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SubjectRe: [PATCH] user_events: Use alloc_pages instead of kzalloc for register pages
On Fri, 11 Mar 2022 14:30:28 -0800
Beau Belgrave <beaub@linux.microsoft.com> wrote:

> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index 2b5e9fdb63a0..59c900789757 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -1587,16 +1587,20 @@ static void set_page_reservations(bool set)
> static int __init trace_events_user_init(void)
> {
> int ret;
> + struct page *register_pages;

The int ret should come last.

>
> /* Zero all bits beside 0 (which is reserved for failures) */
> bitmap_zero(page_bitmap, MAX_EVENTS);
> set_bit(0, page_bitmap);
>
> - register_page_data = kzalloc(MAX_EVENTS, GFP_KERNEL);
> + register_pages = alloc_pages(GFP_KERNEL | __GFP_ZERO,
> + get_order(MAX_EVENTS));
>
> - if (!register_page_data)
> + if (!register_pages)
> return -ENOMEM;
>
> + register_page_data = page_address(register_pages);
> +
> set_page_reservations(true);
>
> ret = create_user_tracefs();
> @@ -1604,7 +1608,7 @@ static int __init trace_events_user_init(void)
> if (ret) {
> pr_warn("user_events could not register with tracefs\n");
> set_page_reservations(false);
> - kfree(register_page_data);
> + __free_pages(register_pages, get_order(MAX_EVENTS));
> return ret;
> }
>

I tried it slightly differently. Why waist bits if MAX_EVENTS is greater
than the order. That is, make MAX_EVENTS depend on the order, not the other
way around.

-- Steve

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 4febc1d6ae72..6941d0794347 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -30,9 +30,10 @@

/*
* Limits how many trace_event calls user processes can create:
- * Must be multiple of PAGE_SIZE.
+ * Must be a power of two of PAGE_SIZE.
*/
-#define MAX_PAGES 1
+#define MAX_PAGE_ORDER 0
+#define MAX_PAGES (1 << MAX_PAGE_ORDER)
#define MAX_EVENTS (MAX_PAGES * PAGE_SIZE)

/* Limit how long of an event name plus args within the subsystem. */
@@ -1606,41 +1607,25 @@ static int create_user_tracefs(void)
return -ENODEV;
}

-static void set_page_reservations(bool set)
-{
- int page;
-
- for (page = 0; page < MAX_PAGES; ++page) {
- void *addr = register_page_data + (PAGE_SIZE * page);
-
- if (set)
- SetPageReserved(virt_to_page(addr));
- else
- ClearPageReserved(virt_to_page(addr));
- }
-}
-
static int __init trace_events_user_init(void)
{
+ struct page pages;
int ret;

/* Zero all bits beside 0 (which is reserved for failures) */
bitmap_zero(page_bitmap, MAX_EVENTS);
set_bit(0, page_bitmap);

- register_page_data = kzalloc(MAX_EVENTS, GFP_KERNEL);
-
- if (!register_page_data)
+ pages = alloc_pages(GFP_KERNEL | __GFP_ZERO, MAX_PAGE_ORDER);
+ if (!pages)
return -ENOMEM;
-
- set_page_reservations(true);
+ register_page_data = page_address(pages);

ret = create_user_tracefs();

if (ret) {
pr_warn("user_events could not register with tracefs\n");
- set_page_reservations(false);
- kfree(register_page_data);
+ free_page((unsigned long)register_page_data);
return ret;
}

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