From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Peter Collingbourne <pcc@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-arm-kernel@lists.infradead.org,
Evgenii Stepanov <eugenis@google.com>,
linux-mm@kvack.org
Subject: Re: [PATCH] mm: introduce reference pages
Date: Mon, 3 Aug 2020 12:32:59 +0300 [thread overview]
Message-ID: <20200803093259.ookknl4y7ee5hun7@box> (raw)
In-Reply-To: <20200731203241.50427-1-pcc@google.com>
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
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next prev parent reply other threads:[~2020-08-03 9:34 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-31 20:32 [PATCH] mm: introduce reference pages Peter Collingbourne
2020-08-03 3:28 ` John Hubbard
2020-08-03 3:51 ` Matthew Wilcox
2020-08-13 22:03 ` Peter Collingbourne
2020-08-13 22:03 ` Peter Collingbourne
2020-08-03 9:32 ` Kirill A. Shutemov [this message]
2020-08-03 12:01 ` Catalin Marinas
2020-08-04 0:50 ` Peter Collingbourne
2020-08-04 15:27 ` Catalin Marinas
2020-08-04 15:48 ` Kirill A. Shutemov
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