On Mon, Jun 20, 2022 at 09:47:22AM +0200, David Hildenbrand wrote:
> On 20.06.22 09:22, Muchun Song wrote:
> > On Sun, Jun 19, 2022 at 09:38:51PM +0800, Muchun Song wrote:
> >> For now, the feature of hugetlb_free_vmemmap is not compatible with the
> >> feature of memory_hotplug.memmap_on_memory, and hugetlb_free_vmemmap
> >> takes precedence over memory_hotplug.memmap_on_memory. However, someone
> >> wants to make memory_hotplug.memmap_on_memory takes precedence over
> >> hugetlb_free_vmemmap since memmap_on_memory makes it more likely to
> >> succeed memory hotplug in close-to-OOM situations. So the decision
> >> of making hugetlb_free_vmemmap take precedence is not wise and elegant.
> >> The proper approach is to have hugetlb_vmemmap.c do the check whether
> >> the section which the HugeTLB pages belong to can be optimized. If
> >> the section's vmemmap pages are allocated from the added memory block
> >> itself, hugetlb_free_vmemmap should refuse to optimize the vmemmap,
> >> otherwise, do the optimization. Then both kernel parameters are
> >> compatible. So this patch introduces VmemmapSelfHosted to mask any
> >> non-optimizable vmemmap pages. The hugetlb_vmemmap can use this flag
> >> to detect if a vmemmap page can be optimized.
> >>
> >> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >> Co-developed-by: Oscar Salvador <osalvador@xxxxxxx>
> >> Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> >> ---
> >> Documentation/admin-guide/kernel-parameters.txt | 22 +++++------
> >> Documentation/admin-guide/sysctl/vm.rst | 5 +--
> >> include/linux/memory_hotplug.h | 9 -----
> >> include/linux/page-flags.h | 11 ++++++
> >> mm/hugetlb_vmemmap.c | 52 +++++++++++++++++++++----
> >> mm/memory_hotplug.c | 27 ++++++-------
> >> 6 files changed, 79 insertions(+), 47 deletions(-)
> >>
> >> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> >> index 8090130b544b..d740e2ed0e61 100644
> >> --- a/Documentation/admin-guide/kernel-parameters.txt
> >> +++ b/Documentation/admin-guide/kernel-parameters.txt
> >> @@ -1722,9 +1722,11 @@
> >> Built with CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON=y,
> >> the default is on.
> >>
> >> - This is not compatible with memory_hotplug.memmap_on_memory.
> >> - If both parameters are enabled, hugetlb_free_vmemmap takes
> >> - precedence over memory_hotplug.memmap_on_memory.
> >> + Note that the vmemmap pages may be allocated from the added
> >> + memory block itself when memory_hotplug.memmap_on_memory is
> >> + enabled, those vmemmap pages cannot be optimized even if this
> >> + feature is enabled. Other vmemmap pages not allocated from
> >> + the added memory block itself do not be affected.
> >>
> >> hung_task_panic=
> >> [KNL] Should the hung task detector generate panics.
> >> @@ -3069,10 +3071,12 @@
> >> [KNL,X86,ARM] Boolean flag to enable this feature.
> >> Format: {on | off (default)}
> >> When enabled, runtime hotplugged memory will
> >> - allocate its internal metadata (struct pages)
> >> - from the hotadded memory which will allow to
> >> - hotadd a lot of memory without requiring
> >> - additional memory to do so.
> >> + allocate its internal metadata (struct pages,
> >> + those vmemmap pages cannot be optimized even
> >> + if hugetlb_free_vmemmap is enabled) from the
> >> + hotadded memory which will allow to hotadd a
> >> + lot of memory without requiring additional
> >> + memory to do so.
> >> This feature is disabled by default because it
> >> has some implication on large (e.g. GB)
> >> allocations in some configurations (e.g. small
> >> @@ -3082,10 +3086,6 @@
> >> Note that even when enabled, there are a few cases where
> >> the feature is not effective.
> >>
> >> - This is not compatible with hugetlb_free_vmemmap. If
> >> - both parameters are enabled, hugetlb_free_vmemmap takes
> >> - precedence over memory_hotplug.memmap_on_memory.
> >> -
> >> memtest= [KNL,X86,ARM,M68K,PPC,RISCV] Enable memtest
> >> Format: <integer>
> >> default : 0 <disable>
> >> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> >> index 5c9aa171a0d3..d7374a1e8ac9 100644
> >> --- a/Documentation/admin-guide/sysctl/vm.rst
> >> +++ b/Documentation/admin-guide/sysctl/vm.rst
> >> @@ -565,9 +565,8 @@ See Documentation/admin-guide/mm/hugetlbpage.rst
> >> hugetlb_optimize_vmemmap
> >> ========================
> >>
> >> -This knob is not available when memory_hotplug.memmap_on_memory (kernel parameter)
> >> -is configured or the size of 'struct page' (a structure defined in
> >> -include/linux/mm_types.h) is not power of two (an unusual system config could
> >> +This knob is not available when the size of 'struct page' (a structure defined
> >> +in include/linux/mm_types.h) is not power of two (an unusual system config could
> >> result in this).
