linux-kernel - Re: [PATCH v7 00/19] gfs2: Fix mmap + page fault deadlocks

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Message-ID: <YTJoqq0fVB+xAB7w@zeniv-ca.linux.org.uk>
Date:   Fri, 3 Sep 2021 18:25:46 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Paul Mackerras <paulus@...abs.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ocfs2-devel@....oracle.com, kvm-ppc@...r.kernel.org
Subject: Re: [PATCH v7 00/19] gfs2: Fix mmap + page fault deadlocks

On Fri, Sep 03, 2021 at 08:52:00AM -0700, Linus Torvalds wrote:
> On Wed, Sep 1, 2021 at 12:53 PM Andreas Gruenbacher <agruenba@...hat.com> wrote:
> >
> > So there's a minor merge conflict between Christoph's iomap_iter
> > conversion and this patch queue now, and I should probably clarify the
> > description of "iomap: Add done_before argument to iomap_dio_rw" that
> > Darrick ran into. Then there are the user copy issues that Al has
> > pointed out. Fixing those will create superficial conflicts with this
> > patch queue, but probably nothing serious.
> >
> > So how should I proceed: do you expect a v8 of this patch queue on top
> > of the current mainline?
> 
> So if you rebase for fixes, it's going to be a "next merge window" thing again.
> 
> Personally, I'm ok with the series as is, and the conflict isn't an
> issue. So I'd take it as is, and then people can fix up niggling
> issues later.
> 
> But if somebody screams loudly..

FWIW, my objections regarding the calling conventions are still there.

Out of 3 callers that do want more than "success/failure", one is gone
(series by tglx) and one more is broken (regardless of the semantics,
btrfs on arm64).  Which leaves 1 caller (fault-in for read in FPU
handling on x86 sigreturn).  That caller turns out to be correct, but
IMO there are fairly strong arguments in favour of *not* using the
normal fault-in in that case.

	"how many bytes can we fault in" is misleading, unless we really
poke into every cacheline in the affected area.  Which might be a primitive
worth having, but it's a lot heavier than needed by the majority of callers.