Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

From: Vlastimil Babka <vbabka@suse.cz>
To: Andrea Arcangeli <aarcange@redhat.com>, Michal Hocko <mhocko@kernel.org>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Andrew Morton <akpm@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Pavel Emelyanov <xemul@virtuozzo.com>,
	linux-mm <linux-mm@kvack.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE
Date: Wed, 31 May 2017 08:30:08 +0200	[thread overview]
Message-ID: <e371b76b-d091-72d0-16c3-5227820595f0@suse.cz> (raw)
In-Reply-To: <20170530160610.GC8412@redhat.com>

On 05/30/2017 06:06 PM, Andrea Arcangeli wrote:
> 
> I'm not sure if it should be considered a bug, the prctl is intended
> to use normally by wrappers so it looks optimal as implemented this
> way: affecting future vmas only, which will all be created after
> execve executed by the wrapper.
> 
> What's the point of messing with the prctl so it mangles over the
> wrapper process own vmas before exec? Messing with those vmas is pure
> wasted CPUs for the wrapper use case which is what the prctl was
> created for.
> 
> Furthermore there would be the risk a program that uses the prctl not
> as a wrapper and then calls the prctl to clear VM_NOHUGEPAGE from
> def_flags assuming the current kABI. The program could assume those
> vmas that were instantiated before disabling the prctl are still with
> VM_NOHUGEPAGE set (they would not after the change you propose).
> 
> Adding a scan of all vmas to PR_SET_THP_DISABLE to clear VM_NOHUGEPAGE
> on existing vmas looks more complex too and less finegrined so
> probably more complex for userland to manage

I would expect the prctl wouldn't iterate all vma's, nor would it modify
def_flags anymore. It would just set a flag somewhere in mm struct that
would be considered in addition to the per-vma flags when deciding
whether to use THP. We could consider whether MADV_HUGEPAGE should be
able to override the prctl or not.

> but ignoring all above
> considerations it would be a functional alternative for CRIU's
> needs. However if you didn't like the complexity of the new madvise
> which is functionally a one-liner equivalent to MADV_NORMAL, I
> wouldn't expect you to prefer to make the prctl even more complex with
> a loop over all vmas that despite being fairly simple it'll still be
> more than a trivial one liner.
> 
> Thanks,
> Andrea
> 
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