linux-kernel - Re: + mm-revert-thp-make-madv_hugepage-check-for-mm-def

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Message-ID: <20140227163405.GA909@redhat.com>
Date:	Thu, 27 Feb 2014 17:34:05 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Alex Thorlton <athorlton@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, schwidefsky@...ibm.com,
	rientjes@...gle.com, riel@...hat.com, mingo@...nel.org,
	mgorman@...e.de, kirill shutemov <kirill.shutemov@...ux.intel.com>,
	heiko carstens <heiko.carstens@...ibm.com>, hannes@...xchg.org,
	gerald schaefer <gerald.schaefer@...ibm.com>,
	ebiederm@...ssion.com, aarcange@...hat.com
Subject: Re: + mm-revert-thp-make-madv_hugepage-check-for-mm-def_flags.patch
	added to -mm tree

On 02/26, Paolo Bonzini wrote:
>
> > +#ifdef CONFIG_S390
> > +		/*
> > +		 * MADV_HUGEPAGE is broken after s390_enable_sie(), qemu
> > +		 * blindly does madvise(MADV_HUGEPAGE) for for all kvm pages
> > +		 * and expects it must fail on s390. Avoid a possible SIGSEGV
> > +		 * until qemu is changed.
> > +		 */
> > +		if (mm_has_pgste(vma->vm_mm))
> > +			return -EINVAL;
> > +#endif
>
> The comment is not quite true.  QEMU doesn't expect that the madvise fails.

Yes, sorry. I didn't mean "it expects -EINVAL".

> It simply expects that the madvise doesn't cause SIGSEGVs or later
> malfunctioning, because (quoting tha man page) madvise "does not influence
> the semantics of the application".

Yes, I understand. But currently this means "MADV_HUGEPAGE should not
actually work", this is what I tried to say.

> There's nothing to fix in QEMU,

I was going to argue, but this is probably true too.

In short: I agree with any comment ;)

Oleg.

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