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Message-ID: <alpine.DEB.2.00.1205231551360.26786@kaball-desktop>
Date: Wed, 23 May 2012 18:37:16 +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] xen: mark local pages as FOREIGN in the m2p_override
On Wed, 23 May 2012, Konrad Rzeszutek Wilk wrote:
> On Mon, May 14, 2012 at 12:23:07PM +0100, Stefano Stabellini wrote:
> > When the frontend and the backend reside on the same domain, even if we
> > add pages to the m2p_override, these pages will never be returned by
> > mfn_to_pfn because the check "get_phys_to_machine(pfn) != mfn" will
> > always fail, so the pfn of the frontend will be returned instead
> > (resulting in a deadlock because the frontend pages are already locked).
> >
> > However m2p_add_override can easily find out whether another pfn
> > corresponding to the mfn exists in the m2p, and can set the FOREIGN bit
> > in the p2m, making sure that mfn_to_pfn returns the pfn of the backend.
> >
> > This allows the backend to perform direct_IO on these pages, but as a
> > side effect prevents the frontend from using get_user_pages_fast on
> > them while they are being shared with the backend.
> >
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > ---
> > arch/x86/xen/p2m.c | 18 ++++++++++++++++++
> > 1 files changed, 18 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> > index 7ece122..c62ae5c 100644
> > --- a/arch/x86/xen/p2m.c
> > +++ b/arch/x86/xen/p2m.c
> > @@ -687,6 +687,7 @@ int m2p_add_override(unsigned long mfn, struct page *page,
> > unsigned long uninitialized_var(address);
> > unsigned level;
> > pte_t *ptep = NULL;
> > + int ret = 0;
> >
> > pfn = page_to_pfn(page);
> > if (!PageHighMem(page)) {
> > @@ -722,6 +723,16 @@ int m2p_add_override(unsigned long mfn, struct page *page,
> > list_add(&page->lru, &m2p_overrides[mfn_hash(mfn)]);
> > spin_unlock_irqrestore(&m2p_override_lock, flags);
> >
> > + /* p2m(m2p(mfn)) == mfn: the mfn is already present somewhere in
> > + * this domain. Set the FOREIGN_FRAME_BIT in the p2m for the other
>
> <nods> In other words, the MFN is local. And you want it to be forced
> to be !local - foreign right.
Yes
> > + * pfn so that the following mfn_to_pfn(mfn) calls will return the
>
> .. the other pfn being. So we want to override p2m(m2p(mfn)) == mfn[frontend]
> so that it becomes p2m(m2p(mfn)) == mfn[backend]? (nods)
Exactly
> What happens if multiple m2p_add_override are called on the same page?
> This would be possible if the xen-blkfront is setup shared for the same
> disk, right?
It is possible even with a single disk, I can see the frontend sharing
three times the same page, before the first request has completed, when
mounting an ext4 partition.
But it is completely safe: m2p_add_override is going to set the page as
FOREIGN the first time and m2p_remove_override is going to remove the
bit only when the last page is unshared (it does so by checking
m2p_find_override(mfn) == NULL).
> Won't we loose the old frontend PFN -> backend MFN information then as
> we would overwrite the old P2M relationship?
No, we are just changing it to be:
old frontend PFN -> (backend MFN | FOREIGN_FRAME_BIT)
> > + * pfn from the m2p_override (the backend pfn) instead.
>
> Can you explain in the comment why we want to do that. I think
> I know, but I am not going to remember it in a month.
Good point.
The reason is that the pages that have been shared by xen-blkfront
are locked (lock_page, called by do_read_cache_page).
When the userspace backend tries to use them with direct_IO, mfn_to_pfn
returns the pfn of the frontend, so do_blockdev_direct_IO is going to
try to lock the same pages again.
I'll add this comment to the commit message.
> > + * As a side effect GUPF might not be safe on the frontend pages
> > + * while they are being shared with the backend. */
>
> How is it not safe?
Because get_user_pages_fast ends up calling mfn_to_pfn that would now
return the backend pfn rather than the frontend pfn.
> > + ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
> > + if (ret >= 0 && get_phys_to_machine(pfn) == mfn)
>
> if (ret == 0)
>
> [get_user only provides -EFAULT or 0]
OK, I'll change the check.
> > + set_phys_to_machine(pfn, FOREIGN_FRAME(mfn));
> > +
> > return 0;
> > }
> > EXPORT_SYMBOL_GPL(m2p_add_override);
> > @@ -733,6 +744,7 @@ int m2p_remove_override(struct page *page, bool clear_pte)
> > unsigned long uninitialized_var(address);
> > unsigned level;
> > pte_t *ptep = NULL;
> > + int ret = 0;
> >
> > pfn = page_to_pfn(page);
> > mfn = get_phys_to_machine(pfn);
> > @@ -802,6 +814,12 @@ int m2p_remove_override(struct page *page, bool clear_pte)
> > } else
> > set_phys_to_machine(pfn, page->index);
> >
>
> You also need a comment here.
OK, I'll add one.
> > + mfn &= ~FOREIGN_FRAME_BIT;
> > + ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
> > + if (ret >= 0 && get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn) &&
>
> ret == 0
OK
> > + m2p_find_override(mfn) == NULL)
> > + set_phys_to_machine(pfn, mfn);
> > +
> > return 0;
> > }
> > EXPORT_SYMBOL_GPL(m2p_remove_override);
> > --
> > 1.7.2.5
>
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