2018-03-21 19:22:36 +00:00
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==============================
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remap_file_pages() system call
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==============================
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2014-06-06 21:38:23 +00:00
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The remap_file_pages() system call is used to create a nonlinear mapping,
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that is, a mapping in which the pages of the file are mapped into a
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nonsequential order in memory. The advantage of using remap_file_pages()
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over using repeated calls to mmap(2) is that the former approach does not
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require the kernel to create additional VMA (Virtual Memory Area) data
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structures.
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Supporting of nonlinear mapping requires significant amount of non-trivial
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code in kernel virtual memory subsystem including hot paths. Also to get
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nonlinear mapping work kernel need a way to distinguish normal page table
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entries from entries with file offset (pte_file). Kernel reserves flag in
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PTE for this purpose. PTE flags are scarce resource especially on some CPU
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architectures. It would be nice to free up the flag for other usage.
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Fortunately, there are not many users of remap_file_pages() in the wild.
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It's only known that one enterprise RDBMS implementation uses the syscall
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on 32-bit systems to map files bigger than can linearly fit into 32-bit
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virtual address space. This use-case is not critical anymore since 64-bit
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systems are widely available.
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mm: replace remap_file_pages() syscall with emulation
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.
Test case:
#define _GNU_SOURCE
#include <assert.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/mman.h>
#define MB (1024UL * 1024)
#define SIZE (4096 * MB)
int main(int argc, char **argv)
{
unsigned long *p;
long i, pass;
for (pass = 0; pass < 10; pass++) {
p = mmap(NULL, SIZE, PROT_READ|PROT_WRITE,
MAP_SHARED | MAP_ANONYMOUS, -1, 0);
if (p == MAP_FAILED) {
perror("mmap");
return -1;
}
for (i = 0; i < SIZE / 4096; i++)
p[i * 4096 / sizeof(*p)] = i;
for (i = 0; i < SIZE / 4096; i++) {
if (remap_file_pages(p + i * 4096 / sizeof(*p), 4096,
0, (SIZE - 4096 * (i + 1)) >> 12, 0)) {
perror("remap_file_pages");
return -1;
}
}
for (i = SIZE / 4096 - 1; i >= 0; i--)
assert(p[i * 4096 / sizeof(*p)] == SIZE / 4096 - i - 1);
munmap(p, SIZE);
}
return 0;
}
[akpm@linux-foundation.org: fix spello]
[sasha.levin@oracle.com: initialize populate before usage]
[sasha.levin@oracle.com: grab file ref to prevent race while mmaping]
Signed-off-by: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Dave Jones <davej@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Armin Rigo <arigo@tunes.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2015-02-10 22:09:46 +00:00
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The syscall is deprecated and replaced it with an emulation now. The
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emulation creates new VMAs instead of nonlinear mappings. It's going to
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work slower for rare users of remap_file_pages() but ABI is preserved.
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2014-06-06 21:38:23 +00:00
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One side effect of emulation (apart from performance) is that user can hit
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vm.max_map_count limit more easily due to additional VMAs. See comment for
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DEFAULT_MAX_MAP_COUNT for more details on the limit.
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