linux-kernel - Re: [Question]: major faults are still triggered after mlockall when numa balancing

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Message-ID: <fbdf3664-b162-471d-82bb-58356cc4f7d2@intel.com>
Date:   Thu, 9 Nov 2023 23:15:30 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "zhangpeng (AS)" <zhangpeng362@...wei.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <lstoakes@...il.com>, <hughd@...gle.com>, <david@...hat.com>,
        <vbabka@...e.cz>, <mgorman@...e.de>, <mingo@...hat.com>,
        <riel@...hat.com>, <ying.huang@...el.com>, <hannes@...xchg.org>,
        Nanyong Sun <sunnanyong@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [Question]: major faults are still triggered after mlockall when
 numa balancing



On 11/9/2023 10:29 PM, Matthew Wilcox wrote:
> On Thu, Nov 09, 2023 at 03:11:41PM +0100, Peter Zijlstra wrote:
>> On Thu, Nov 09, 2023 at 09:47:24PM +0800, zhangpeng (AS) wrote:
>>> Is there any way to avoid such a major fault?
>>
>> man madvise
> 
> but from the mlockall manpage:
> 
>        mlockall() locks all pages mapped into the address space of the calling
>        process.  This includes the pages of the code, data, and stack segment,
>        as well as shared libraries, user space kernel data, shared memory, and
>        memory-mapped files.  All mapped pages are guaranteed to be resident in
>        RAM  when  the  call  returns successfully; the pages are guaranteed to
>        stay in RAM until later unlocked.
> 
> https://pubs.opengroup.org/onlinepubs/9699919799/functions/mlockall.html
> isn't quite so explicit, but I do think that page cache should be locked
> into memory.

Here is my understanding. It's related with write to a mlocked private file
mapping. From Peng:
"For the data segment, the global variable area is a private mapping".
So it's data segment of ELF file and mapped privately by ELF loader.

For this case, even ELF loader is updated to mlock the data segment, a
write will trigger COW and a new anonymous page will be allocated and
mlocked. The original file mapped page will be munlocked in 
    do_wp_page()
        wp_page_copy()
            if (old_folio) {
                page_remove_rmap()
            }
So it's possible the original file mapped page is reclaimed and later
accessing will trigger major fault.


Regards
Yin, Fengwei