linux-kernel - Re: [PATCH] /proc/kpagecount: return 0 for special pages that are never mapped

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Message-ID: <d9fa0325-690d-47cc-fc49-2606855f6a4f@oracle.com>
Date:   Wed, 5 Dec 2018 16:44:15 -0800
From:   Anthony Yznaga <anthony.yznaga@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        adobriyan@...il.com, akpm@...ux-foundation.org, vbabka@...e.cz,
        sfr@...b.auug.org.au, kirill.shutemov@...ux.intel.com,
        rppt@...ux.vnet.ibm.com, mhocko@...e.com,
        alexander.h.duyck@...ux.intel.com, hannes@...xchg.org,
        miles.chen@...iatek.com, n-horiguchi@...jp.nec.com
Subject: Re: [PATCH] /proc/kpagecount: return 0 for special pages that are
 never mapped



On 12/05/2018 11:44 AM, Matthew Wilcox wrote:
> On Wed, Dec 05, 2018 at 11:40:51AM -0800, Anthony Yznaga wrote:
>> On 12/04/2018 05:25 PM, Matthew Wilcox wrote:
>>> On Tue, Dec 04, 2018 at 05:18:32PM -0800, anthony.yznaga@...cle.com wrote:
>>>> On 12/04/2018 04:48 PM, Matthew Wilcox wrote:
>>>>> On Tue, Dec 04, 2018 at 02:45:26PM -0800, Anthony Yznaga wrote:
>>>>>> +static inline int page_has_type(struct page *page)
>>>>>> +{
>>>>>> +	return (PageType(page, 0) &&
>>>>>> +	       ((page->page_type & PAGE_TYPE_ALL) != PAGE_TYPE_ALL));
>>>>>> +}
>>>>>> +
>>>>> I think this is a bit complex, and a bit of a pain to update as we add
>>>>> new page types.  How about this?
>>>>>
>>>>> 	return (int)page_type < -128;
>>>>>
>>>>> (I'm open to appropriate #defines to make this more obvious that it's ~0x7F)
>>>> I thought about having this:
>>>>
>>>> #define PAGE_TYPE_END    0xffffff80
>>>>
>>>> static int inline page_has_type(struct page *page)
>>>> {
>>>>     return page->page_type > PAGE_TYPE_BASE &&
>>>>            page->page_type < PAGE_TYPE_END;
>>>> }
>>>>
>>>> But I opted for the additional complexity to avoid more false-positives from
>>>> possibly corrupted values.  I'm certainly fine with a simple approach, though.
>>> The way I'm thinking about this field is that usually it's _mapcount
>>> which is 0xffffffff to represent 0.  We allow a certain small amount
>>> of underflow and still treat it as a mapcount.  We also allow for some
>>> amount of overflow.  So to be utterly precise, what you had there would
>>> have been fine, but for simplicity, I'd rather just do a signed compare
>>> against -128.
>> The signed compare does not allow for mapcount overflow.  Is that acceptable?
>> False-positives would be benign for /proc/kpagecount though from a debug
>> perspective it could be helpful to see overflowed mapcounts.  Some future
>> caller would need separate consideration.
> Nobody seems terribly interested in mapcount overflows.  I got no response
> to https://lkml.org/lkml/2018/3/2/991

Okay.  Thanks for the background.

How about this, then:

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 50ce1bddaf56..39b4494e29f1 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -669,6 +669,7 @@ static inline int TestClearPageDoubleMap(struct page *page)
 
 #define PAGE_TYPE_BASE    0xf0000000
 /* Reserve        0x0000007f to catch underflows of page_mapcount */
+#define PAGE_MAPCOUNT_RESERVE    -128
 #define PG_buddy    0x00000080
 #define PG_balloon    0x00000100
 #define PG_kmemcg    0x00000200
@@ -677,6 +678,11 @@ static inline int TestClearPageDoubleMap(struct page *page)
 #define PageType(page, flag)                        \
     ((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
 
+static inline int page_has_type(struct page *page)
+{
+    return (int)page->page_type < PAGE_MAPCOUNT_RESERVE;
+}
+
 #define PAGE_TYPE_OPS(uname, lname)                    \
 static __always_inline int Page##uname(struct page *page)        \
 {                                    \