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Subject: Re: [Patch v2 1/4] mm: enable dump several reasons for __dump_page()
To:     Wei Yang <richardw.yang@linux.intel.com>
Cc:     akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, rientjes@google.com
References: <20200120030415.15925-1-richardw.yang@linux.intel.com>
 <20200120030415.15925-2-richardw.yang@linux.intel.com>
 <8426f31b-606e-deca-acbe-dd59b193e113@arm.com>
 <20200120085530.GB18028@richard>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <1c2e1cd6-5b65-79d7-f332-b866d5446c71@arm.com>
Date:   Tue, 21 Jan 2020 10:50:41 +0530
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On 01/20/2020 02:25 PM, Wei Yang wrote:
> On Mon, Jan 20, 2020 at 11:42:30AM +0530, Anshuman Khandual wrote:
>>
>>
>> On 01/20/2020 08:34 AM, Wei Yang wrote:
>>> This is a preparation to dump all reasons during check page.
>>
>> This really makes sense rather then just picking the reason from
>> the last "if" statement.
>>
>>>
>>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>>> ---
>>>  include/linux/mmdebug.h |  2 +-
>>>  mm/debug.c              | 11 ++++++-----
>>>  mm/page_alloc.c         |  2 +-
>>>  3 files changed, 8 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
>>> index 2ad72d2c8cc5..f0a612db8bae 100644
>>> --- a/include/linux/mmdebug.h
>>> +++ b/include/linux/mmdebug.h
>>> @@ -10,7 +10,7 @@ struct vm_area_struct;
>>>  struct mm_struct;
>>>  
>>>  extern void dump_page(struct page *page, const char *reason);
>>> -extern void __dump_page(struct page *page, const char *reason);
>>> +extern void __dump_page(struct page *page, int num, const char **reason);
>>>  void dump_vma(const struct vm_area_struct *vma);
>>>  void dump_mm(const struct mm_struct *mm);
>>>  
>>> diff --git a/mm/debug.c b/mm/debug.c
>>> index 0461df1207cb..a8ac6f951f9f 100644
>>> --- a/mm/debug.c
>>> +++ b/mm/debug.c
>>> @@ -42,11 +42,11 @@ const struct trace_print_flags vmaflag_names[] = {
>>>  	{0, NULL}
>>>  };
>>>  
>>> -void __dump_page(struct page *page, const char *reason)
>>> +void __dump_page(struct page *page, int num, const char **reason)
>>>  {
>>>  	struct address_space *mapping;
>>>  	bool page_poisoned = PagePoisoned(page);
>>> -	int mapcount;
>>> +	int mapcount, i;
>>>  
>>>  	/*
>>>  	 * If struct page is poisoned don't access Page*() functions as that
>>> @@ -97,8 +97,9 @@ void __dump_page(struct page *page, const char *reason)
>>>  			sizeof(unsigned long), page,
>>>  			sizeof(struct page), false);
>>>  
>>> -	if (reason)
>>> -		pr_warn("page dumped because: %s\n", reason);
>>> +	pr_warn("page dumped because:\n");
>>> +	for (i = 0; i < num; i++)
>>> +		pr_warn("\t%s\n", reason[i]);
>>
>> We should have a NR_BAD_PAGE_REASONS or something to cap this iteration
>> and also check reason[i] for non-NULL before trying to print the array.
>> There might be call sites like the following which will be problematic
>> otherwise.
>>
>> split_huge_page_to_list() -> dump_page(head, NULL)
>>
> 
> You are right, I missed this case.
> 
>>>  
>>>  #ifdef CONFIG_MEMCG
>>>  	if (!page_poisoned && page->mem_cgroup)
>>
>> While here, will it be better to move the above debug print block after
>> mem_cgroup block instead ?
>>
> 
> Not sure, let's see whether others have some idea.
> 
>>> @@ -108,7 +109,7 @@ void __dump_page(struct page *page, const char *reason)
>>>  
>>>  void dump_page(struct page *page, const char *reason)
>>>  {
>>> -	__dump_page(page, reason);
>>> +	__dump_page(page, 1, &reason);
>>>  	dump_page_owner(page);
>>>  }
>>>  EXPORT_SYMBOL(dump_page);
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index d047bf7d8fd4..0cf6218aaba7 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -638,7 +638,7 @@ static void bad_page(struct page *page, const char *reason,
>>>  
>>>  	pr_alert("BUG: Bad page state in process %s  pfn:%05lx\n",
>>>  		current->comm, page_to_pfn(page));
>>> -	__dump_page(page, reason);
>>> +	__dump_page(page, 1, &reason);
>>>  	bad_flags &= page->flags;
>>>  	if (bad_flags)
>>>  		pr_alert("bad because of flags: %#lx(%pGp)\n",
>>>
>>
>> Do we still need to have bad_flags ? After consolidating all reasons making
>> a page bad should not we just print page->flags unconditionally each time and
>> let the user decipher it instead. __dump_page() will print page->flags for
>> each case (atleast after the new patch from Vlastimil). AFAICS, the only
>> place currently consuming bad_flags is bad_page() which seems redundant after
>> first calling __dump_page().
> 
> Hmm... I don't catch this. The work in __dump_page() seems a little different
> from this one. Not sure we could remove it.

Lets look at 'bad_flags' as it exists today without this series.

It gets evaluated in free_pages_check_bad() and check_new_page_bad() before
being passed into bad_page(). All other call sites for bad_page() just pass
0 for 'bad_flags'. Now in bad_page(), we have

        __dump_page(page, reason);
        bad_flags &= page->flags;
        if (bad_flags)
                pr_alert("bad because of flags: %#lx(%pGp)\n",
                                                bad_flags, &bad_flags);

Here, bad_flags &= page->flags will always be positive when 'reason'
is either

"PAGE_FLAGS_CHECK_AT_FREE flag(s) set"

or

"PAGE_FLAGS_CHECK_AT_PREP flag set"

The point here is we dont need to print bad_flags here as __dump_page()
already prints page->flags universally along with the "bad_reason"
after the following change.

[1] https://patchwork.kernel.org/patch/11332035/