Re: [PATCH] fs/kernfs: raise sb->maxbytes to MAX_LFS_FILESIZE

[jane . chu]

On 11/24/2025 9:27 AM, Greg KH wrote:
> On Mon, Nov 24, 2025 at 09:06:21AM -0800, jane.chu@xxxxxxxxxx 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