Re: [PATCH v27 3/4] mm/page_poison: expose page_poisoning_enabled to kernel modules

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On 02/08/2018 02:34 AM, Michael S. Tsirkin wrote:
On Wed, Feb 07, 2018 at 02:54:30PM +0800, Wei Wang wrote:
In some usages, e.g. virtio-balloon, a kernel module needs to know if
page poisoning is in use. This patch exposes the page_poisoning_enabled
function to kernel modules.

Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
  mm/page_poison.c | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/mm/page_poison.c b/mm/page_poison.c
index e83fd44..c08d02a 100644
--- a/mm/page_poison.c
+++ b/mm/page_poison.c
@@ -30,6 +30,11 @@ bool page_poisoning_enabled(void)
  		debug_pagealloc_enabled()));
  }
+/**
+ * page_poisoning_enabled - check if page poisoning is enabled
+ *
+ * Return true if page poisoning is enabled, or false if not.
+ */
  static void poison_page(struct page *page)
  {
  	void *addr = kmap_atomic(page);
@@ -37,6 +42,7 @@ static void poison_page(struct page *page)
  	memset(addr, PAGE_POISON, PAGE_SIZE);
  	kunmap_atomic(addr);
  }
+EXPORT_SYMBOL_GPL(page_poisoning_enabled);
static void poison_pages(struct page *page, int n)
  {
Looks like both the comment and the export are in the wrong place.

Thanks. Will be more careful.

I'm a bit concerned that callers also in fact poke at the
PAGE_POISON - exporting that seems to be more of an accident
as it's only used without page_poisoning.c - it might be
better to have page_poisoning_enabled get u8 * and set it.


PAGE_POISON is a macro defined in the header, why would callers using it be a concern?

Do you suggest to have:

bool page_poisoning_get(u8 *val)
{
    if (page_poisoning_enabled()) {
        *val = PAGE_POISON;
        return true;
    }

    return false;
}
EXPORT_SYMBOL_GPL(page_poisoning_get);


Best,
Wei



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