[PATCH] Argo: don't obtain excess page references

[PATCH] Argo: don't obtain excess page references

[Jan Beulich]

find_ring_mfn() already holds a page reference when trying to obtain a
writable type reference. We shouldn't make assumptions on the general
reference count limit being effectively "infinity". Obtain merely a type
ref, re-using the general ref by only dropping the previously acquired
one in the case of an error.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
I further question the log-dirty check there: The present P2M type of a
page doesn't really matter for writing to the page (plus it's stale by
the time it is looked at). Instead I think every write to such a page
needs to be accompanied by a call to paging_mark_dirty().

--- a/xen/common/argo.c
+++ b/xen/common/argo.c
@@ -1429,10 +1429,11 @@ find_ring_mfn(struct domain *d, gfn_t gf
         ret = -EAGAIN;
 #endif
     else if ( (p2mt != p2m_ram_rw) ||
-              !get_page_and_type(page, d, PGT_writable_page) )
+              !get_page_type(page, PGT_writable_page) )
         ret = -EINVAL;
 
-    put_page(page);
+    if ( unlikely(ret) )
+        put_page(page);
 
     return ret;
 }

Ping: [PATCH] Argo: don't obtain excess page references

[Jan Beulich]

On 11.10.2022 11:28, Jan Beulich wrote:
> find_ring_mfn() already holds a page reference when trying to obtain a
> writable type reference. We shouldn't make assumptions on the general
> reference count limit being effectively "infinity". Obtain merely a type
> ref, re-using the general ref by only dropping the previously acquired
> one in the case of an error.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Ping?

> ---
> I further question the log-dirty check there: The present P2M type of a
> page doesn't really matter for writing to the page (plus it's stale by
> the time it is looked at). Instead I think every write to such a page
> needs to be accompanied by a call to paging_mark_dirty().
> 
> --- a/xen/common/argo.c
> +++ b/xen/common/argo.c
> @@ -1429,10 +1429,11 @@ find_ring_mfn(struct domain *d, gfn_t gf
>          ret = -EAGAIN;
>  #endif
>      else if ( (p2mt != p2m_ram_rw) ||
> -              !get_page_and_type(page, d, PGT_writable_page) )
> +              !get_page_type(page, PGT_writable_page) )
>          ret = -EINVAL;
>  
> -    put_page(page);
> +    if ( unlikely(ret) )
> +        put_page(page);
>  
>      return ret;
>  }
> 

Re: Ping: [PATCH] Argo: don't obtain excess page references

[Christopher Clark]

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On Mon, Nov 21, 2022 at 4:41 AM Jan Beulich <jbeulich@suse.com> wrote:

> On 11.10.2022 11:28, Jan Beulich wrote:
> > find_ring_mfn() already holds a page reference when trying to obtain a
> > writable type reference. We shouldn't make assumptions on the general
> > reference count limit being effectively "infinity". Obtain merely a type
> > ref, re-using the general ref by only dropping the previously acquired
> > one in the case of an error.
> >
> > Signed-off-by: Jan Beulich <jbeulich@suse.com>
>
> Ping?
>

Message received -- I will look at this. Thanks.

Christopher


>
> > ---
> > I further question the log-dirty check there: The present P2M type of a
> > page doesn't really matter for writing to the page (plus it's stale by
> > the time it is looked at). Instead I think every write to such a page
> > needs to be accompanied by a call to paging_mark_dirty().
> >
> > --- a/xen/common/argo.c
> > +++ b/xen/common/argo.c
> > @@ -1429,10 +1429,11 @@ find_ring_mfn(struct domain *d, gfn_t gf
> >          ret = -EAGAIN;
> >  #endif
> >      else if ( (p2mt != p2m_ram_rw) ||
> > -              !get_page_and_type(page, d, PGT_writable_page) )
> > +              !get_page_type(page, PGT_writable_page) )
> >          ret = -EINVAL;
> >
> > -    put_page(page);
> > +    if ( unlikely(ret) )
> > +        put_page(page);
> >
> >      return ret;
> >  }
> >
>
>

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Re: Ping: [PATCH] Argo: don't obtain excess page references

[Christopher Clark]

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On Mon, Nov 21, 2022 at 4:41 AM Jan Beulich <jbeulich@suse.com> wrote:

> On 11.10.2022 11:28, Jan Beulich wrote:
> > find_ring_mfn() already holds a page reference when trying to obtain a
> > writable type reference. We shouldn't make assumptions on the general
> > reference count limit being effectively "infinity". Obtain merely a type
> > ref, re-using the general ref by only dropping the previously acquired
> > one in the case of an error.
> >
> > Signed-off-by: Jan Beulich <jbeulich@suse.com>
>
> Ping?
>

Hi Jan,

Sorry it has taken me so long to review this patch and thank-you for
posting it. The points raised are helpful.

