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Date: Mon, 22 May 2017 11:12:25 +0300
From: Mike Rapoport <rppt@linux.vnet.ibm.com>
To: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>, linux-mm <linux-mm@kvack.org>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE
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On Mon, May 22, 2017 at 12:56:45PM +0530, Anshuman Khandual wrote:
> On 05/22/2017 11:42 AM, Mike Rapoport wrote:
> > Currently applications can explicitly enable or disable THP for a memory
> > region using MADV_HUGEPAGE or MADV_NOHUGEPAGE. However, once either of
> > these advises is used, the region will always have
> > VM_HUGEPAGE/VM_NOHUGEPAGE flag set in vma->vm_flags.
> > The MADV_CLR_HUGEPAGE resets both these flags and allows managing THP in
> > the region according to system-wide settings.
> 
> Invoking madvise() for the first time with either MADV_HUGEPAGE or
> MADV_NOHUGEPAGE on the buffer will unsubscribe it from the system
> wide behavior for good. I am not saying we should not have a way
> to put it back into system wide mode but are there no other functions
> through madvise() or any other interface which may have the same
> situation.

There are madvise() interfaces that set or clear some of the vma->vm_flags,
e.g MADV_*FORK or MADV_*DUMP. The difference with MADV_*HUGEPAGE is that
it is using two flags and with current madvise() interface either of them
has to be set, but there is no interface to clear them both.

--
Sincerely yours,
Mike.