On 13/05/2022 03:22, Penny Zheng wrote:
Hi Julien
Hi Penny,
-----Original Message-----
From: Julien Grall <jul...@xen.org>
Sent: Thursday, May 12, 2022 6:14 PM
To: Penny Zheng <penny.zh...@arm.com>; xen-devel@lists.xenproject.org
Cc: Wei Chen <wei.c...@arm.com>; Stefano Stabellini
<sstabell...@kernel.org>; Bertrand Marquis <bertrand.marq...@arm.com>;
Volodymyr Babchuk <volodymyr_babc...@epam.com>
Subject: Re: [PATCH v3 4/8] xen/arm: introduce put_page_nr and get_page_nr
On 12/05/2022 10:11, Penny Zheng wrote:
Later, we need to add the right amount of references, which should be
the number of borrower domains, to the owner domain. Since we only
have
get_page() to increment the page reference by 1, a loop is needed per
page, which is inefficient and time-consuming.
To save the loop time, this commit introduces a set of new helpers
put_page_nr() and get_page_nr() to increment/drop the page reference by
nr.
Signed-off-by: Penny Zheng <penny.zh...@arm.com>
---
v3 changes:
- check overflow with "n"
- remove spurious change
- bring back the check that we enter the loop only when count_info is
greater than 0
---
v2 change:
- new commit
---
xen/arch/arm/include/asm/mm.h | 4 ++++
xen/arch/arm/mm.c | 36 ++++++++++++++++++++++++++---------
2 files changed, 31 insertions(+), 9 deletions(-)
diff --git a/xen/arch/arm/include/asm/mm.h
b/xen/arch/arm/include/asm/mm.h index 424aaf2823..c737d51e4d 100644
--- a/xen/arch/arm/include/asm/mm.h
+++ b/xen/arch/arm/include/asm/mm.h
@@ -347,6 +347,10 @@ void free_init_memory(void);
int guest_physmap_mark_populate_on_demand(struct domain *d,
unsigned long gfn,
unsigned int order);
+extern bool get_page_nr(struct page_info *page, const struct domain
*domain,
+ unsigned long nr); extern void
+put_page_nr(struct page_info *page, unsigned long nr);
+
extern void put_page_type(struct page_info *page);
static inline void put_page_and_type(struct page_info *page)
{
diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c index
7b1f2f4906..a9461e07aa 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1537,7 +1537,8 @@ long arch_memory_op(int op,
XEN_GUEST_HANDLE_PARAM(void) arg)
return 0;
}
-struct domain *page_get_owner_and_reference(struct page_info *page)
+static struct domain *page_get_owner_and_nr_reference(struct page_info
*page,
+ unsigned long
+nr)
{
unsigned long x, y = page->count_info;
struct domain *owner;
@@ -1548,10 +1549,10 @@ struct domain
*page_get_owner_and_reference(struct page_info *page)
* Count == 0: Page is not allocated, so we cannot take a reference.
* Count == -1: Reference count would wrap, which is invalid.
*/
- if ( unlikely(((x + 1) & PGC_count_mask) <= 1) )
+ if ( unlikely(((x + nr) & PGC_count_mask) <= 1) )
This check looks wrong to me. You want to make sure that the right equation
return is at least equal to n otherwise.
Right, right, I haven't considered thoroughly! A thousand thanks for the
following
detailed explanation~
Furthermore, I think we need to restrict 'nr' to PGC_count_mask to fully catch
any overflow.
Before the loop, the code would look like:
/* Restrict nr to avoid "double" overflow */ if ( nr >= PGC_count_mask ) {
ASSERT_UNREACHABLE();
return NULL;
}
The check in the loop would look like:
if ( unlikely((x + nr) & PGC_count_mask) <= n )
That said, it might be easier to read the overflow check if we do:
count = x & PGC_count_mask;
if ( !count || ((PGC_count_mask - count) <= n) )
I haven't measured and check which of the two options would result to better
code and performance (get_page() is often called).
Correct me if I understand wrongly:
IMO, only option two is actually catching any overflow? Let (PGC_count_mask -
count) <= nr
stay in the loop, not before the loop like option 1, to cover the changeable
page->count_info.
Both option should catch the overflow. In option 1, this was 2 part check:
if ( nr >= PGC_count_mask )
...
do
{
if ( unlikely((x + nr) & PGC_count_mask <= n )
return NULL;
...
} while (...);
Cheers,
--
Julien Grall
