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

From: Jan Kara <jack@suse.cz>
To: Ira Weiny <ira.weiny@intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Peter Xu <peterx@redhat.com>,
	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>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Date: Fri, 18 Sep 2020 11:36:30 +0200	[thread overview]
Message-ID: <20200918093630.GC18920@quack2.suse.cz> (raw)
In-Reply-To: <20200917224857.GF2540965@iweiny-DESK2.sc.intel.com>

On Thu 17-09-20 15:48:57, Ira Weiny wrote:
> On Thu, Sep 17, 2020 at 07:09:00PM -0300, Jason Gunthorpe wrote:
> > On Thu, Sep 17, 2020 at 05:40:59PM -0400, Peter Xu wrote:
> > > On Thu, Sep 17, 2020 at 01:35:56PM -0700, Linus Torvalds wrote:
> > > > For that to happen, we'd need to have the vma flag so that we wouldn't
> > > > have any worry about non-pinners, but as you suggested, I think even
> > > > just a mm-wide counter - or flag - to deal with the fast-bup case is
> > > > likely perfectly sufficient.
> > > 
> > > Would mm_struct.pinned_vm suffice?
> > 
> > I think that could be a good long term goal
> > 
> > IIRC last time we dug into the locked_vm vs pinned_vm mess it didn't
> > get fixed. There is a mix of both kinds, as you saw, and some
> > resistance I don't clearly remember to changing it.
> > 
> > My advice for this -rc fix is to go with a single bit in the mm_struct
> > set on any call to pin_user_pages*
> > 
> > Then only users using pin_user_pages and forking are the only ones who
> > would ever do extra COW on fork. I think that is OK for -rc, this
> > workload should be rare due to the various historical issues. Anyhow,
> > a slow down regression is better than a it is broken regression.
> > 
> > This can be improved into a counter later. Due to the pinned_vm
> > accounting all call sites should have the mm_struct at unpin, but I
> > have a feeling it will take a alot of driver patches to sort it all
> > out.
> 
> Agreed.  The HFI1 driver for example increments/decrements pinned_vm on it's
> own.  I've kind of always felt dirty for that...
> 
> I think long term it would be better to move this accounting to
> pin_user_pages() but Jason is correct that I think that is going to be too
> complex for an rc.

Moving accounting to pin_user_pages() won't be simple because you need to
unaccount on unpin. And that can happen from a different task context (e.g.
IRQ handler for direct IO) so we won't have proper mm_struct available.

> Could we move pinned_vm out of the drivers/rdma subsystem?

I'd love to because IMO it's a mess...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR