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RE: [PATCH v7 9/9] xen: retrieve reserved pages on populate_physmap
- To: Julien Grall <julien@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Penny Zheng <Penny.Zheng@xxxxxxx>
- Date: Mon, 27 Jun 2022 10:11:20 +0000
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- Cc: Wei Chen <Wei.Chen@xxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Mon, 27 Jun 2022 10:11:42 +0000
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- Thread-topic: [PATCH v7 9/9] xen: retrieve reserved pages on populate_physmap
Hi Julien
> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: Saturday, June 25, 2022 3:51 AM
> To: Penny Zheng <Penny.Zheng@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Wei Chen <Wei.Chen@xxxxxxx>; Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; George Dunlap <george.dunlap@xxxxxxxxxx>;
> Jan Beulich <jbeulich@xxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>;
> Wei Liu <wl@xxxxxxx>
> Subject: Re: [PATCH v7 9/9] xen: retrieve reserved pages on
> populate_physmap
>
> Hi Penny,
>
> On 20/06/2022 03:44, Penny Zheng wrote:
> > When a static domain populates memory through populate_physmap at
> > runtime, it shall retrieve reserved pages from resv_page_list to make
> > sure that guest RAM is still restricted in statically configured memory
> regions.
> > This commit also introduces a new helper acquire_reserved_page to make
> it work.
> >
> > Signed-off-by: Penny Zheng <penny.zheng@xxxxxxx>
> > ---
> > v7 changes:
> > - remove the lock, since we add the page to rsv_page_list after it has
> > been totally freed.
>
> Hmmm... Adding the page after it has been totally freed doesn't mean you
> can get away with the lock. AFAICT you can still have concurrent free/allocate
> that could modify the list.
>
> Therefore if you add/remove pages without the list, you would end up to
> corrupt the list.
>
> If you disagree, then please point out which lock (or mechanism) will prevent
> concurrent access.
>
Ok. Combined with the last serie comments, actually, you suggest that we need
to add
two locks, right?
One is the lock for concurrent free/allocation on page_info, and we will use
heap_lock, one stays in free_staticmem_pages, one stays in its reversed function
prepare_staticmem_pages.
The other is for concurrent free/allocation on resv_page_list, and we will use
d->page_alloc_lock tp guard it. One stays in put_static_page, and another
stays in reversed function acquire_reserved_page.
> Cheers,
>
> --
> Julien Grall
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