Re: [PATCHv3 1/2] mm/memblock: extend the limit inferior of bottom-up after parsing hotplug attr

From: Mike Rapoport
Date: Wed Jan 02 2019 - 04:28:07 EST

On Wed, Jan 02, 2019 at 02:47:34PM +0800, Pingfan Liu wrote:
> On Mon, Dec 31, 2018 at 4:40 PM Mike Rapoport <rppt@xxxxxxxxxxxxx> wrote:
> >
> > On Fri, Dec 28, 2018 at 11:00:01AM +0800, Pingfan Liu wrote:
> > > The bottom-up allocation style is introduced to cope with movable_node,
> > > where the limit inferior of allocation starts from kernel's end, due to
> > > lack of knowledge of memory hotplug info at this early time. But if later,
> > > hotplug info has been got, the limit inferior can be extend to 0.
> > > 'kexec -c' prefers to reuse this style to alloc mem at lower address,
> > > since if the reserved region is beyond 4G, then it requires extra mem
> > > (default is 16M) for swiotlb.
> >
> > I fail to understand why the availability of memory hotplug information
> > would allow to extend the lower limit of bottom-up memblock allocations
> > below the kernel. The memory in the physical range [0, kernel_start) can be
> > allocated as soon as the kernel memory is reserved.
> >
> Yes, the [0, kernel_start) can be allocated at this time by some func
> e.g. memblock_reserve(). But there is trick. For the func like
> memblock_find_in_range(), this is hotplug attr checking ,,it will
> check the hotmovable attr in __next_mem_range()
> {
> if (movable_node_is_enabled() && memblock_is_hotpluggable(m))
> continue
> }. So the movable memory can be safely skipped.

I still don't see the connection between allocating memory below
kernel_start and the hotplug info.

The check for 'end > kernel_end' in

if (memblock_bottom_up() && end > kernel_end)

does not protect against allocation in a hotplugable area.
If memblock_find_in_range() is called before hotplug info is parsed it can
return a range in a hotplugable area.

The point I'd like to clarify is why allocating memory in the range [0,
kernel_start) cannot be done before hotplug info is available and why it is
safe to allocate that memory afterwards?

