mm: add ksys_readahead() helper; remove in-kernel calls to sys_readahead()

Using this helper allows us to avoid the in-kernel calls to the
sys_readahead() syscall. The ksys_ prefix denotes that this function is
meant as a drop-in replacement for the syscall. In particular, it uses the
same calling convention as sys_readahead().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

arch/mips/kernel/linux32.c

@@ -131,7 +131,7 @@ SYSCALL_DEFINE1(32_personality, unsigned long, personality)
 asmlinkage ssize_t sys32_readahead(int fd, u32 pad0, u64 a2, u64 a3,
 				   size_t count)
 {
-	return sys_readahead(fd, merge_64(a2, a3), count);
+	return ksys_readahead(fd, merge_64(a2, a3), count);
 }
 
 asmlinkage long sys32_sync_file_range(int fd, int __pad,

arch/parisc/kernel/sys_parisc.c

@@ -345,7 +345,7 @@ asmlinkage ssize_t parisc_pwrite64(unsigned int fd, const char __user *buf,
 asmlinkage ssize_t parisc_readahead(int fd, unsigned int high, unsigned int low,
 		                    size_t count)
 {
-	return sys_readahead(fd, (loff_t)high << 32 | low, count);
+	return ksys_readahead(fd, (loff_t)high << 32 | low, count);
 }
 
 asmlinkage long parisc_fadvise64_64(int fd,

arch/powerpc/kernel/sys_ppc32.c

@@ -88,7 +88,7 @@ compat_ssize_t compat_sys_pwrite64(unsigned int fd, const char __user *ubuf, com
 
 compat_ssize_t compat_sys_readahead(int fd, u32 r4, u32 offhi, u32 offlo, u32 count)
 {
-	return sys_readahead(fd, ((loff_t)offhi << 32) | offlo, count);
+	return ksys_readahead(fd, ((loff_t)offhi << 32) | offlo, count);
 }
 
 asmlinkage int compat_sys_truncate64(const char __user * path, u32 reg4,

arch/s390/kernel/compat_linux.c

@@ -328,7 +328,7 @@ COMPAT_SYSCALL_DEFINE5(s390_pwrite64, unsigned int, fd, const char __user *, ubu
 
 COMPAT_SYSCALL_DEFINE4(s390_readahead, int, fd, u32, high, u32, low, s32, count)
 {
-	return sys_readahead(fd, (unsigned long)high << 32 | low, count);
+	return ksys_readahead(fd, (unsigned long)high << 32 | low, count);
 }
 
 struct stat64_emu31 {

arch/sparc/kernel/sys_sparc32.c

@@ -217,7 +217,7 @@ asmlinkage long compat_sys_readahead(int fd,
 				     unsigned long offlo,
 				     compat_size_t count)
 {
-	return sys_readahead(fd, (offhi << 32) | offlo, count);
+	return ksys_readahead(fd, (offhi << 32) | offlo, count);
 }
 
 long compat_sys_fadvise64(int fd,

arch/x86/ia32/sys_ia32.c

@@ -203,7 +203,7 @@ COMPAT_SYSCALL_DEFINE6(x86_fadvise64_64, int, fd, __u32, offset_low,
 COMPAT_SYSCALL_DEFINE4(x86_readahead, int, fd, unsigned int, off_lo,
 		       unsigned int, off_hi, size_t, count)
 {
-	return sys_readahead(fd, ((u64)off_hi << 32) | off_lo, count);
+	return ksys_readahead(fd, ((u64)off_hi << 32) | off_lo, count);
 }
 
 COMPAT_SYSCALL_DEFINE6(x86_sync_file_range, int, fd, unsigned int, off_low,

include/linux/syscalls.h

@@ -982,6 +982,7 @@ static inline int ksys_fadvise64_64(int fd, loff_t offset, loff_t len,
 unsigned long ksys_mmap_pgoff(unsigned long addr, unsigned long len,
 			      unsigned long prot, unsigned long flags,
 			      unsigned long fd, unsigned long pgoff);
+ssize_t ksys_readahead(int fd, loff_t offset, size_t count);
 
 /*
  * The following kernel syscall equivalents are just wrappers to fs-internal

mm/readahead.c

@@ -573,7 +573,7 @@ do_readahead(struct address_space *mapping, struct file *filp,
 	return force_page_cache_readahead(mapping, filp, index, nr);
 }
 
-SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
+ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
 {
 	ssize_t ret;
 	struct fd f;
@@ -592,3 +592,8 @@ SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
 	}
 	return ret;
 }
+
+SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
+{
+	return ksys_readahead(fd, offset, count);
+}