linux-kernel - Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to deferred split list

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Date: Fri, 26 Apr 2024 22:15:55 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Yang Shi <shy828301@...il.com>, Ryan Roberts <ryan.roberts@....com>,
 Barry Song <21cnbao@...il.com>, Lance Yang <ioworker0@...il.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On 26.04.24 21:20, Zi Yan wrote:
> On 26 Apr 2024, at 15:08, David Hildenbrand wrote:
> 
>> On 26.04.24 21:02, Zi Yan wrote:
>>> From: Zi Yan <ziy@...dia.com>
>>>
>>> In __folio_remove_rmap(), a large folio is added to deferred split list
>>> if any page in a folio loses its final mapping. But it is possible that
>>> the folio is fully unmapped and adding it to deferred split list is
>>> unnecessary.
>>>
>>> For PMD-mapped THPs, that was not really an issue, because removing the
>>> last PMD mapping in the absence of PTE mappings would not have added the
>>> folio to the deferred split queue.
>>>
>>> However, for PTE-mapped THPs, which are now more prominent due to mTHP,
>>> they are always added to the deferred split queue. One side effect
>>> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be
>>> unintentionally increased, making it look like there are many partially
>>> mapped folios -- although the whole folio is fully unmapped stepwise.
>>>
>>> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs
>>> where possible starting from commit b06dc281aa99 ("mm/rmap: introduce
>>> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapped
>>> folio is unmapped in one go and can avoid being added to deferred split
>>> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be
>>> noise when we cannot batch-unmap a complete PTE-mapped folio in one go
>>> -- or where this type of batching is not implemented yet, e.g., migration.
>>>
>>> To avoid the unnecessary addition, folio->_nr_pages_mapped is checked
>>> to tell if the whole folio is unmapped. If the folio is already on
>>> deferred split list, it will be skipped, too.
>>>
>>> Note: commit 98046944a159 ("mm: huge_memory: add the missing
>>> folio_test_pmd_mappable() for THP split statistics") tried to exclude
>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
>>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
>>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
>>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
>>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable().
>>>
>>> Suggested-by: David Hildenbrand <david@...hat.com>
>>> Signed-off-by: Zi Yan <ziy@...dia.com>
>>> ---
>>>    mm/rmap.c | 12 +++++++++---
>>>    1 file changed, 9 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 2608c40dffad..a9bd64ebdd9a 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1495,6 +1495,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>    {
>>>    	atomic_t *mapped = &folio->_nr_pages_mapped;
>>>    	int last, nr = 0, nr_pmdmapped = 0;
>>> +	bool partially_mapped = false;
>>>    	enum node_stat_item idx;
>>>     	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
>>> @@ -1515,6 +1516,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>    					nr++;
>>>    			}
>>>    		} while (page++, --nr_pages > 0);
>>> +
>>> +		partially_mapped = !!nr && !!atomic_read(mapped);
>>
>> Nit: The && should remove the need for both !!.
> 
> My impression was that !! is needed to convert from int to bool and I do
> find "!!int && !!int" use in the kernel. 

I might be wrong about this, but if you wouldn't write

	if (!!nr && !!atomic_read(mapped))

then

bool partially_mapped = nr && atomic_read(mapped);

is sufficient.

&& would make sure that the result is either 0 or 1, which
you can store safely in a bool, no matter which underlying type
is used to store that value.

But I *think* nowdays, the compiler will always handle that
correctly, even without the "&&" (ever since C99 added _Bool).

Likely, also

	bool partially_mapped = nr & atomic_read(mapped);

Would nowadays work, but looks stupid.


Related: https://lkml.org/lkml/2013/8/31/138

---
#include <stdio.h>
#include <stdbool.h>
#include <stdint.h>
#include <inttypes.h>

volatile uint64_t a = 0x8000000000000000ull;

void main (void) {
         printf("uint64_t a = a: 0x%" PRIx64 "\n", a);

         int i1 = a;
         printf("int i1 = a: %d\n", i1);

         int i2 = !!a;
         printf("int i2 = !!a: %d\n", i2);

         bool b1 = a;
         printf("bool b1 = a: %d\n", b1);

         bool b2 = !!a;
         printf("bool b2 = !!a: %d\n", b2);
}
---
$ ./test
uint64_t a = a: 0x8000000000000000
int i1 = a: 0
int i2 = !!a: 1
bool b1 = a: 1
bool b2 = !!a: 1
---

Note that if bool would be defined as "int", you would need the !!, otherwise you
would lose information.

But even for b1, the gcc generates now:

  40118c:       48 8b 05 7d 2e 00 00    mov    0x2e7d(%rip),%rax        # 404010 <a>
  401193:       48 85 c0                test   %rax,%rax
  401196:       0f 95 c0                setne  %al


My stdbool.h contains

#define bool	_Bool

And I think C99 added _Bool that makes that work.

But I didn't read the standard, and it's time for the weekend :)

-- 
Cheers,

David / dhildenb