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SubjectRe: [External] Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page
On Fri, Nov 20, 2020 at 4:42 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Fri 20-11-20 14:43:04, Muchun Song wrote:
> [...]
>
> Thanks for improving the cover letter and providing some numbers. I have
> only glanced through the patchset because I didn't really have more time
> to dive depply into them.
>
> Overall it looks promissing. To summarize. I would prefer to not have
> the feature enablement controlled by compile time option and the kernel
> command line option should be opt-in. I also do not like that freeing
> the pool can trigger the oom killer or even shut the system down if no
> oom victim is eligible.

Hi Michal,

I have replied to you about those questions on the other mail thread.

Thanks.

>
> One thing that I didn't really get to think hard about is what is the
> effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be
> invalid when racing with the split. How do we enforce that this won't
> blow up?

This feature depends on the CONFIG_SPARSEMEM_VMEMMAP,
in this case, the pfn_to_page can work. The return value of the
pfn_to_page is actually the address of it's struct page struct.
I can not figure out where the problem is. Can you describe the
problem in detail please? Thanks.

>
> I have also asked in a previous version whether the vmemmap manipulation
> should be really unconditional. E.g. shortlived hugetlb pages allocated
> from the buddy allocator directly rather than for a pool. Maybe it
> should be restricted for the pool allocation as those are considered
> long term and therefore the overhead will be amortized and freeing path
> restrictions better understandable.

Yeah, I agree with you. This can be an optimization. And we can
add it to the todo list and implement it in the future. Now the patch
series is already huge.

>
> > Documentation/admin-guide/kernel-parameters.txt | 9 +
> > Documentation/admin-guide/mm/hugetlbpage.rst | 3 +
> > arch/x86/include/asm/hugetlb.h | 17 +
> > arch/x86/include/asm/pgtable_64_types.h | 8 +
> > arch/x86/mm/init_64.c | 7 +-
> > fs/Kconfig | 14 +
> > include/linux/bootmem_info.h | 78 +++
> > include/linux/hugetlb.h | 19 +
> > include/linux/hugetlb_cgroup.h | 15 +-
> > include/linux/memory_hotplug.h | 27 -
> > mm/Makefile | 2 +
> > mm/bootmem_info.c | 124 ++++
> > mm/hugetlb.c | 163 ++++-
> > mm/hugetlb_vmemmap.c | 765 ++++++++++++++++++++++++
> > mm/hugetlb_vmemmap.h | 103 ++++
>
> I will need to look closer but I suspect that a non-trivial part of the
> vmemmap manipulation really belongs to mm/sparse-vmemmap.c because the
> split and remapping shouldn't really be hugetlb specific. Sure hugetlb
> knows how to split but all the splitting should be implemented in
> vmemmap proper.
>
> > mm/memory_hotplug.c | 116 ----
> > mm/sparse.c | 5 +-
> > 17 files changed, 1295 insertions(+), 180 deletions(-)
> > create mode 100644 include/linux/bootmem_info.h
> > create mode 100644 mm/bootmem_info.c
> > create mode 100644 mm/hugetlb_vmemmap.c
> > create mode 100644 mm/hugetlb_vmemmap.h
>
> Thanks!
> --
> Michal Hocko
> SUSE Labs



--
Yours,
Muchun

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