linux-kernel - Re: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user

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Message-ID: <CAL1RGDVBR49QrAbkZ0Wa9Gh98HTwjtsQbFQ4Ws3Ra7rEjT1Mng@mail.gmail.com>
Date:	Fri, 27 Jan 2012 09:28:03 -0800
From:	Roland Dreier <roland@...nel.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	linux-rdma@...r.kernel.org, Andrea Arcangeli <aarcange@...hat.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters

> Sigh, what a mess ... it seems what we really want to do is know
> if userspace might trigger a COW because or not, and only do a
> preemptive COW in that case.  (We're not really concerned with
> userspace fork()ing and setting up a COW in the future, since that's
> what we have MADV_DONTFORK for)
>
> The status quo works for userspace anonymous mappings but
> it doesn't work for my case of mapping a kernel buffer read-only
> into userspace.  And fixing my case breaks the anonymous case.
> Do you see a way out of this dilemma?  Do we need to add yet
> another flag to get_user_pages()?

So thinking about this a bit more... it seems what we want is at least
to first order that we do the equivalent of write==1 exactly when the vma
for a mapping has VM_WRITE set (or is it VMA_MAYWRITE / force==1?
I don't quite understand the distinction between WRITE and MAYWRITE).

Right now, one call to get_user_pages() might involve more than one vma,
but we could simulate the above by doing find_vma() and making sure our
call to get_user_pages() goes one vma at a time.  Of course that would be
inefficient since get_user_pages() will redo the find_vma() internally, so it
would I guess make sense to add another FOLL_ flag to tell
get_user_pages() to do this?

Am I all wet, or am I becoming an MM hacker?

Thanks,
  Roland
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