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

Wei Yang
2020-01-20T00:33:53Z

On Sun, Jan 19, 2020 at 02:06:18PM -0800, David Rientjes wrote:
>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.
>

I have thought about this. And curious about the order of those reasons.

>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?

Sure, thanks for the suggestion.

--
Wei Yang
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