linux-kernel - Re: [PATCH v7 17/19] gup: Introduce FOLL

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Date:   Thu, 9 Sep 2021 12:36:14 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>, cluster-devel@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v7 17/19] gup: Introduce FOLL_NOFAULT flag to disable
 page faults

On Fri, Aug 27, 2021 at 06:49:24PM +0200, Andreas Gruenbacher wrote:
> Introduce a new FOLL_NOFAULT flag that causes get_user_pages to return
> -EFAULT when it would otherwise trigger a page fault.  This is roughly
> similar to FOLL_FAST_ONLY but available on all architectures, and less
> fragile.

So, FOLL_FAST_ONLY only has one single user through
get_user_pages_fast_only (pin_user_pages_fast_only is entirely unused,
which makes totally sense given that give up on fault and pin are not
exactly useful semantics).

But it looks like they want to call it from atomic context, so we can't
really share it.  Sight, I hate all these single-user FOLL flags that
make gup.c a complete mess.

But otherwise this looks fine.