Re: [PATCH 5/7] mm: free user PTE page table pages

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 7/19/21 6:01 AM, Kirill A. Shutemov wrote:
> On Sun, Jul 18, 2021 at 12:30:31PM +0800, Qi Zheng wrote:
>> 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.
> 
> The patch is very hard to review.
> 
> Could you split up introduction of the new API in the separate patch? With
> a proper documentation of the API.

Good idea, i will do it.

> 
> Why pte_refcount is atomic? Looks like you do everything under pmd_lock().
> Do I miss something?

When we do pte_get_unless_zero(), we hold pmd_lock to protect against
free_pte_table(). But we don't need to hold the pmd lock when we do
pte_get()/pte_put() in mapping/unmapping routine.

> 
> And performance numbers should be included. I don't expect pmd_lock() in
> all hotpaths to scale well.
> 

Yeah, so we use rcu lock to replace the pmd lock in some routines in the
subsequent patch (mm: defer freeing PTE page table for a grace period).

Thanks,

Qi