linux-kernel - Re: [PATCH v4] mm: fix is_pinnable

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Date:   Mon, 23 May 2022 23:22:11 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Dias <joaodias@...gle.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

On 5/23/22 10:16 PM, Minchan Kim wrote:
> On Mon, May 23, 2022 at 07:55:25PM -0700, John Hubbard wrote:
>> On 5/23/22 09:33, Minchan Kim wrote:
>> ...
>>>> So then:
>>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 0e42038382c1..b404f87e2682 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -482,7 +482,12 @@ unsigned long __get_pfnblock_flags_mask(const struct page *page,
>>>>           word_bitidx = bitidx / BITS_PER_LONG;
>>>>           bitidx &= (BITS_PER_LONG-1);
>>>>
>>>> -       word = bitmap[word_bitidx];
>>>> +       /*
>>>> +        * This races, without locks, with set_pageblock_migratetype(). Ensure
>>>                                                set_pfnblock_flags_mask would be better?
>>>> +        * a consistent (non-tearing) read of the memory array, so that results,
>>>
>>> Thanks for proceeding and suggestion, John.
>>>
>>> IIUC, the load tearing wouldn't be an issue since [1] fixed the issue.
>>
>> Did it? [1] fixed something, but I'm not sure we can claim that that
>> code is now safe against tearing in all possible cases, especially given
>> the recent discussion here. Specifically, having this code do a read,
>> then follow that up with calculations, seems correct. Anything else is
> 
> The load tearing you are trying to explain in the comment would be
> solved by [1] since the bits will always align on a word and accessing
> word size based on word aligned address is always atomic so there is
> no load tearing problem IIUC.
> 
> Instead of the tearing problem, what we are trying to solve with
> READ_ONCE is to prevent refetching when the function would be
> inlined in the future.
> 

I'm perhaps using "tearing" as too broad of a term, maybe just removing
the "(non-tearing)" part would fix up the comment.

>> sketchy...
>>
>>>
>>> The concern in our dicussion was aggressive compiler(e.g., LTO) or code refactoring
>>> to make the code inline in *future* could potentially cause forcing refetching(i.e.,
>>> re-read) tie bitmap[word_bitidx].
>>>
>>> If so, shouldn't the comment be the one you helped before?
>>
>> Well, maybe updated to something like this?
>>
>> /*
>>   * This races, without locks, with set_pageblock_migratetype(). Ensure
> 
> set_pageblock_migratetype is more upper level function so it would
> be better fit to say set_pfnblock_flags_mask.

OK

>                                       
>>   * a consistent (non-tearing) read of the memory array, so that results,
> 
> So tearing problem should't already happen by [1] so I am trying to
> explain refetching(or re-read) problem in the comment.
> 
>>   * even though racy, are not corrupted--even if this function is
> 
> The value is already atomic so I don't think it could be corrupted
> even though it would be inlined in the future.
> 
> Please correct me if I miss something.
> 
>>   * refactored and/or inlined.
>>   */
> 

thanks,

-- 
John Hubbard
NVIDIA