Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

From: Peter Xu <peterx@redhat.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: 20200918161902.GX8409@ziepe.ca,
	Linus Torvalds <torvalds@linux-foundation.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Leon Romanovsky <leonro@nvidia.com>,
	Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"Maya B . Gokhale" <gokhale2@llnl.gov>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Marty Mcfadden <mcfadden8@llnl.gov>,
	Kirill Shutemov <kirill@shutemov.name>,
	Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
	Jan Kara <jack@suse.cz>, Kirill Tkhai <ktkhai@virtuozzo.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Zhang, Wei" <wzam@amazon.com>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Date: Tue, 2 Feb 2021 12:05:36 -0500	[thread overview]
Message-ID: <20210202170536.GE6468@xz-x1> (raw)
In-Reply-To: <20210202164420.GL4718@ziepe.ca>

On Tue, Feb 02, 2021 at 12:44:20PM -0400, Jason Gunthorpe wrote:
> On Tue, Feb 02, 2021 at 11:31:27AM -0500, Peter Xu wrote:
> > On Tue, Feb 02, 2021 at 04:40:33PM +0200, Gal Pressman wrote:
> > > Hi Peter & Jason,
> > 
> > Hi, Gal, Jason,
> > 
> > > 
> > > It seems the hugetlb part was overlooked?
> > > We're testing if the RDMA fork MADV_DONTFORK stuff can be removed on appropriate
> > > kernels, but our tests still fail due to lacking explicit huge pages support [1].
> > 
> > I didn't think it high priority only because I think most hugetlbfs users
> > should be using it shared, but maybe I'm wrong..  Then it got lost indeed.
> 
> It turns out people are doing this:
> 
> mmap(NULL, SEND_BUFF_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB, -1, 0)
> 
> Which makes some sense...

Yes, thanks Jason.  Though my understanding is that hugetlb pages are normally
reserved in production, used with careful pre-provisioning on which app would
consume how much (either 2M or 1G).  Such an app (especially if it forks
randomly) could actually easily exaust the huge page pool.

> 
> Gal, you could also MADV_DONTFORK this range if you are explicitly
> allocating them via special mmap.

Yeah I wanted to mention this one too but I just forgot when reply: the issue
thread previously pasted smells like some people would like to drop
MADV_DONTFORK, but if it's able to still be applied I don't know why not.. It
should still be better than depending on the coming patch imho - it marks the
region as "not necessary for the fork" then we skip the whole hugetlbfs chunk.
It should at least be more efficient if applicable.

Thanks,

-- 
Peter Xu