Re: [PATCH v2] x86/mm: Fix incorrect for loop count calculation in sync_global_pgds

[Dan Williams]

On Mon, May 1, 2017 at 8:55 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> Jeff Moyer reported that on his system with two memory regions 0~64G and
> 1T~1T+192G, and kernel option "memmap=192G!1024G" added, enabling kaslr
> will make system hang intermittently during boot. While adding 'nokaslr'
> won't.
>
> This is because the for loop count calculation in sync_global_pgds is
> not correct. When a mapping area crosses pgd entries, we should
> calculate the starting address of region which next pgd covers and assign
> it to next for loop count, but not add PGDIR_SIZE directly. The old
> code works right only if the mapping area is times of PGDIR_SIZE,
> otherwize the end region could be skipped so that it can't be synchronized
> to all other processes from kernel pgd init_mm.pgd.
>
> In Jeff's system, emulated pmem area [1024G, 1216G) is smaller than
> PGDIR_SIZE. While 'nokaslr' works because PAGE_OFFSET is 1T aligned, it
> makes this area be mapped inside one pgd entry. With kaslr enabled,
> this area could cross two pgd entries, then the next pgd entry won't
> be synced to all other processes. That is why we saw empty PGD.
>
> Fix it in this patch.
>
[..]
>
> Reported-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
[..]

Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>