Re: [External] [RFC PATCH 0/6] Introduce Copy-On-Write to Page Table

[Qi Zheng]

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