linux-kernel - Re: [PATCH 2/2] mm: replace remap_file

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Date:	Mon, 12 May 2014 11:11:48 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	peterz@...radead.org, mingo@...nel.org
Subject: Re: [PATCH 2/2] mm: replace remap_file_pages() syscall with emulation

On 05/08/2014 05:57 PM, Andrew Morton wrote:
> On Thu,  8 May 2014 15:41:28 +0300 "Kirill A. Shutemov" <kirill.shutemov@...ux.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.

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.


Thanks,
Sasha
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