Re: [PATCH v2 6/9] mm: free user PTE page table pages

[David Hildenbrand]

On 19.08.21 05:18, 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.
>
> While we access ->pte_refcount of a PTE page table, any of the
> following ensures the pmd entry corresponding to the PTE page
> table stability:
>
> 	- mmap_lock
> 	- anon_lock
> 	- i_mmap_lock
> 	- parallel threads are excluded by other means which
> 	  can make ->pmd stable(e.g. gup case)
>
> This patch does not support THP temporarily, it will be
> supported in the next patch.

Can you clarify (and document here) who exactly takes a reference on the 
page table? Do I understand correctly that

a) each !pte_none() entry inside a page table take a reference to the 
page it's containted in.
b) each page table walker temporarily grabs a page table reference
c) The PMD tables the PTE is referenced in (->currently only ever a 
single one) does *not* take a reference.

So if there are no PTE entries left and nobody walks the page tables, 
you can remove it? You should really extend the 
description/documentation to make it clearer how exactly it's supposed 
to work.


It feels kind of strange to not introduce the CONFIG_FREE_USER_PTE 
Kconfig option in this patch. At least it took me a while to identify it 
in the previous patch.

Maybe you should introduce the empty stubs and use them in a separate 
patch, and then have this patch just introduce CONFIG_FREE_USER_PTE 
along with the actual refcounting magic inside the !stub implementation.

--
Thanks,

David / dhildenb