Re: [PATCH 1/2] mm/migrate: optimize migrate_vma_setup() for holes

[Ralph Campbell]

On 7/9/20 11:35 PM, Bharata B Rao wrote:
> On Thu, Jul 09, 2020 at 09:57:10AM -0700, Ralph Campbell wrote:
> > When migrating system memory to device private memory, if the source
> > address range is a valid VMA range and there is no memory or a zero page,
> > the source PFN array is marked as valid but with no PFN. This lets the
> > device driver allocate private memory and clear it, then insert the new
> > device private struct page into the CPU's page tables when
> > migrate_vma_pages() is called. migrate_vma_pages() only inserts the
> > new page if the VMA is an anonymous range. There is no point in telling
> > the device driver to allocate device private memory and then not migrate
> > the page. Instead, mark the source PFN array entries as not migrating to
> > avoid this overhead.
> >
> > Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> > ---
> >   mm/migrate.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index b0125c082549..8aa434691577 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -2204,9 +2204,13 @@ static int migrate_vma_collect_hole(unsigned long start,
> >   {
> >   	struct migrate_vma *migrate = walk->private;
> >   	unsigned long addr;
> > +	unsigned long flags;
> > +
> > +	/* Only allow populating anonymous memory. */
> > +	flags = vma_is_anonymous(walk->vma) ? MIGRATE_PFN_MIGRATE : 0;
> >   
> >   	for (addr = start; addr < end; addr += PAGE_SIZE) {
> > -		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
> > +		migrate->src[migrate->npages] = flags;
>
> I see a few other such cases where we directly populate MIGRATE_PFN_MIGRATE
> w/o a pfn in migrate_vma_collect_pmd() and wonder why the vma_is_anonymous()
> check can't help there as well?
>
> 1. pte_none() check in migrate_vma_collect_pmd()
> 2. is_zero_pfn() check in migrate_vma_collect_pmd()
>
> Regards,
> Bharata.

For case 1, this seems like a useful addition.
For case 2, the zero page is only inserted if the VMA is marked read-only and
anonymous so I don't think the check is needed.
I'll post a v2 with the change.

Thanks for the suggestions!