Re: [PATCH 2/2] mm: replace remap_file_pages() syscall with emulation

[Kirill A. Shutemov]

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@xxxxxxxxxxxxxxx> 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?

I would like to get rid of it completely, but I thought it's not an option
for compatibility reason.

-- 
 Kirill A. Shutemov
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