> Thanks for your kindly review.
>
> Regards,
> Pingfan
>
> > The extents of the memory node hosting the kernel image can be used to
> > limit memblok allocations from that particular node, even in top-down mode.
> >
> > > Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> > > Cc: Tang Chen <tangchen@xxxxxxxxxxxxxx>
> > > Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
> > > Cc: Len Brown <lenb@xxxxxxxxxx>
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > > Cc: Yaowei Bai <baiyaowei@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> > > Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> > > Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > > Cc: Daniel Vacek <neelx@xxxxxxxxxx>
> > > Cc: Mathieu Malaterre <malat@xxxxxxxxxx>
> > > Cc: Stefan Agner <stefan@xxxxxxxx>
> > > Cc: Dave Young <dyoung@xxxxxxxxxx>
> > > Cc: Baoquan He <bhe@xxxxxxxxxx>
> > > Cc: yinghai@xxxxxxxxxx,
> > > Cc: vgoyal@xxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > ---
> > > drivers/acpi/numa.c | 4 ++++
> > > include/linux/memblock.h | 1 +
> > > mm/memblock.c | 58 +++++++++++++++++++++++++++++-------------------
> > > 3 files changed, 40 insertions(+), 23 deletions(-)
> > >
> > > diff --git a/drivers/acpi/numa.c b/drivers/acpi/numa.c
> > > index 2746994..3eea4e4 100644
> > > --- a/drivers/acpi/numa.c
> > > +++ b/drivers/acpi/numa.c
> > > @@ -462,6 +462,10 @@ int __init acpi_numa_init(void)
> > >
> > > cnt = acpi_table_parse_srat(ACPI_SRAT_TYPE_MEMORY_AFFINITY,
> > > acpi_parse_memory_affinity, 0);
> > > +
> > > +#if defined(CONFIG_X86) || defined(CONFIG_ARM64)
> > > + mark_mem_hotplug_parsed();
> > > +#endif
> > > }
> > >
> > > /* SLIT: System Locality Information Table */
> > > diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> > > index aee299a..d89ed9e 100644
> > > --- a/include/linux/memblock.h
> > > +++ b/include/linux/memblock.h
> > > @@ -125,6 +125,7 @@ int memblock_reserve(phys_addr_t base, phys_addr_t size);
> > > void memblock_trim_memory(phys_addr_t align);
> > > bool memblock_overlaps_region(struct memblock_type *type,
> > > phys_addr_t base, phys_addr_t size);
> > > +void mark_mem_hotplug_parsed(void);
> > > int memblock_mark_hotplug(phys_addr_t base, phys_addr_t size);
> > > int memblock_clear_hotplug(phys_addr_t base, phys_addr_t size);
> > > int memblock_mark_mirror(phys_addr_t base, phys_addr_t size);
> > > diff --git a/mm/memblock.c b/mm/memblock.c
> > > index 81ae63c..a3f5e46 100644
> > > --- a/mm/memblock.c
> > > +++ b/mm/memblock.c
> > > @@ -231,6 +231,12 @@ __memblock_find_range_top_down(phys_addr_t start, phys_addr_t end,
> > > return 0;
> > > }
> > >
> > > +static bool mem_hotmovable_parsed __initdata_memblock;
> > > +void __init_memblock mark_mem_hotplug_parsed(void)
> > > +{
> > > + mem_hotmovable_parsed = true;
> > > +}
> > > +
> > > /**
> > > * memblock_find_in_range_node - find free area in given range and node
> > > * @size: size of free area to find
> > > @@ -259,7 +265,7 @@ phys_addr_t __init_memblock memblock_find_in_range_node(phys_addr_t size,
> > > phys_addr_t end, int nid,
> > > enum memblock_flags flags)
> > > {
> > > - phys_addr_t kernel_end, ret;
> > > + phys_addr_t kernel_end, ret = 0;
> > >
> > > /* pump up @end */
> > > if (end == MEMBLOCK_ALLOC_ACCESSIBLE)
> > > @@ -270,34 +276,40 @@ phys_addr_t __init_memblock memblock_find_in_range_node(phys_addr_t size,
> > > end = max(start, end);
> > > kernel_end = __pa_symbol(_end);
> > >
> > > - /*
> > > - * try bottom-up allocation only when bottom-up mode
> > > - * is set and @end is above the kernel image.
> > > - */
> > > - if (memblock_bottom_up() && end > kernel_end) {
> > > - phys_addr_t bottom_up_start;
> > > + if (memblock_bottom_up()) {
> > > + phys_addr_t bottom_up_start = start;
> > >
> > > - /* make sure we will allocate above the kernel */
> > > - bottom_up_start = max(start, kernel_end);
> > > -
> > > - /* ok, try bottom-up allocation first */
> > > - ret = __memblock_find_range_bottom_up(bottom_up_start, end,
> > > - size, align, nid, flags);
> > > - if (ret)
> > > + if (mem_hotmovable_parsed) {
> > > + ret = __memblock_find_range_bottom_up(
> > > + bottom_up_start, end, size, align, nid,
> > > + flags);
> > > return ret;
> > >
> > > /*
> > > - * we always limit bottom-up allocation above the kernel,
> > > - * but top-down allocation doesn't have the limit, so
> > > - * retrying top-down allocation may succeed when bottom-up
> > > - * allocation failed.
> > > - *
> > > - * bottom-up allocation is expected to be fail very rarely,
> > > - * so we use WARN_ONCE() here to see the stack trace if
> > > - * fail happens.
> > > + * if mem hotplug info is not parsed yet, try bottom-up
> > > + * allocation with @end above the kernel image.
> > > */
> > > - WARN_ONCE(IS_ENABLED(CONFIG_MEMORY_HOTREMOVE),
> > > + } else if (!mem_hotmovable_parsed && end > kernel_end) {
> > > + /* make sure we will allocate above the kernel */
> > > + bottom_up_start = max(start, kernel_end);
> > > + ret = __memblock_find_range_bottom_up(
> > > + bottom_up_start, end, size, align, nid,
> > > + flags);
> > > + if (ret)
> > > + return ret;
> > > + /*
> > > + * we always limit bottom-up allocation above the
> > > + * kernel, but top-down allocation doesn't have
> > > + * the limit, so retrying top-down allocation may
> > > + * succeed when bottom-up allocation failed.
> > > + *
> > > + * bottom-up allocation is expected to be fail
> > > + * very rarely, so we use WARN_ONCE() here to see
> > > + * the stack trace if fail happens.
> > > + */
> > > + WARN_ONCE(IS_ENABLED(CONFIG_MEMORY_HOTREMOVE),
> > > "memblock: bottom-up allocation failed, memory hotremove may be affected\n");
> > > + }
> > > }
> > >
> > > return __memblock_find_range_top_down(start, end, size, align, nid,
> > > --
> > > 2.7.4
> > >
> >
> > --
> > Sincerely yours,
> > Mike.
> >
>

--
Sincerely yours,
Mike.