> >>
> >> Enable (set to 1) or disable (set to 0) the feature of optimizing vmemmap pages
> >> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> >> index 20d7edf62a6a..e0b2209ab71c 100644
> >> --- a/include/linux/memory_hotplug.h
> >> +++ b/include/linux/memory_hotplug.h
> >> @@ -351,13 +351,4 @@ void arch_remove_linear_mapping(u64 start, u64 size);
> >> extern bool mhp_supports_memmap_on_memory(unsigned long size);
> >> #endif /* CONFIG_MEMORY_HOTPLUG */
> >>
> >> -#ifdef CONFIG_MHP_MEMMAP_ON_MEMORY
> >> -bool mhp_memmap_on_memory(void);
> >> -#else
> >> -static inline bool mhp_memmap_on_memory(void)
> >> -{
> >> - return false;
> >> -}
> >> -#endif
> >> -
> >> #endif /* __LINUX_MEMORY_HOTPLUG_H */
> >> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> >> index e66f7aa3191d..2aa5dcbfe468 100644
> >> --- a/include/linux/page-flags.h
> >> +++ b/include/linux/page-flags.h
> >> @@ -193,6 +193,11 @@ enum pageflags {
> >>
> >> /* Only valid for buddy pages. Used to track pages that are reported */
> >> PG_reported = PG_uptodate,
> >> +
> >> +#ifdef CONFIG_MEMORY_HOTPLUG
> >> + /* For self-hosted memmap pages */
> >> + PG_vmemmap_self_hosted = PG_owner_priv_1,
> >> +#endif
> >> };
> >>
> >> #define PAGEFLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1)
> >> @@ -628,6 +633,12 @@ PAGEFLAG_FALSE(SkipKASanPoison, skip_kasan_poison)
> >> */
> >> __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
> >>
> >> +#ifdef CONFIG_MEMORY_HOTPLUG
> >> +PAGEFLAG(VmemmapSelfHosted, vmemmap_self_hosted, PF_ANY)
> >> +#else
> >> +PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
> >> +#endif
> >> +
> >> /*
> >> * On an anonymous page mapped into a user virtual memory area,
> >> * page->mapping points to its anon_vma, not to a struct address_space;
> >> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> >> index 1089ea8a9c98..73bfbb47f6a4 100644
> >> --- a/mm/hugetlb_vmemmap.c
> >> +++ b/mm/hugetlb_vmemmap.c
> >> @@ -10,7 +10,7 @@
> >> */
> >> #define pr_fmt(fmt) "HugeTLB: " fmt
> >>
> >> -#include <linux/memory_hotplug.h>
> >> +#include <linux/memory.h>
> >> #include "hugetlb_vmemmap.h"
> >>
> >> /*
> >> @@ -97,18 +97,54 @@ int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head)
> >> return ret;
> >> }
> >>
> >> +static unsigned int vmemmap_optimizable_pages(struct hstate *h,
> >> + struct page *head)
> >> +{
> >> + if (READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF)
> >> + return 0;
> >> +
> >> + if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
> >> + unsigned long pfn = page_to_pfn(head);
> >> +
> >> + /*
> >> + * Due to HugeTLB alignment requirements and the vmemmap pages
> >> + * being at the start of the hotplugged memory region in
> >> + * memory_hotplug.memmap_on_memory case. Checking the first
> >> + * vmemmap page's vmemmap if it is marked as VmemmapSelfHosted
> >> + * is sufficient.
> >> + *
> >> + * [ hotplugged memory ]
> >> + * [ section ][...][ section ]
> >> + * [ vmemmap ][ usable memory ]
> >> + * ^ | | |
> >> + * +---+ | |
> >> + * ^ | |
> >> + * +-------+ |
> >> + * ^ |
> >> + * +-------------------------------------------+
> >> + *
> >> + * Hotplugged memory block never has non-present sections, while
> >> + * boot memory block can have one or more. So pfn_valid() is
> >> + * used to filter out the non-present section which also cannot
> >> + * be memmap_on_memory.
> >> + */
> >> + pfn = ALIGN_DOWN(pfn, PHYS_PFN(memory_block_size_bytes()));
> >> + if (pfn_valid(pfn) && PageVmemmapSelfHosted(pfn_to_page(pfn)))
> >
> > Although it works, I think PageVmemmapSelfHosted() check for the 1st pfn's
> > vmemmap page is not always reliable. Since we reused PG_owner_priv_1
> > as PG_vmemmap_self_hosted, the test is noly reliable for vmemmap page's
> > vmemmap page. Other non-vmemmap page can be flagged with PG_owner_priv_1.
> > So this check can be false-positive. Maybe the following code snippet is
> > the solution.
>
> How could that happen for pages used for backing a vmemmap?
>
It cannot happen for memmap_on_memory case. Howwver, it can happen for other
cases. E.g. the 1st pfn (of boot memory block) whose vmemmap page may be flagged
as PG_owner_priv_1 (if PG_swapcache is set). Then, the check is false-positive.
> >
> > Any thoughts? Oscar or David.
>
> First of all, I think you should really avoid using
> memory_block_size_bytes(); when using memory_block_size_bytes(), you
> wouldn't need PageVmemmapSelfHosted(), you can just check if the vmemmap
> of the page is itself. But I think we should try making this independent
> of the memory block size.
>
Agree.
> If virt_to_page(page) doesn't work, maybe just traverse the direct map
> to find the page backing page directly?
>
Yeah, now I have tried to walk page tables to get the backing page.
I'll update a new version.
Thanks.
> --
> Thanks,
>
> David / dhildenb
>
>
