[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [PATCH] x86/mm: re-implement get_page_light() using an atomic increment
The current usage of a cmpxchg loop to increase the value of page count is not optimal on amd64, as there's already an instruction to do an atomic add to a 64bit integer. Switch the code in get_page_light() to use an atomic increment, as that avoids a loop construct. This slightly changes the order of the checks, as current code will crash before modifying the page count_info if the conditions are not correct, while with the proposed change the crash will happen immediately after having carried the counter increase. Since we are crashing anyway, I don't believe the re-ordering to have any meaningful impact. Note that the page must already have a non-zero reference count which prevents the flags from changing, and the previous usage of the cmpxchg loop didn't guarantee that the rest of the fields in count_info didn't change while updating the reference count. Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx> --- xen/arch/x86/mm.c | 12 +++--------- 1 file changed, 3 insertions(+), 9 deletions(-) diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c index 4d6d7bfe4f89..2aff6d4b5338 100644 --- a/xen/arch/x86/mm.c +++ b/xen/arch/x86/mm.c @@ -2580,16 +2580,10 @@ bool get_page(struct page_info *page, const struct domain *domain) */ static void get_page_light(struct page_info *page) { - unsigned long x, nx, y = page->count_info; + unsigned long old_pgc = arch_fetch_and_add(&page->count_info, 1); - do { - x = y; - nx = x + 1; - BUG_ON(!(x & PGC_count_mask)); /* Not allocated? */ - BUG_ON(!(nx & PGC_count_mask)); /* Overflow? */ - y = cmpxchg(&page->count_info, x, nx); - } - while ( unlikely(y != x) ); + BUG_ON(!(old_pgc & PGC_count_mask)); /* Not allocated? */ + BUG_ON(!((old_pgc + 1) & PGC_count_mask)); /* Overflow? */ } static int validate_page(struct page_info *page, unsigned long type, -- 2.44.0
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