[PATCH] mm: introduce reference pages

[Catalin Marinas]

On Mon, Aug 03, 2020 at 12:32:59PM +0300, Kirill A. Shutemov wrote:
> 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.

I think this would work even for the arm64 MTE (though I haven't tried):
use memfd_create() to get such file descriptor, mmap() it as MAP_SHARED
to populate the initial pattern, mmap() it as MAP_PRIVATE for any
subsequent mapping that needs to be copied-on-write.

-- 
Catalin