Re: [RFC PATCH] mm: align anon mmap for THP

From: "Harrosh, Boaz" <Boaz.Harrosh@netapp.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Mike Kravetz <mike.kravetz@oracle.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Hugh Dickins <hughd@google.com>, Michal Hocko <mhocko@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Matthew Wilcox <willy@infradead.org>,
	Toshi Kani <toshi.kani@hpe.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH] mm: align anon mmap for THP
Date: Mon, 14 Jan 2019 16:29:29 +0000	[thread overview]
Message-ID: <MWHPR06MB2896ACD09C21B2939959C8A8EE800@MWHPR06MB2896.namprd06.prod.outlook.com> (raw)
In-Reply-To: <20190114135001.w2wpql53zitellus@kshutemo-mobl1>

 Kirill A. Shutemov <kirill@shutemov.name> wrote:
> On Fri, Jan 11, 2019 at 03:28:37PM -0800, Mike Kravetz wrote:
>> Ok, I just wanted to ask the question.  I've seen application code doing
>> the 'mmap sufficiently large area' then unmap to get desired alignment
>> trick.  Was wondering if there was something we could do to help.
>
> Application may want to get aligned allocation for different reasons.
> It should be okay for userspace to ask for size + (alignment - PAGE_SIZE)
> and then round up the address to get the alignment. We basically do the
> same on kernel side.
>

This is what we do and will need to keep doing for old Kernels.
But it is a pity that those holes can not be reused for small maps, and most important
that we cannot have "mapping holes" around the mapping that catch memory
overruns

> For THP, I believe, kernel already does The Right Thing™ for most users.
> User still may want to get speific range as THP (to avoid false sharing or
> something).

I'm an OK Kernel programmer.  But I was not able to create a HugePage mapping
against /dev/shm/ in a reliable way. I think it only worked on Fedora 28/29
but not on any other distro/version. (MMAP_HUGE)

We run with our own compiled Kernel on various distros, THP is configured
in but mmap against /dev/shm/ never gives me Huge pages. Does it only
work with unanimous mmap ? (I think it is mount dependent which is not
in the application control)

Just a rant. One day I will figure this out. Meanwhile I do this ugly
user mode aligns the pointers, and try to sleep at night ...

> But still I believe userspace has all required tools to get it
> right.
>

I still wish that if I ask for an mmap size aligned on 2M that I would automatically
get a 2M pointer. I don't see how the system can benefit from having both ends
of the VMA cross Huge page boundary.

> --
> Kirill A. Shutemov

Thanks
Boaz