Re: [PATCH v3 00/15] Free user PTE page table pages

From: Qi Zheng
Date: Wed Nov 10 2021 - 03:52:20 EST

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Hi all,

I’m sorry, something went wrong when sending this patch set, I will resend the whole patch later.

Thanks,
Qi

On 11/10/21 4:40 PM, Qi Zheng wrote:

Hi,

This patch series aims to free user PTE page table pages when all PTE entries
are empty.

The beginning of this story is that some malloc libraries(e.g. jemalloc or
tcmalloc) usually allocate the amount of VAs by mmap() and do not unmap those VAs.
They will use madvise(MADV_DONTNEED) to free physical memory if they want.
But the page tables do not be freed by madvise(), so it can produce many
page tables when the process touches an enormous virtual address space.

The following figures are a memory usage snapshot of one process which actually
happened on our server:

VIRT: 55t
RES: 590g
VmPTE: 110g

As we can see, the PTE page tables size is 110g, while the RES is 590g. In
theory, the process only need 1.2g PTE page tables to map those physical
memory. The reason why PTE page tables occupy a lot of memory is that
madvise(MADV_DONTNEED) only empty the PTE and free physical memory but
doesn't free the PTE page table pages. So we can free those empty PTE page
tables to save memory. In the above cases, we can save memory about 108g(best
case). And the larger the difference between the size of VIRT and RES, the
more memory we save.

In this patch series, we add a pte_refcount field to the struct page of page
table to track how many users of PTE page table. Similar to the mechanism of
page refcount, the user of PTE page table should hold a refcount to it before
accessing. The PTE page table page will be freed when the last refcount is
dropped.

Testing:

The following code snippet can show the effect of optimization:

mmap 50G
while (1) {
for (; i < 1024 * 25; i++) {
touch 2M memory
madvise MADV_DONTNEED 2M
}
}

As we can see, the memory usage of VmPTE is reduced:

before after
VIRT 50.0 GB 50.0 GB
RES 3.1 MB 3.6 MB
VmPTE 102640 kB 248 kB

I also have tested the stability by LTP[1] for several weeks. I have not seen
any crash so far.

The performance of page fault can be affected because of the allocation/freeing
of PTE page table pages. The following is the test result by using a micro
benchmark[2]:

root@~# perf stat -e page-faults --repeat 5 ./multi-fault $threads:

threads before (pf/min) after (pf/min)
1 32,085,255 31,880,833 (-0.64%)
8 101,674,967 100,588,311 (-1.17%)
16 113,207,000 112,801,832 (-0.36%)

(The "pfn/min" means how many page faults in one minute.)

The performance of page fault is ~1% slower than before.

And there are no obvious changes in perf hot spots:

before:
19.29% [kernel] [k] clear_page_rep
16.12% [kernel] [k] do_user_addr_fault
9.57% [kernel] [k] _raw_spin_unlock_irqrestore
6.16% [kernel] [k] get_page_from_freelist
5.03% [kernel] [k] __handle_mm_fault
3.53% [kernel] [k] __rcu_read_unlock
3.45% [kernel] [k] handle_mm_fault
3.38% [kernel] [k] down_read_trylock
2.74% [kernel] [k] free_unref_page_list
2.17% [kernel] [k] up_read
1.93% [kernel] [k] charge_memcg
1.73% [kernel] [k] try_charge_memcg
1.71% [kernel] [k] __alloc_pages
1.69% [kernel] [k] ___perf_sw_event
1.44% [kernel] [k] get_mem_cgroup_from_mm

after:
18.19% [kernel] [k] clear_page_rep
16.28% [kernel] [k] do_user_addr_fault
8.39% [kernel] [k] _raw_spin_unlock_irqrestore
5.12% [kernel] [k] get_page_from_freelist
4.81% [kernel] [k] __handle_mm_fault
4.68% [kernel] [k] down_read_trylock
3.80% [kernel] [k] handle_mm_fault
3.59% [kernel] [k] get_mem_cgroup_from_mm
2.49% [kernel] [k] free_unref_page_list
2.41% [kernel] [k] up_read
2.16% [kernel] [k] charge_memcg
1.92% [kernel] [k] __rcu_read_unlock
1.88% [kernel] [k] ___perf_sw_event
1.70% [kernel] [k] pte_get_unless_zero

This series is based on next-20211108.

Comments and suggestions are welcome.

