenty of
non-obvious ways for that to end up being costly, even assuming it
works correctly in all cases.

	O_DIRECT case is a PITA - there we use GUP and there's no way
to tell GUP that in the current situation we do *NOT* want to hit
->fault()/->page_mkwrite()/etc.  pagefault_disable() won't be even
noticed there...