Re: [PATCH 1/4] mm/page_alloc.c: extract commom part to check page

David Rientjes
2020-01-19T22:06:23Z

On Sun, 19 Jan 2020, Wei Yang wrote:

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d047bf7d8fd4..8cd06729169f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1025,13 +1025,9 @@ static inline bool page_expected_state(struct page *page,
> return true;
> }
>
> -static void free_pages_check_bad(struct page *page)
> +static inline const char *__check_page(struct page *page)
> {
> - const char *bad_reason;
> - unsigned long bad_flags;
> -
> - bad_reason = NULL;
> - bad_flags = 0;
> + const char *bad_reason = NULL;
>
> if (unlikely(atomic_read(&page->_mapcount) != -1))
> bad_reason = "nonzero mapcount";
> @@ -1039,14 +1035,23 @@ static void free_pages_check_bad(struct page *page)
> bad_reason = "non-NULL mapping";
> if (unlikely(page_ref_count(page) != 0))
> bad_reason = "nonzero _refcount";
> - if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_FREE)) {
> - bad_reason = "PAGE_FLAGS_CHECK_AT_FREE flag(s) set";
> - bad_flags = PAGE_FLAGS_CHECK_AT_FREE;
> - }
> #ifdef CONFIG_MEMCG
> if (unlikely(page->mem_cgroup))
> bad_reason = "page still charged to cgroup";
> #endif
> + return bad_reason;
> +}
> +
> +static void free_pages_check_bad(struct page *page)
> +{
> + const char *bad_reason = NULL;
> + unsigned long bad_flags = 0;
> +
> + bad_reason = __check_page(page);
> + if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_FREE)) {
> + bad_reason = "PAGE_FLAGS_CHECK_AT_FREE flag(s) set";
> + bad_flags = PAGE_FLAGS_CHECK_AT_FREE;
> + }
> bad_page(page, bad_reason, bad_flags);
> }
>
> @@ -2044,12 +2049,7 @@ static void check_new_page_bad(struct page *page)
> const char *bad_reason = NULL;
> unsigned long bad_flags = 0;
>
> - if (unlikely(atomic_read(&page->_mapcount) != -1))
> - bad_reason = "nonzero mapcount";
> - if (unlikely(page->mapping != NULL))
> - bad_reason = "non-NULL mapping";
> - if (unlikely(page_ref_count(page) != 0))
> - bad_reason = "nonzero _refcount";
> + bad_reason = __check_page(page);
> if (unlikely(page->flags & __PG_HWPOISON)) {
> bad_reason = "HWPoisoned (hardware-corrupted)";
> bad_flags = __PG_HWPOISON;
> @@ -2061,10 +2061,6 @@ static void check_new_page_bad(struct page *page)
> bad_reason = "PAGE_FLAGS_CHECK_AT_PREP flag set";
> bad_flags = PAGE_FLAGS_CHECK_AT_PREP;
> }
> -#ifdef CONFIG_MEMCG
> - if (unlikely(page->mem_cgroup))
> - bad_reason = "page still charged to cgroup";
> -#endif
> bad_page(page, bad_reason, bad_flags);
> }
>

I think this is compounding a previous problem in these functions: these
are all "if" clauses, not "else if" clauses so they are presumably ordered
based on least significant to most significant (we only see the last
bad_reason that we find). For the page->mem_cgroup check, this leaves
bad_flags set but it doesn't match bad_reason.

Could you instead fix the problem with these functions so that we actually
list *all* the problems with the page rather than only the last
conditional that is true?