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authorJeff Dike <jdike@addtoit.com>2007-05-06 17:51:32 -0400
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-05-07 15:13:03 -0400
commit3d564047a5f45cb628ec72514f68076e532988f3 (patch)
tree3a4247baed8e66bfe5d159f058a88c1a5b7e7ed1 /arch/um/os-Linux/file.c
parentf9d6e5f83b40d8ff73a74d4bba2c5f51d6048b12 (diff)
uml: start fixing os_read_file and os_write_file
This patch starts the removal of a very old, very broken piece of code. This stems from the problem of passing a userspace buffer into read() or write() on the host. If that buffer had not yet been faulted in, read and write will return -EFAULT. To avoid this problem, the solution was to fault the buffer in before the system call by touching the pages that hold the buffer by doing a copy-user of a byte to each page. This is obviously bogus, but it does usually work, in tt mode, since the kernel and process are in the same address space and userspace addresses can be accessed directly in the kernel. In skas mode, where the kernel and process are in separate address spaces, it is completely bogus because the userspace address, which is invalid in the kernel, is passed into the system call instead of the corresponding physical address, which would be valid. Here, it appears that this code, on every host read() or write(), tries to fault in a random process page. This doesn't seem to cause any correctness problems, but there is a performance impact. This patch, and the ones following, result in a 10-15% performance gain on a kernel build. This code can't be immediately tossed out because when it is, you can't log in. Apparently, there is some code in the console driver which depends on this somehow. However, we can start removing it by switching the code which does I/O using kernel addresses to using plain read() and write(). This patch introduces os_read_file_k and os_write_file_k for use with kernel buffers and converts all call locations which use obvious kernel buffers to use them. These include I/O using buffers which are local variables which are on the stack or kmalloc-ed. Later patches will handle the less obvious cases, followed by a mass conversion back to the original interface. Signed-off-by: Jeff Dike <jdike@linux.intel.com> Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'arch/um/os-Linux/file.c')
-rw-r--r--arch/um/os-Linux/file.c18
1 files changed, 18 insertions, 0 deletions
diff --git a/arch/um/os-Linux/file.c b/arch/um/os-Linux/file.c
index 4a9510c67622..5e9b8dcf34d4 100644
--- a/arch/um/os-Linux/file.c
+++ b/arch/um/os-Linux/file.c
@@ -334,12 +334,30 @@ int os_read_file(int fd, void *buf, int len)
334 copy_from_user_proc); 334 copy_from_user_proc);
335} 335}
336 336
337int os_read_file_k(int fd, void *buf, int len)
338{
339 int n = read(fd, buf, len);
340
341 if(n < 0)
342 return -errno;
343 return n;
344}
345
337int os_write_file(int fd, const void *buf, int len) 346int os_write_file(int fd, const void *buf, int len)
338{ 347{
339 return file_io(fd, (void *) buf, len, 348 return file_io(fd, (void *) buf, len,
340 (int (*)(int, void *, int)) write, copy_to_user_proc); 349 (int (*)(int, void *, int)) write, copy_to_user_proc);
341} 350}
342 351
352int os_write_file_k(int fd, const void *buf, int len)
353{
354 int n = write(fd, (void *) buf, len);
355
356 if(n < 0)
357 return -errno;
358 return n;
359}
360
343int os_file_size(char *file, unsigned long long *size_out) 361int os_file_size(char *file, unsigned long long *size_out)
344{ 362{
345 struct uml_stat buf; 363 struct uml_stat buf;