Re: [PATCH] rdma: don't make pages writeable if not requiested
From: Michael S. Tsirkin
Date: Thu Mar 21 2013 - 03:03:34 EST
On Wed, Mar 20, 2013 at 11:55:54PM -0700, Roland Dreier wrote:
> On Wed, Mar 20, 2013 at 11:18 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > core/umem.c seems to get the arguments to get_user_pages
> > in the reverse order: it sets writeable flag and
> > breaks COW for MAP_SHARED if and only if hardware needs to
> > write the page.
> > This breaks memory overcommit for users such as KVM:
> > each time we try to register a page to send it to remote, this
> > breaks COW. It seems that for applications that only have
> > REMOTE_READ permission, there is no reason to break COW at all.
> I proposed a similar (but not exactly the same, see below) patch a
> while ago: https://lkml.org/lkml/2012/1/26/7 but read the thread,
> especially https://lkml.org/lkml/2012/2/6/265
> I think this change will break the case where userspace tries to
> register an MR with read-only permission, but intends locally through
> the CPU to write to the memory.
Shouldn't it set LOCAL_WRITE then?
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