Re: [Qemu-devel] [PATCH] rdma: don't make pages writeable if not requiested

[Michael S. Tsirkin]

On Thu, Mar 21, 2013 at 08:23:48AM -0400, Michael R. Hines wrote:
> Yes, I'd be happy to try the patch.
> 
> Got meetings all day...... but will dive in soon.

The patch is unlikely to be the final version. In particular
you need to change !umem->writable to umem->writable.

> On 03/21/2013 02:18 AM, Michael S. Tsirkin 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.
> >
> >If the page that is COW has lots of copies, this makes the user process
> >quickly exceed the cgroups memory limit.  This makes RDMA mostly useless
> >for virtualization, thus the stable tag.
> >
> >Reported-by: "Michael R. Hines" <address@hidden>
> >Cc: address@hidden
> >Signed-off-by: Michael S. Tsirkin <address@hidden>
> >---
> >
> >Note: compile-tested only, I don't have RDMA hardware at the moment.
> >Michael, could you please try this patch (also fixing your
> >usespace code not to request write access) and report?
> >
> >Note2: grep for get_user_pages in infiniband drivers turns up
> >lots of users who set write to 1 unconditionally.
> >These might be bugs too, should be checked.
> >
> >  drivers/infiniband/core/umem.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
> >index a841123..5929598 100644
> >--- a/drivers/infiniband/core/umem.c
> >+++ b/drivers/infiniband/core/umem.c
> >@@ -152,7 +152,7 @@ struct ib_umem *ib_umem_get(struct ib_ucontext *context, 
> >unsigned long addr,
> >             ret = get_user_pages(current, current->mm, cur_base,
> >                                  min_t(unsigned long, npages,
> >                                        PAGE_SIZE / sizeof (struct page *)),
> >-                                 1, !umem->writable, page_list, vma_list);
> >+                                 !umem->writable, 1, page_list, vma_list);
> >
> >             if (ret < 0)
> >                     goto out;