linux-kernel - Re: [PATCH v3 1/2] xen: enter/exit lazy_mmu_mode around m2p

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Date:	Tue, 17 Apr 2012 15:26:05 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] xen: enter/exit lazy_mmu_mode around m2p_override
 calls

On Tue, 17 Apr 2012, Konrad Rzeszutek Wilk wrote:
> On Tue, Apr 10, 2012 at 05:29:34PM +0100, Stefano Stabellini wrote:
> > This patch is a significant performance improvement for the
> > m2p_override: about 6% using the gntdev device.
> > 
> > Each m2p_add/remove_override call issues a MULTI_grant_table_op and a
> > __flush_tlb_single if kmap_op != NULL.  Batching all the calls together
> > is a great performance benefit because it means issuing one hypercall
> > total rather than two hypercall per page.
> > If paravirt_lazy_mode is set PARAVIRT_LAZY_MMU, all these calls are
> > going to be batched together, otherwise they are issued one at a time.
> > 
> > Adding arch_enter_lazy_mmu_mode/arch_leave_lazy_mmu_mode around the
> > m2p_add/remove_override calls forces paravirt_lazy_mode to
> > PARAVIRT_LAZY_MMU, therefore makes sure that they are always batched.
> > 
> > 
> > Changes in v3:
> > - do not call arch_enter/leave_lazy_mmu_mode in xen_blkbk_unmap, that
> > can be called in interrupt context.
> 
> This is with RHEL5 (somehow the pvops kernels don't trigger this):

Do you mean RHEL5 as a guest? Are you using blkback or qdisk?
How can I repro the bug?


In any case it looks like a similar kind of issue to the one I fixed
before: paravirt_enter_lazy_mmu is probably called with
paravirt_lazy_mode == PARAVIRT_LAZY_MMU, in this case by
gnttab_unmap_refs.
But I don't understand how gnttab_unmap_refs can be called when you seem
to be using blkback.

Anyhow gnttab_unmap_refs can be called by:

- mmu_notifier on invalidate_range_start;
- vm_operations_struct on close;
- file_operations on release;
- the unmap gntdev ioctl.

I guess the mmu_notifier or vm_operations_struct could be the ones
calling gnttab_unmap_refs with lazy_mode set to PARAVIRT_LAZY_MMU.

I suspect that something like the following code would fix the issue but
it is pretty ugly and I need to test it before a submitting it as a fix:


@@ -780,7 +780,7 @@ EXPORT_SYMBOL_GPL(gnttab_map_refs);
 int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
                      struct page **pages, unsigned int count, bool clear_pte)
 {
-       int i, ret;
+       int i, ret, lazy = 0;
 
        ret = HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref, unmap_ops, count);
        if (ret)

@@ -789,7 +789,10 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
        if (xen_feature(XENFEAT_auto_translated_physmap))
                return ret;
 
-       arch_enter_lazy_mmu_mode();
+       if (!in_interrupt() && paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
+               arch_enter_lazy_mmu_mode();
+               lazy = 1;
+       }
 
        for (i = 0; i < count; i++) {
                ret = m2p_remove_override(pages[i], clear_pte);
@@ -797,7 +800,8 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
                        return ret;
        }
 
-       arch_leave_lazy_mmu_mode();
+       if (lazy)
+               arch_leave_lazy_mmu_mode();
 
        return ret;
 }

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