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

[Kirill A. Shutemov]

On Mon, May 22, 2017 at 10:11:51AM -0400, Jerome Glisse wrote:
> On Mon, May 22, 2017 at 04:12:15PM +0300, Kirill A. Shutemov wrote:
> > 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.
> 
> Well then how do i avoid the BUG_ON above ? Because the init_mm pgd is
> clear but none of the stall entry in any other mm. So if i unplug memory
> and replug memory at exact same address it tries to allocate new p4d/pud
> for struct page area and then when sync_global_pgds() is call it goes
> over the list of pgd and BUG_ON() :
> 
> if (!p4d_none(*p4d_ref) && !p4d_none(*p4d))
>     BUG_ON(p4d_page_vaddr(*p4d) != p4d_page_vaddr(*p4d_ref));
> 
> 
> So to me either above check need to go and we should overwritte pgd no
> matter what or we should restore previous behavior. I don't mind either
> one.

I would prefer to drop the BUG_ON.

Ingo?

-- 
 Kirill A. Shutemov