[PATCH] mm: introduce reference pages

[Kirill A. Shutemov]

On Fri, Jul 31, 2020 at 01:32:41PM -0700, Peter Collingbourne wrote:
> Introduce a new mmap flag, MAP_REFPAGE, that creates a mapping similar
> to an anonymous mapping, but instead of clean pages being backed by the
> zero page, they are instead backed by a so-called reference page, whose
> address is specified using the offset argument to mmap. Loads from
> the mapping will load directly from the reference page, and initial
> stores to the mapping will copy-on-write from the reference page.
> 
> Reference pages are useful in circumstances where anonymous mappings
> combined with manual stores to memory would impose undesirable costs,
> either in terms of performance or RSS. Use cases are focused on heap
> allocators and include:
> 
> - Pattern initialization for the heap. This is where malloc(3) gives
>   you memory whose contents are filled with a non-zero pattern
>   byte, in order to help detect and mitigate bugs involving use
>   of uninitialized memory. Typically this is implemented by having
>   the allocator memset the allocation with the pattern byte before
>   returning it to the user, but for large allocations this can result
>   in a significant increase in RSS, especially for allocations that
>   are used sparsely. Even for dense allocations there is a needless
>   impact to startup performance when it may be better to amortize it
>   throughout the program. By creating allocations using a reference
>   page filled with the pattern byte, we can avoid these costs.
> 
> - Pre-tagged heap memory. Memory tagging [1] is an upcoming ARMv8.5
>   feature which allows for memory to be tagged in order to detect
>   certain kinds of memory errors with low overhead. In order to set
>   up an allocation to allow memory errors to be detected, the entire
>   allocation needs to have the same tag. The issue here is similar to
>   pattern initialization in the sense that large tagged allocations
>   will be expensive if the tagging is done up front. The idea is that
>   the allocator would create reference pages with each of the possible
>   memory tags, and use those reference pages for the large allocations.

Looks like it's wrong layer to implement the functionality. Just have a
special fd that would return the same page for all vm_ops->fault and map
the fd with normal mmap(MAP_PRIVATE, fd). It will get you what you want
without touching core-mm.

-- 
 Kirill A. Shutemov