Re: Speeding up wineserver syncronization objects with shared memory

[Alexandre Julliard]

Gavriel State  writes:
> Looking at the kernel sources, it appears to be page-alignment, not
> filesystem block size that mmap needs to match.  Rereading old wine-devel
> archives, it appears that the notion that the filesystem block size is 
> involved is either from a very old kernel, or a misreading of some kernel
> comments.

Page alignment is needed for the address in memory, not for the offset
inside the file on disk; since section virtual addresses in PE files
are always page-aligned the memory address is never a problem. The
only problem comes from the alignment of the data inside the PE file,
and this is where we only need block-size alignment to make mmap
possible.

Speeding up wineserver syncronization objects with shared memory

[David Howells]

> Page alignment is needed for the address in memory, not for the offset
> inside the file on disk; since section virtual addresses in PE files
> are always page-aligned the memory address is never a problem. The
> only problem comes from the alignment of the data inside the PE file,
> and this is where we only need block-size alignment to make mmap
> possible.

Not so... The file offset must also be page aligned!

Try the following program:

#include <stdio.h>
    #include <unistd.h>
    #include <fcntl.h>
    #include <sys/mman.h>

int main(int argc, char **argv)
    {
    	void *addr;
    	int fd;

fd = open("/bin/sh",O_RDONLY);
    	if (fd<0) { perror("mmap"); return 1; }

addr = mmap(NULL,512,PROT_READ,MAP_PRIVATE,fd,argc==1?512:4096);
    	if (addr==MAP_FAILED) { perror("mmap"); return 1; }

return 0;
    }

If you don't run it with an argument, it fails (Invalid Argument), and if you
do give it an argument, it works.

If you look at the kernel:

[arch/i386/kernel/sys_i386.c]
    asmlinkage int old_mmap(struct mmap_arg_struct *arg)
    {
    	struct mmap_arg_struct a;
    	int err = -EFAULT;

if (copy_from_user(&a, arg, sizeof(a)))
    		goto out;

err = -EINVAL;
    	if (a.offset & ~PAGE_MASK)
    		goto out;

err = do_mmap2(a.addr, a.len, a.prot, a.flags, a.fd, a.offset >> PAGE_SHIFT);
    out:
    	return err;
    }

Note that "a.offset" (the file offset) is checked for page alignment. Note
also that the internal kernel routine do_mmap2() deals with the offset in
terms of pages, not bytes.

David