linux-kernel - Re: [PATCHv2] mm: Account pud page tables

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Message-ID: <20170925135305.ydeeyapav2s36ifj@dhcp22.suse.cz>
Date:   Mon, 25 Sep 2017 15:53:05 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCHv2] mm: Account pud page tables

On Mon 25-09-17 16:07:15, Kirill A. Shutemov wrote:
> On Mon, Sep 25, 2017 at 01:54:30PM +0200, Michal Hocko wrote:
> > On Mon 25-09-17 10:39:13, Kirill A. Shutemov wrote:
> > > On machine with 5-level paging support a process can allocate
> > > significant amount of memory and stay unnoticed by oom-killer and
> > > memory cgroup. The trick is to allocate a lot of PUD page tables.
> > > We don't account PUD page tables, only PMD and PTE.
> > > 
> > > We already addressed the same issue for PMD page tables, see
> > > dc6c9a35b66b ("mm: account pmd page tables to the process").
> > > Introduction 5-level paging bring the same issue for PUD page tables.
> > > 
> > > The patch expands accounting to PUD level.
> > 
> > OK, we definitely need this or something like that but I really do not
> > like how much code we actually need for each pte level for accounting.
> > Do we really need to distinguish each level? Do we have any arch that
> > would use a different number of pages to back pte/pmd/pud?
> 
> Looks like we actually do. At least on mips. See PMD_ORDER/PUD_ORDER.

Hmm, but then oom_badness does consider them a single page which is
wrong. I haven't checked other users. Anyway even if we've had different
sizes why cannot we deal with this in callers. They know which level of
page table they allocate/free, no?
-- 
Michal Hocko
SUSE Labs