Phil Nelson wrote: > On Sunday 25 March 2007 07:07, [EMAIL PROTECTED] wrote: >> This would probably work well for systems without mmap(), >> it is what you meant with "OS-specific"? >> >> Otherwise I fear that we'd miss possible modifications done via mmap(). >> It may be used though as another "shortcut" check if there were no mmap()s. > > On the systems that support mmap(), we can check the protection argument > for the flag PROT_WRITE. If it is mapped with write, the kernel module would > have to assume a write. (I don't think we want to try to walk the buffer > cache in each supported system trying to find out if any of the pages were > modified.) >
At least on Linux, the kernel maps all writable pages as read-only. When
a user writes to such a page, the kernel takes the fault and remaps as
read-write. Once the dirty page is cleaned, the page is remapped back to
read-only. It is through this mechanism that writes via mmap can be
detected.
For the Linux coda module, I think this could be implemented with just
a bit of difficulty.
1. Add a "was this ever written?" flag into the code_inode_info struct,
and a spot to hold the host file's vma->vm_ops.page_mkwrite pointer.
2. Devise a mechanism for getting the coda_inode from a corresponding
vma. Note that this only needs to remember inodes that have
a read-write mmap and have not yet been written to.
3. Write the coda_page_mkwrite function in some way like this:
static int coda_page_mkwrite(struct vm_area_struct *vma, struct page *page)
{
/* TODO: get coda_inode from host vma */
struct coda_inode_info *cii = ITOC(coda_inode);
/* TODO: set "written" flag in cii */
/* TODO: reset the host vma's vm_ops back
since we don't need to be called again */
if (host vma's page_mkwrite exists) {
call it and return its value
} else {
return 0;
}
}
4. In coda_file_mmap, do the saving and substitution of coda_page_mkwrite
in the vm_ops for the vma. Also save the vma to coda_inode mapping.
This is the most fragile piece of the puzzle, since we have no way of
knowing if our vm_ops is replaced by someone else later.
Making sure this can be done reliably is the hardest part of this.
Adam
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