From mboxrd@z Thu Jan  1 00:00:00 1970
From: Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH 2/4] mm: speed up mremap by 500x on large regions (v2)
Date: Tue, 16 Oct 2018 13:29:52 +0200
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 <20181013013200.206928-3-joel@joelfernandes.org>
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Cc: linux-mips@linux-mips.org, Rich Felker <dalias@libc.org>,
 linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org,
 Peter Zijlstra <peterz@infradead.org>,
 Catalin Marinas <catalin.marinas@arm.com>,
 Dave Hansen <dave.hansen@linux.intel.com>, Will Deacon <will.deacon@arm.com>,
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 Yoshinori Sato <ysato@users.sourceforge.jp>, Max Filippov <jcmvbkbc@gmail.com>,
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 "maintainer:X86 ARCHITECTURE 32-BIT AND 64-BIT" <x86@kernel.org>,
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 kasan-dev@googlegroups.com, elfring@users.sourceforge.net,
 Ingo Molnar <mingo@redhat.com>, Geert Uytter
To: Joel Fernandes <joel@joelfernandes.org>,
 Christoph Hellwig <hch@infradead.org>
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Errors-To: linux-snps-arc-bounces+gla-linux-snps-arc=m.gmane.org@lists.infradead.org

On 10/16/18 12:33 AM, Joel Fernandes wrote:
> On Mon, Oct 15, 2018 at 02:42:09AM -0700, Christoph Hellwig wrote:
>> On Fri, Oct 12, 2018 at 06:31:58PM -0700, Joel Fernandes (Google) wrote:
>>> Android needs to mremap large regions of memory during memory management
>>> related operations.
>>
>> Just curious: why?
> 
> In Android we have a requirement of moving a large (up to a GB now, but may
> grow bigger in future) memory range from one location to another.

I think Christoph's "why?" was about the requirement, not why it hurts
applications. I admit I'm now also curious :)

> This move
> operation has to happen when the application threads are paused for this
> operation. Therefore, an inefficient move like it is now (for example 250ms
> on arm64) will cause response time issues for applications, which is not
> acceptable. Huge pages cannot be used in such memory ranges to avoid this
> inefficiency as (when the application threads are running) our fault handlers
> are designed to process 4KB pages at a time, to keep response times low. So
> using huge pages in this context can, again, cause response time issues.
> 
> Also, the mremap syscall waiting for quarter of a second for a large mremap
> is quite weird and we ought to improve it where possible.