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

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Date:   Mon, 22 May 2017 10:11:51 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] x86/mm: pgds getting out of sync after memory hot remove

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.

Cheers,
Jérôme