Re: [PATCH] x86/mm: pgds getting out of sync after memory hot remove

[Kirill A. Shutemov]

On Fri, May 19, 2017 at 02:01:26PM -0400, Jérôme Glisse wrote:
> After memory hot remove it seems we do not synchronize pgds for kernel
> virtual memory range (on vmemmap_free()). This seems bogus to me as it
> means we are left with stall entry for process with mm != mm_init
> 
> Yet i am puzzle by the fact that i am only now hitting this issue. It
> never was an issue with 4.12 or before ie HMM never triggered following
> BUG_ON inside sync_global_pgds():
> 
> if (!p4d_none(*p4d_ref) && !p4d_none(*p4d))
>    BUG_ON(p4d_page_vaddr(*p4d) != p4d_page_vaddr(*p4d_ref));
> 
> 
> It seems that Kirill 5 level page table changes play a role in this
> behavior change. I could not bisect because HMM is painfull to rebase
> for each bisection step so that is just my best guess.
> 
> 
> Am i missing something here ? Am i wrong in assuming that should sync
> pgd on vmemmap_free() ? If so anyone have a good guess on why i am now
> seeing the above BUG_ON ?

What would we gain by syncing pgd on free? Stale pgds are fine as long as
they are not referenced (use-after-free case). Syncing is addtional work.

See af2cf278ef4f ("x86/mm/hotplug: Don't remove PGD entries in remove_pagetable()")
and 5372e155a28f ("x86/mm: Drop unused argument 'removed' from sync_global_pgds()").

-- 
 Kirill A. Shutemov