Re: [RFC PATCH] mm/page_reporting: Adjust threshold according to MAX_ORDER

[Gavin Shan]

On 6/2/21 10:03 AM, Andrew Morton wrote:
> On Tue,  1 Jun 2021 11:33:19 +0800 Gavin Shan <gshan@xxxxxxxxxx> wrote:
>
> > The PAGE_REPORTING_MIN_ORDER is equal to @pageblock_order, taken as
> > minimal order (threshold) to trigger page reporting. The page reporting
> > is never triggered with the following configurations and settings on
> > aarch64. In the particular scenario, the page reporting won't be triggered
> > until the largest (2 ^ (MAX_ORDER-1)) free area is achieved from the
> > page freeing. The condition is very hard, or even impossible to be met.
> >
> >    CONFIG_ARM64_PAGE_SHIFT:              16
> >    CONFIG_HUGETLB_PAGE:                  Y
> >    CONFIG_HUGETLB_PAGE_SIZE_VARIABLE:    N
> >    pageblock_order:                      13
> >    CONFIG_FORCE_MAX_ZONEORDER:           14
> >    MAX_ORDER:                            14
> >
> > The issue can be reproduced in VM, running kernel with above configurations
> > and settings. The 'memhog' is used inside the VM to access 512MB anonymous
> > area. The QEMU's RSS doesn't drop accordingly after 'memhog' exits.
> >
> >    /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64          \
> >    -accel kvm -machine virt,gic-version=host                        \
> >    -cpu host -smp 8,sockets=2,cores=4,threads=1 -m 4096M,maxmem=64G \
> >    -object memory-backend-ram,id=mem0,size=2048M                    \
> >    -object memory-backend-ram,id=mem1,size=2048M                    \
> >    -numa node,nodeid=0,cpus=0-3,memdev=mem0                         \
> >    -numa node,nodeid=1,cpus=4-7,memdev=mem1                         \
> >      :                                                              \
> >    -device virtio-balloon-pci,id=balloon0,free-page-reporting=yes
> >
> > This tries to fix the issue by adjusting the threshold to the smaller value
> > of @pageblock_order and (MAX_ORDER/2). With this applied, the QEMU's RSS
> > drops after 'memhog' exits.
>
> Sounds promising, but...
>
> > --- a/mm/page_reporting.h
> > +++ b/mm/page_reporting.h
> > @@ -10,9 +10,10 @@
> >   #include <linux/pgtable.h>
> >   #include <linux/scatterlist.h>
> >   
> > -#define PAGE_REPORTING_MIN_ORDER	pageblock_order
> > -
> >   #ifdef CONFIG_PAGE_REPORTING
> > +#define PAGE_REPORTING_MIN_ORDER	\
> > +	min_t(unsigned int, pageblock_order, (MAX_ORDER / 2))
> > +
> >   DECLARE_STATIC_KEY_FALSE(page_reporting_enabled);
> >   void __page_reporting_notify(void);
>
> Could you please redo this as a regular old static function in
> page_reporting.c?  Bonus points for commenting its design ;)

Thanks for your time to review, Andrew. Sorry for the delay and late
reply. Yes, I will do if needed after the discussion with David is
finalized.

Thanks,
Gavin