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

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Message-ID: <aafc0954-80df-dceb-03f2-2caea29cbbda@nvidia.com>
Date:   Wed, 11 May 2022 19:18:56 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Minchan Kim <minchan@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
CC:     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 5/11/22 18:08, John Hubbard wrote:
> On 5/11/22 18:03, Minchan Kim wrote:
>>>
>>> Or there might be some code path that really hates a READ_ONCE() in
>>> that place.
>>
>> My worry about chaning __get_pfnblock_flags_mask is it's called
>> multiple hot places in mm codes so I didn't want to add overhead
>> to them.
> 
> ...unless it really does generate the same code as is already there,
> right? Let me check that real quick.
> 

It does change the generated code slightly. I don't know if this will
affect performance here or not. But just for completeness, here you go:

free_one_page() originally has this (just showing the changed parts):

     mov    0x8(%rdx,%rax,8),%rbx
     and    $0x3f,%ecx
     shr    %cl,%rbx
     and    $0x7,


And after applying this diff:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0e42038382c1..df1f8e9a294f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -482,7 +482,7 @@ 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];
+       word = READ_ONCE(bitmap[word_bitidx]);
         return (word >> bitidx) & mask;
  }


...it now does an extra memory dereference:

     lea    0x8(%rdx,%rax,8),%rax
     and    $0x3f,%ecx
     mov    (%rax),%rbx
     shr    %cl,%rbx
     and    $0x7,%ebx


thanks,
-- 
John Hubbard
NVIDIA