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
