From: Sasha Levin <sasha.levin@oracle.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
peterz@infradead.org, mingo@kernel.org
Subject: Re: [PATCH 2/2] mm: replace remap_file_pages() syscall with emulation
Date: Wed, 14 May 2014 16:52:17 -0400 [thread overview]
Message-ID: <5373D781.7020109@oracle.com> (raw)
In-Reply-To: <20140512170514.GA28227@node.dhcp.inet.fi>
On 05/12/2014 01:05 PM, Kirill A. Shutemov wrote:
> On Mon, May 12, 2014 at 11:11:48AM -0400, Sasha Levin wrote:
>> On 05/08/2014 05:57 PM, Andrew Morton wrote:
>>> On Thu, 8 May 2014 15:41:28 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
>>>
>>>>> remap_file_pages(2) was invented to be able efficiently map parts of
>>>>> huge file into limited 32-bit virtual address space such as in database
>>>>> workloads.
>>>>>
>>>>> Nonlinear mappings are pain to support and it seems there's no
>>>>> legitimate use-cases nowadays since 64-bit systems are widely available.
>>>>>
>>>>> Let's drop it and get rid of all these special-cased code.
>>>>>
>>>>> The patch replaces the syscall with emulation which creates new VMA on
>>>>> each remap_file_pages(), unless they it can be merged with an adjacent
>>>>> one.
>>>>>
>>>>> I didn't find *any* real code that uses remap_file_pages(2) to test
>>>>> emulation impact on. I've checked Debian code search and source of all
>>>>> packages in ALT Linux. No real users: libc wrappers, mentions in strace,
>>>>> gdb, valgrind and this kind of stuff.
>>>>>
>>>>> There are few basic tests in LTP for the syscall. They work just fine
>>>>> with emulation.
>>>>>
>>>>> To test performance impact, I've written small test case which
>>>>> demonstrate pretty much worst case scenario: map 4G shmfs file, write to
>>>>> begin of every page pgoff of the page, remap pages in reverse order,
>>>>> read every page.
>>>>>
>>>>> The test creates 1 million of VMAs if emulation is in use, so I had to
>>>>> set vm.max_map_count to 1100000 to avoid -ENOMEM.
>>>>>
>>>>> Before: 23.3 ( +- 4.31% ) seconds
>>>>> After: 43.9 ( +- 0.85% ) seconds
>>>>> Slowdown: 1.88x
>>>>>
>>>>> I believe we can live with that.
>>>>>
>>> There's still all the special-case goop around the place to be cleaned
>>> up - VM_NONLINEAR is a decent search term. As is "grep nonlinear
>>> mm/*.c". And although this cleanup is the main reason for the
>>> patchset, let's not do it now - we can do all that if/after this patch
>>> get merged.
>>>
>>> I'll queue the patches for some linux-next exposure and shall send
>>> [1/2] Linuswards for 3.16 if nothing terrible happens. Once we've
>>> sorted out the too-many-vmas issue we'll need to work out when to merge
>>> [2/2].
>>
>> It seems that since no one is really using it, it's also impossible to
>> properly test it. I've sent a fix that deals with panics in error paths
>> that are very easy to trigger, but I'm worried that there are a lot more
>> of those hiding over there.
>
> Sorry for that.
>
>> Since we can't find any actual users, testing suites are very incomplete
>> w.r.t this syscall, and the amount of work required to "remove" it is
>> non-trivial, can we just kill this syscall off?
>>
>> It sounds to me like a better option than to ship a new, buggy and possibly
>> security dangerous version which we can't even test.
>
> Taking into account your employment, is it possible to check how the RDBMS
> (old but it still supported 32-bit versions) would react on -ENOSYS here?
Alrighty, I got an answer:
1. remap_file_pages() only works when the "VLM" feature of the db is enabled,
so those databases can work just fine without it, but be limited to 3-4GB of
memory. This is not needed at all on 64bit machines.
2. As of OL7 (kernel 3.8), there will not be a 32bit kernel build. I'm still
waiting for an answer whether there will do a 32bit DB build for a 64bit kernel,
but that never happened before and seems unlikely.
3. They're basically saying that by the time upstream releases a kernel without
remap_file_pages() no one will need it here.
To sum it up, they're fine with removing remap_file_pages().
Thanks,
Sasha
next prev parent reply other threads:[~2014-05-14 20:53 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-08 12:41 [PATCHv2 0/2] remap_file_pages() decommission Kirill A. Shutemov
2014-05-08 12:41 ` [PATCH 1/2] mm: mark remap_file_pages() syscall as deprecated Kirill A. Shutemov
2014-06-12 5:48 ` Michael Kerrisk
2014-06-12 9:40 ` Kirill A. Shutemov
2014-06-12 9:44 ` Michael Kerrisk (man-pages)
2014-05-08 12:41 ` [PATCH 2/2] mm: replace remap_file_pages() syscall with emulation Kirill A. Shutemov
2014-05-08 21:57 ` Andrew Morton
2014-05-12 15:11 ` Sasha Levin
2014-05-12 17:05 ` Kirill A. Shutemov
2014-05-14 20:52 ` Sasha Levin [this message]
2014-05-14 21:17 ` Kirill A. Shutemov
2014-05-14 21:40 ` Andrew Morton
2014-05-13 7:32 ` Armin Rigo
2014-05-13 12:57 ` Sasha Levin
2014-05-08 15:35 ` [PATCHv2 0/2] remap_file_pages() decommission Linus Torvalds
2014-05-08 15:44 ` Armin Rigo
2014-05-08 16:02 ` Kirill A. Shutemov
2014-05-08 16:08 ` Linus Torvalds
2014-05-09 14:05 ` Kirill A. Shutemov
2014-05-09 15:14 ` Linus Torvalds
2014-05-09 18:19 ` Kirill A. Shutemov
2014-05-12 12:43 ` Kirill A. Shutemov
2014-05-12 14:59 ` Konstantin Khlebnikov
2014-05-12 3:36 ` Andi Kleen
2014-05-12 5:16 ` Konstantin Khlebnikov
2014-05-12 7:50 ` Armin Rigo
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