Thanks,
Qi.

[1] https://github.com/linux-test-project/ltp
[2] https://lore.kernel.org/lkml/20100106160614.ff756f82.kamezawa.hiroyu@xxxxxxxxxxxxxx/2-multi-fault-all.c

Changelog in v2 -> v3:
- Refactored this patch series:
- [PATCH v3 6/15]: Introduce the new dummy helpers first
- [PATCH v3 7-12/15]: Convert each subsystem individually
- [PATCH v3 13/15]: Implement the actual logic to the dummy helpers
And thanks for the advice from David and Jason.
- Add a document.

Changelog in v1 -> v2:
- Change pte_install() to pmd_install().
- Fix some typo and code style problems.
- Split [PATCH v1 5/7] into [PATCH v2 4/9], [PATCH v2 5/9]，[PATCH v2 6/9]
and [PATCH v2 7/9].

Qi Zheng (15):
mm: do code cleanups to filemap_map_pmd()
mm: introduce is_huge_pmd() helper
mm: move pte_offset_map_lock() to pgtable.h
mm: rework the parameter of lock_page_or_retry()
mm: add pmd_installed_type return for __pte_alloc() and other friends
mm: introduce refcount for user PTE page table page
mm/pte_ref: add support for user PTE page table page allocation
mm/pte_ref: initialize the refcount of the withdrawn PTE page table
page
mm/pte_ref: add support for the map/unmap of user PTE page table page
mm/pte_ref: add support for page fault path
mm/pte_ref: take a refcount before accessing the PTE page table page
mm/pte_ref: update the pmd entry in move_normal_pmd()
mm/pte_ref: free user PTE page table pages
Documentation: add document for pte_ref
mm/pte_ref: use mmu_gather to free PTE page table pages

Documentation/vm/pte_ref.rst | 216 ++++++++++++++++++++++++++++++++++++
arch/x86/Kconfig | 2 +-
fs/proc/task_mmu.c | 24 +++-
fs/userfaultfd.c | 9 +-
include/linux/huge_mm.h | 10 +-
include/linux/mm.h | 170 ++++-------------------------
include/linux/mm_types.h | 6 +-
include/linux/pagemap.h | 8 +-
include/linux/pgtable.h | 152 +++++++++++++++++++++++++-
include/linux/pte_ref.h | 146 +++++++++++++++++++++++++
include/linux/rmap.h | 2 +
kernel/events/uprobes.c | 2 +
mm/Kconfig | 4 +
mm/Makefile | 4 +-
mm/damon/vaddr.c | 12 +-
mm/debug_vm_pgtable.c | 5 +-
mm/filemap.c | 45 +++++---
mm/gup.c | 25 ++++-
mm/hmm.c | 5 +-
mm/huge_memory.c | 3 +-
mm/internal.h | 4 +-
mm/khugepaged.c | 21 +++-
mm/ksm.c | 6 +-
mm/madvise.c | 21 +++-
mm/memcontrol.c | 12 +-
mm/memory-failure.c | 11 +-
mm/memory.c | 254 ++++++++++++++++++++++++++++++++-----------
mm/mempolicy.c | 6 +-
mm/migrate.c | 54 ++++-----
mm/mincore.c | 7 +-
mm/mlock.c | 1 +
mm/mmu_gather.c | 40 +++----
mm/mprotect.c | 11 +-
mm/mremap.c | 14 ++-
mm/page_vma_mapped.c | 4 +
mm/pagewalk.c | 15 ++-
mm/pgtable-generic.c | 1 +
mm/pte_ref.c | 141 ++++++++++++++++++++++++
mm/rmap.c | 10 ++
mm/swapfile.c | 3 +
mm/userfaultfd.c | 40 +++++--
41 files changed, 1186 insertions(+), 340 deletions(-)
create mode 100644 Documentation/vm/pte_ref.rst
create mode 100644 include/linux/pte_ref.h
create mode 100644 mm/pte_ref.c

Next message: Michael.Wu: "RE: checkpatch warns a symbolic link has not a newline"
Previous message: Huang, Ying: "Re: [PATCH] mm: migrate: Support multiple target nodes demotion"
In reply to: Qi Zheng: "[PATCH v3 09/15] mm/pte_ref: add support for the map/unmap of user PTE page table page"
Next in thread: Qi Zheng: "[PATCH v3 00/15] Free user PTE page table pages"
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