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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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.

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

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

-- 
 Kirill A. Shutemov