linux-kernel - Re: [HMM v13 06/18] mm/ZONE_DEVICE/unaddressable: add special swap for unaddressable

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Message-ID: <20161121124218.GF2392@redhat.com>
Date:   Mon, 21 Nov 2016 07:42:19 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [HMM v13 06/18] mm/ZONE_DEVICE/unaddressable: add special swap
 for unaddressable

On Mon, Nov 21, 2016 at 04:28:04PM +0530, Anshuman Khandual wrote:
> On 11/18/2016 11:48 PM, Jérôme Glisse wrote:
> > To allow use of device un-addressable memory inside a process add a
> > special swap type. Also add a new callback to handle page fault on
> > such entry.
> 
> IIUC this swap type is required only for the mirror cases and its
> not a requirement for migration. If it's required for mirroring
> purpose where we intercept each page fault, the commit message
> here should clearly elaborate on that more.

It is only require for un-addressable memory. The mirroring has nothing to do
with it. I will clarify commit message.

[...]

> > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> > index b6f03e9..d584c74 100644
> > --- a/include/linux/memremap.h
> > +++ b/include/linux/memremap.h
> > @@ -47,6 +47,11 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
> >   */
> >  struct dev_pagemap {
> >  	void (*free_devpage)(struct page *page, void *data);
> > +	int (*fault)(struct vm_area_struct *vma,
> > +		     unsigned long addr,
> > +		     struct page *page,
> > +		     unsigned flags,
> > +		     pmd_t *pmdp);
> 
> We are extending the dev_pagemap once again to accommodate device driver
> specific fault routines for these pages. Wondering if this extension and
> the new swap type should be in the same patch.

It make sense to have it in one single patch as i also change page fault code
path to deal with the new special swap entry and those make use of this new
callback.


> > +int device_entry_fault(struct vm_area_struct *vma,
> > +		       unsigned long addr,
> > +		       swp_entry_t entry,
> > +		       unsigned flags,
> > +		       pmd_t *pmdp)
> > +{
> > +	struct page *page = device_entry_to_page(entry);
> > +
> 
> A BUG_ON() if page->pgmap->fault has not been populated by the driver.
> 

Ok

Cheers,
Jérôme