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>

1 files changed, 69 insertions, 0 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 7f684d5a8087..e023dc5e59a8 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2634,6 +2634,75 @@ SYSCALL_DEFINE2(munmap, unsigned long, addr, size_t, len)
         return vm_munmap(addr, len);
 }
 
+
+/*
+ * Emulation of deprecated remap_file_pages() syscall.
+ */
+SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
+                unsigned long, prot, unsigned long, pgoff, unsigned long, flags)
+{
+
+        struct mm_struct *mm = current->mm;
+        struct vm_area_struct *vma;
+        unsigned long populate = 0;
+        unsigned long ret = -EINVAL;
+        struct file *file;
+
+        pr_warn_once("%s (%d) uses deprecated remap_file_pages() syscall. "
+                        "See Documentation/vm/remap_file_pages.txt.\n",
+                        current->comm, current->pid);
+
+        if (prot)
+                return ret;
+        start = start & PAGE_MASK;
+        size = size & PAGE_MASK;
+
+        if (start + size <= start)
+                return ret;
+
+        /* Does pgoff wrap? */
+        if (pgoff + (size >> PAGE_SHIFT) < pgoff)
+                return ret;
+
+        down_write(&mm->mmap_sem);
+        vma = find_vma(mm, start);
+
+        if (!vma || !(vma->vm_flags & VM_SHARED))
+                goto out;
+
+        if (start < vma->vm_start || start + size > vma->vm_end)
+                goto out;
+
+        if (pgoff == linear_page_index(vma, start)) {
+                ret = 0;
+                goto out;
+        }
+
+        prot |= vma->vm_flags & VM_READ ? PROT_READ : 0;
+        prot |= vma->vm_flags & VM_WRITE ? PROT_WRITE : 0;
+        prot |= vma->vm_flags & VM_EXEC ? PROT_EXEC : 0;
+
+        flags &= MAP_NONBLOCK;
+        flags |= MAP_SHARED | MAP_FIXED | MAP_POPULATE;
+        if (vma->vm_flags & VM_LOCKED) {
+                flags |= MAP_LOCKED;
+                /* drop PG_Mlocked flag for over-mapped range */
+                munlock_vma_pages_range(vma, start, start + size);
+        }
+
+        file = get_file(vma->vm_file);
+        ret = do_mmap_pgoff(vma->vm_file, start, size,
+                        prot, flags, pgoff, &populate);
+        fput(file);
+out:
+        up_write(&mm->mmap_sem);
+        if (populate)
+                mm_populate(ret, populate);
+        if (!IS_ERR_VALUE(ret))
+                ret = 0;
+        return ret;
+}
+
 static inline void verify_mm_writelocked(struct mm_struct *mm)
 {
 #ifdef CONFIG_DEBUG_VM