linux-kernel - Re: [HMM 12/15] mm/migrate: new memory migration helper for use with device memory v4

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Date:   Tue, 11 Jul 2017 14:49:19 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Evgeny Baskakov <ebaskakov@...dia.com>
Cc:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Sherry Cheung <SCheung@...dia.com>,
        Subhash Gutti <sgutti@...dia.com>
Subject: Re: [HMM 12/15] mm/migrate: new memory migration helper for use with
 device memory v4

On Tue, Jul 11, 2017 at 11:42:20AM -0700, Evgeny Baskakov wrote:
> On 7/11/17 11:29 AM, Jerome Glisse wrote:
> > Can you test if attached patch helps ? I am having trouble reproducing
> > this
> > from inside a vm.
> > 
> > My theory is that 2 concurrent CPU page fault happens. First one manage to
> > start the migration back to system memory but second one see the migration
> > special entry and call migration_entry_wait() which increase page refcount
> > and this happen before first one check page refcount are ok for migration.
> > 
> > For regular migration such scenario is ok as the migration bails out and
> > because page is CPU accessible there is no need to kick again the migration
> > for other thread that CPU fault to migrate.
> > 
> > I am looking into how i can change migration_entry_wait() not to refcount
> > pages. Let me know if the attached patch helps.
> > 
> > Thank you
> > Jérôme
> 
> Hi Jerome,
> 
> Thanks for the update.
> 
> Unfortunately, the patch does not help. I just applied it and recompiled the
> kernel. Please find attached a new kernel log and an app log.
> 

What are the symptoms ? The program just stop making any progress and you
trigger a sysrequest to dump current states of each threads ? In this
log i don't see migration_entry_wait() anymore but it seems to be waiting
on page lock so there might be 2 issues here.

Jérôme