Re: [PATCH v14 4/5] mm: support reporting free page blocks

[Wei Wang]

On 08/18/2017 09:46 PM, Michal Hocko wrote:
> On Thu 17-08-17 11:26:55, Wei Wang wrote:
> > This patch adds support to walk through the free page blocks in the
> > system and report them via a callback function. Some page blocks may
> > leave the free list after zone->lock is released, so it is the caller's
> > responsibility to either detect or prevent the use of such pages.
> This could see more details to be honest. Especially the usecase you are
> going to use this for. This will help us to understand the motivation
> in future when the current user might be gone a new ones largely diverge
> into a different usage. This wouldn't be the first time I have seen
> something like that.

OK, I will more details here about how it's used to accelerate live 
migration.

> > Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> > Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > ---
> >   include/linux/mm.h |  6 ++++++
> >   mm/page_alloc.c    | 44 ++++++++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 50 insertions(+)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 46b9ac5..cd29b9f 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1835,6 +1835,12 @@ extern void free_area_init_node(int nid, unsigned long * zones_size,
> >   		unsigned long zone_start_pfn, unsigned long *zholes_size);
> >   extern void free_initmem(void);
> >   
> > +extern void walk_free_mem_block(void *opaque1,
> > +				unsigned int min_order,
> > +				void (*visit)(void *opaque2,
> > +					      unsigned long pfn,
> > +					      unsigned long nr_pages));
> > +
> >   /*
> >    * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK)
> >    * into the buddy system. The freed pages will be poisoned with pattern
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 6d00f74..a721a35 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4762,6 +4762,50 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
> >   	show_swap_cache_info();
> >   }
> >   
> > +/**
> > + * walk_free_mem_block - Walk through the free page blocks in the system
> > + * @opaque1: the context passed from the caller
> > + * @min_order: the minimum order of free lists to check
> > + * @visit: the callback function given by the caller
> The original suggestion for using visit was motivated by a visit design
> pattern but I can see how this can be confusing. Maybe a more explicit
> name wold be better. What about report_free_range.


I'm afraid that name would be too long to fit in nicely.
How about simply naming it "report"?


> > + *
> > + * The function is used to walk through the free page blocks in the system,
> > + * and each free page block is reported to the caller via the @visit callback.
> > + * Please note:
> > + * 1) The function is used to report hints of free pages, so the caller should
> > + * not use those reported pages after the callback returns.
> > + * 2) The callback is invoked with the zone->lock being held, so it should not
> > + * block and should finish as soon as possible.
> I think that the explicit note about zone->lock is not really need. This
> can change in future and I would even bet that somebody might rely on
> the lock being held for some purpose and silently get broken with the
> change. Instead I would much rather see something like the following:
> "
> Please note that there are no locking guarantees for the callback

Just a little confused with this one:

The callback is invoked within zone->lock, why would we claim it "no
locking guarantees for the callback"?

> and
> that the reported pfn range might be freed or disappear after the
> callback returns so the caller has to be very careful how it is used.
>
> The callback itself must not sleep or perform any operations which would
> require any memory allocations directly (not even GFP_NOWAIT/GFP_ATOMIC)
> or via any lock dependency. It is generally advisable to implement
> the callback as simple as possible and defer any heavy lifting to a
> different context.
>
> There is no guarantee that each free range will be reported only once
> during one walk_free_mem_block invocation.
>
> pfn_to_page on the given range is strongly discouraged and if there is
> an absolute need for that make sure to contact MM people to discuss
> potential problems.
>
> The function itself might sleep so it cannot be called from atomic
> contexts.
>
> In general low orders tend to be very volatile and so it makes more
> sense to query larger ones for various optimizations which like
> ballooning etc... This will reduce the overhead as well.
> "

I think it looks quite comprehensive. Thanks.


Best,
Wei