[PATCH v5] mm: introduce reference pages

[David Hildenbrand]

On 17.07.21 04:57, Peter Collingbourne wrote:
> Introduce a new syscall, refpage_create, which returns a file
> descriptor which may be mapped using mmap. Such a mapping is 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
> contents are specified using an argument to refpage_create. 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.

I'm wondering, does the target use case really require the COW 
optimization like we have for the shared zeropage?

If we'd avoid having a reference page at all and only store the pattern, 
we could significantly reduce the memory consumption when using a lot of 
reference pages, especially per process multiple ones. I'm asking 
because ...

> 
> 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.

... I assume the first *sane* access to such a page is a write, and not 
a read.

> 
> - 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.

... and here as well.

Having a first access being a read sound more like an actual BUG (e.g., 
detect and mitigate bugs), which doesn't scream for needing a 
performance improvement or sacrificing a whole (unmovable/unswappable) 
reference page.

So, what would you lose when not populating a real reference pages at 
all and instead only populating the pattern when populating a fresh 
page? (and populating a fresh page even on read faults)


-- 
Thanks,

David / dhildenb