linux-kernel - Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user

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Date:   Mon, 2 Jul 2018 08:52:51 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Jan Kara <jack@...e.cz>
Cc:     John Hubbard <jhubbard@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Michal Hocko <mhocko@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <john.hubbard@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()

On Thu, Jun 28, 2018 at 11:17:43AM +0200, Jan Kara wrote:
> On Wed 27-06-18 19:42:01, John Hubbard wrote:
> > On 06/27/2018 10:02 AM, Jan Kara wrote:
> > > On Wed 27-06-18 08:57:18, Jason Gunthorpe wrote:
> > >> On Wed, Jun 27, 2018 at 02:42:55PM +0200, Jan Kara wrote:
> > >>> On Wed 27-06-18 13:59:27, Michal Hocko wrote:
> > >>>> On Wed 27-06-18 13:53:49, Jan Kara wrote:
> > >>>>> On Wed 27-06-18 13:32:21, Michal Hocko wrote:
> > >>>> [...]
> > >>>>>> Appart from that, do we really care about 32b here? Big DIO, IB users
> > >>>>>> seem to be 64b only AFAIU.
> > >>>>>
> > >>>>> IMO it is a bad habit to leave unpriviledged-user-triggerable oops in the
> > >>>>> kernel even for uncommon platforms...
> > >>>>
> > >>>> Absolutely agreed! I didn't mean to keep the blow up for 32b. I just
> > >>>> wanted to say that we can stay with a simple solution for 32b. I thought
> > >>>> the g-u-p-longterm has plugged the most obvious breakage already. But
> > >>>> maybe I just misunderstood.
> > >>>
> > >>> Most yes, but if you try hard enough, you can still trigger the oops e.g.
> > >>> with appropriately set up direct IO when racing with writeback / reclaim.
> > >>
> > >> gup longterm is only different from normal gup if you have DAX and few
> > >> people do, which really means it doesn't help at all.. AFAIK??
> > >
> > > Right, what I wrote works only for DAX. For non-DAX situation g-u-p
> > > longterm does not currently help at all. Sorry for confusion.
> > >
> >
> > OK, I've got an early version of this up and running, reusing the page->lru
> > fields. I'll clean it up and do some heavier testing, and post as a PATCH v2.
>
> Cool.
>
> > One question though: I'm still vague on the best actions to take in the
> > following functions:
> >
> >     page_mkclean_one
> >     try_to_unmap_one
> >
> > At the moment, they are both just doing an evil little early-out:
> >
> > 	if (PageDmaPinned(page))
> > 		return false;
> >
> > ...but we talked about maybe waiting for the condition to clear, instead?
> > Thoughts?
>
> What needs to happen in page_mkclean() depends on the caller. Most of the
> callers really need to be sure the page is write-protected once
> page_mkclean() returns. Those are:
>
>   pagecache_isize_extended()
>   fb_deferred_io_work()
>   clear_page_dirty_for_io() if called for data-integrity writeback - which
>     is currently known only in its caller (e.g. write_cache_pages()) where
>     it can be determined as wbc->sync_mode == WB_SYNC_ALL. Getting this
>     information into page_mkclean() will require some plumbing and
>     clear_page_dirty_for_io() has some 50 callers but it's doable.
>
> clear_page_dirty_for_io() for cleaning writeback (wbc->sync_mode !=
> WB_SYNC_ALL) can just skip pinned pages and we probably need to do that as
> otherwise memory cleaning would get stuck on pinned pages until RDMA
> drivers release its pins.

Sorry for naive question, but won't it create too much dirty pages
so writeback will be called "non-stop" to rebalance watermarks without
ability to progress?

Thanks

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