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

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Message-ID: <2025112410-shingle-wackiness-d25c@gregkh>
Date: Mon, 24 Nov 2025 18:27:00 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: jane.chu@...cle.com
Cc: tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/kernfs: raise sb->maxbytes to MAX_LFS_FILESIZE

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?

> 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?

thanks,

greg k-h