On Thu, 8 Feb 2007 11:21:02 +0100 Jan Kara <jack@xxxxxxx> wrote:
> On Thu 08-02-07 01:45:29, Andrew Morton wrote:
> <snip>
> > > I though Andreas meant "any write changes" - i.e. you check that noone
> > > has open file descriptor for writing and block any new open for writing.
> > > That can be done quite easily.
> > > Anyway, I agree with you that userspace solution to a possible page
> > > cache pollution is preferable after thinking about it for a while.
> > > As I've been thinking about it, we could actually do the copying
> > > from user space. We could do something like:
> > > block any writes to file (as I described above)
> > > craft new inode with blocks allocated as we want (using preallocation,
> > > we should mostly have the kernel infrastructure we need)
> > > copy data using splice syscall
> > > call the kernel to switch data
> > >
> >
> > I don't think we need to block any writes to any file or anything.
> >
> > To move a page within a file:
> >
> > fd = open(file);
> > p = mmap(fd);
> > the_page_was_in_core = mincore(p, offset);
> > munmap(p);
> > ioctl(fd, ..., new_block);
> >
> > <kernel>
> > read_cache_page(inode, offset);
> > lock_page(page);
> > if (try_to_free_buffers(page)) {
> > <relocate the page>
> > set_page_dirty(page);
> > }
> > unlock_page(page);
> >
> > if (the_page_was_in_core) {
> > sync_file_range(fd, offset SYNC_FILE_RANGE_WAIT_BEFORE|
> > SYNC_FILE_RANGE_WRITE|
> > SYNC_FILE_RANGE_WAIT_AFTER);
> > fadvise(fd, offset, FADV_DONTNEED);
> > }
> >
> > completely coherent with pagecache, quite safe in the presence of mmap,
> > mlock, O_DIRECT, everything else. Also fully journallable in-kernel.
> Yes, this is the simple way. But I see two disadvantages:
> 1) You'd like to relocate metadata (indirect blocks) too.
Well. Do we really? Are we looking for a 100% solution here, or a 90% one?
Relocating data is the main thing. After that, yeah, relocating metadata,
inodes and directories is probably a second-order thing.
> For that you need
> a different mechanism.
I suspect a similar approach will work there: load and lock the
buffer_heads (or maybe just the top-level buffer_head) and then alter their
contents. It could be that verify_chain() will just magically do the right
thing there, but some changes might be needed.
> In my approach, you can mostly assume you've got
> sanely laid out metadata and so the existence of such mechanism is not
> so important.
> 2) You'd like to allocate new blocks in big chunks. So your kernel function
> should rather take a range. Also when you fail in the middle of
> relocating a file (for example the block you'd like to use is already
> taken by someone else), I find it nice if you can return at least to the
> original state. But that's probably not important.
Well yes, that was a minimal sketch.
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