linux-kernel - Re: [PATCH] mm: introduce MADV_CLR

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Date:   Thu, 1 Jun 2017 09:58:39 +0300
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Michal Hocko <mhocko@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

On Wed, May 31, 2017 at 04:18:09PM +0200, Andrea Arcangeli wrote:
> On Wed, May 31, 2017 at 03:39:22PM +0300, Mike Rapoport wrote:
> > For the CRIU usecase, disabling THP for a while and re-enabling it
> > back will do the trick, provided VMAs flags are not affected, like
> > in the patch you've sent.  Moreover, we may even get away with
> 
> Are you going to check uname -r to know when the kABI changed in your
> favor (so CRIU cannot ever work with enterprise backports unless you
> expand the uname -r coverage), or how do you know the patch is
> applied?

CRIU does not rely on uname -r. We have code that checks what kernel
features we can actually use. For instance, we use UFFDIO_API to see if we
can do post-copy at all.
 
> Optimistically assuming people is going to run new CRIU code only on
> new kernels looks very risky, it would leads to silent random memory
> corruption, so I doubt you can get away without a uname -r check.
> 
> This is fairly simple change too, its main cons is that it adds a
> branch to the page fault fast path, the old behavior of the prctl and
> the new madvise were both zero cost.
> 
> Still if the prctl is preferred despite the added branch, to avoid
> uname -r clashes, to me it sounds better to add a new prctl ID and
> keep the old one too. The old one could be implemented the same way as
> the new one if you want to save a few bytes of .text. But the old one
> should probably do a printk_once to print a deprecation warning so the
> old ID with weaker (zero runtime cost) semantics can be removed later.
>