Wrt to the patch - I can't ack because:
the general ref that is already held is from the page owner, and it may
actually be foreign; so the second ref acquire is currently ensuring that
it is a match for the owner of the ring. That needs addressing.

Am supportive of points raised:
- review + limit ref counts taken
    - better to not need two general page refs
- a type ref rather than general may be sufficient to hold for the ring
lifetime?
- paging_mark_dirty at writes
- p2m log dirty would be better to be allowed than EAGAIN
- allowing mapping of foreign pages may have uses though likely also
challenging

I should let you know that my time available is extremely limited at the
moment, sorry.

Christopher




>
> > ---
> > I further question the log-dirty check there: The present P2M type of a
> > page doesn't really matter for writing to the page (plus it's stale by
> > the time it is looked at). Instead I think every write to such a page
> > needs to be accompanied by a call to paging_mark_dirty().
> >
> > --- a/xen/common/argo.c
> > +++ b/xen/common/argo.c
> > @@ -1429,10 +1429,11 @@ find_ring_mfn(struct domain *d, gfn_t gf
> >          ret = -EAGAIN;
> >  #endif
> >      else if ( (p2mt != p2m_ram_rw) ||
> > -              !get_page_and_type(page, d, PGT_writable_page) )
> > +              !get_page_type(page, PGT_writable_page) )
> >          ret = -EINVAL;
> >
> > -    put_page(page);
> > +    if ( unlikely(ret) )
> > +        put_page(page);
> >
> >      return ret;
> >  }
> >
>
>

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Re: Ping: [PATCH] Argo: don't obtain excess page references

[Jan Beulich]

On 30.01.2023 05:35, Christopher Clark wrote:
> On Mon, Nov 21, 2022 at 4:41 AM Jan Beulich <jbeulich@suse.com> wrote:
> 
>> On 11.10.2022 11:28, Jan Beulich wrote:
>>> find_ring_mfn() already holds a page reference when trying to obtain a
>>> writable type reference. We shouldn't make assumptions on the general
>>> reference count limit being effectively "infinity". Obtain merely a type
>>> ref, re-using the general ref by only dropping the previously acquired
>>> one in the case of an error.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>
>> Ping?
> 
> Sorry it has taken me so long to review this patch and thank-you for
> posting it. The points raised are helpful.
> 
> Wrt to the patch - I can't ack because:
> the general ref that is already held is from the page owner, and it may
> actually be foreign; so the second ref acquire is currently ensuring that
> it is a match for the owner of the ring. That needs addressing.

I'm afraid I may not understand your reply: Are you saying there's something
wrong with the change? Or are you saying there's something wrong that merely
becomes apparent due to the change? Or yet something else?

> Am supportive of points raised:
> - review + limit ref counts taken
>     - better to not need two general page refs
> - a type ref rather than general may be sufficient to hold for the ring
> lifetime?
> - paging_mark_dirty at writes
> - p2m log dirty would be better to be allowed than EAGAIN

I can certainly extend the patch to a series, but that'll make sense only
if ...

> - allowing mapping of foreign pages may have uses though likely also
> challenging
> 
> I should let you know that my time available is extremely limited at the
> moment, sorry.

... it would then also be looked at within a reasonable amount of time. I'm
already sitting on way too many unreviewed patches.

Jan

Re: Ping: [PATCH] Argo: don't obtain excess page references

[Jan Beulich]

On 30.01.2023 09:03, Jan Beulich wrote:
> On 30.01.2023 05:35, Christopher Clark wrote:
>> On Mon, Nov 21, 2022 at 4:41 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>>> On 11.10.2022 11:28, Jan Beulich wrote:
>>>> find_ring_mfn() already holds a page reference when trying to obtain a
>>>> writable type reference. We shouldn't make assumptions on the general
>>>> reference count limit being effectively "infinity". Obtain merely a type
>>>> ref, re-using the general ref by only dropping the previously acquired
>>>> one in the case of an error.
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>>
>>> Ping?
>>
>> Sorry it has taken me so long to review this patch and thank-you for
>> posting it. The points raised are helpful.
>>
>> Wrt to the patch - I can't ack because:
>> the general ref that is already held is from the page owner, and it may
>> actually be foreign; so the second ref acquire is currently ensuring that
>> it is a match for the owner of the ring. That needs addressing.
> 
> I'm afraid I may not understand your reply: Are you saying there's something
> wrong with the change? Or are you saying there's something wrong that merely
> becomes apparent due to the change? Or yet something else?

And to extend on the questions: What notion of "foreign" are you referring
to here? The earlier check_get_page_from_gfn() is passed the very same
domain. And anyway, get_page() wouldn't allow to acquire a ref with a
domain other than the page owner. Plus the sole caller of find_ring_mfns()
passes currd.

Jan