From: Peter Collingbourne <pcc@google.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
Andrew Morton <akpm@linux-foundation.org>,
Linux ARM <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 17:50:32 -0700 [thread overview]
Message-ID: <CAMn1gO54gTG5f1dVfZHAaXWJRWeOe8BwTAvbk376_pjzcLYRSQ@mail.gmail.com> (raw)
In-Reply-To: <20200803120134.GD6132@gaia>
On Mon, Aug 3, 2020 at 5:01 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> 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.
Thanks, I like this idea. I will try to implement it.
> 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.
That would require a separate mmap() (i.e. separate VMA) for each
page, no? That sounds like it could be expensive both in terms of VMAs
and the number of mmap syscalls required (i.e. N/PAGE_SIZE). You could
decrease these costs by increasing the size of the memfd files to more
than a page, but that would also increase the amount of memory
required for the reference pages.
Peter
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next prev parent reply other threads:[~2020-08-04 0:52 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
2020-08-03 12:01 ` Catalin Marinas
2020-08-04 0:50 ` Peter Collingbourne [this message]
2020-08-04 15:27 ` Catalin Marinas
2020-08-04 15:48 ` Kirill A. Shutemov
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