linux-kernel - Re: [PATCH] fs/kernfs: raise sb->maxbytes to MAX_LFS

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Message-ID: <1c701f3a-ded4-44a5-aa42-9d0223b86915@oracle.com>
Date: Mon, 24 Nov 2025 09:54:59 -0800
From: jane.chu@...cle.com
To: Greg KH <gregkh@...uxfoundation.org>
Cc: tj@...nel.org, linux-kernel@...r.kernel.org, jane.chu@...cle.com
Subject: Re: [PATCH] fs/kernfs: raise sb->maxbytes to MAX_LFS_FILESIZE


On 11/24/2025 9:27 AM, Greg KH wrote:
> On Mon, Nov 24, 2025 at 09:06:21AM -0800, jane.chu@...cle.com wrote:
>> Hi, Greg,
>>
>> On 11/24/2025 8:17 AM, Greg KH wrote:
>>> On Tue, Nov 11, 2025 at 01:26:06PM -0700, Jane Chu wrote:
>>>> On an ARM64 A1 system, it's possible to have physical memory span
>>>> up to the 64T boundary, like below
>>>>
>>>> $ lsmem -b -r -n -o range,size
>>>> 0x0000000080000000-0x00000000bfffffff 1073741824
>>>> 0x0000080000000000-0x000008007fffffff 2147483648
>>>> 0x00000800c0000000-0x0000087fffffffff 546534588416
>>>> 0x0000400000000000-0x00004000bfffffff 3221225472
>>>> 0x0000400100000000-0x0000407fffffffff 545460846592
>>>>
>>>> So it's time to extend /sys/kernel/mm/page_idle/bitmap to be able
>>>> to account for >2G number of pages, by raising the kernfs file size
>>>> limit.
>>>
>>> Wait, we are having sysfs files that are bigger than >2G?  Which files
>>> exactly?
>>
>> This file:  /sys/kernel/mm/page_idle/bitmap
>> that tracks idle pages, 1 bit per page.
> 
> Why is that a sysfs file and not a debugfs file?

The bitmap file  was introduced by
   33c3fc71c8cf "(mm: introduce idle page tracking)"
for idle page tracking.

See also
   https://docs.kernel.org/admin-guide/mm/idle_page_tracking.html

> 
>> Because of the above memory span, so even though the system has < 64TiB
>> memory, we still need to be able to seek beyond the 2GiB point in the
>> /sys/kernel/mm/page_idle/bitmap file.
> 
> What uses this file?  It's not on my systems, what arch uses it?

Our use case is for production, not for debug purpose.
The file is on my dual-socket Intel Ice Lake system w/ Linux v6.12.x, 
the issue was originally reported on an ARM64 A1 system.

thanks,
-jane

> 
> thanks,
> 
> greg k-h