On 2022/5/20 2:31 AM, Chih-En Lin wrote:
When creating the user process, it usually uses the Copy-On-Write (COW)
mechanism to save the memory usage and the cost of time for copying.
COW defers the work of copying private memory and shares it across the
processes as read-only. If either process wants to write in these
memories, it will page fault and copy the shared memory, so the process
will now get its private memory right here, which is called break COW.
Presently this kind of technology is only used as the mapping memory.
It still needs to copy the entire page table from the parent.
It might cost a lot of time and memory to copy each page table when the
parent already has a lot of page tables allocated. For example, here is
the state table for mapping the 1 GB memory of forking.
mmap before fork mmap after fork
MemTotal: 32746776 kB 32746776 kB
MemFree: 31468152 kB 31463244 kB
AnonPages: 1073836 kB 1073628 kB
Mapped: 39520 kB 39992 kB
PageTables: 3356 kB 5432 kB
This patch introduces Copy-On-Write to the page table. This patch only
implements the COW on the PTE level. It's based on the paper
On-Demand Fork [1]. Summary of the implementation for the paper:
- Only implements the COW to the anonymous mapping
- Only do COW to the PTE table which the range is all covered by a
single VMA.
- Use the reference count to control the COW PTE table lifetime.
Decrease the counter when breaking COW or dereference the COW PTE
table. When the counter reduces to zero, free the PTE table.
Hi,
To reduce the empty user PTE tables, I also introduced a reference
count (pte_ref) for user PTE tables in my patch[1][2], It is used
to track the usage of each user PTE tables.
The following people will hold a pte_ref:
- The !pte_none() entry, such as regular page table entry that map
physical pages, or swap entry, or migrate entry, etc.
- Visitor to the PTE page table entries, such as page table walker.
With COW PTE, a new holder (the process using the COW PTE) is added.
It's funny, it leads me to see more meaning of pte_ref.
Thanks,
Qi
[1] [RFC PATCH 00/18] Try to free user PTE page table pages
link:
https://lore.kernel.org/lkml/20220429133552.33768-1-zhengqi.arch@xxxxxxxxxxxxx/
(percpu_ref version)
[2] [PATCH v3 00/15] Free user PTE page table pages
link:
https://lore.kernel.org/lkml/20211110105428.32458-1-zhengqi.arch@xxxxxxxxxxxxx/
(atomic count version)
--
Thanks,
Qi
