On Wed, 28 Nov 2012, Linus Torvalds wrote:
> On Wed, Nov 28, 2012 at 12:03 PM, Linus Torvalds > <torva...@linux-foundation.org> wrote: > > > > mmap() is in *no* way special. The exact same thing happens for > > regular read/write. Yet somehow the mmap code is special-cased, while > > the normal read-write code is not. > > I just double-checked, because it's been a long time since I actually > looked at the code. > > But yeah, block device read/write uses the pure page cache functions. > IOW, it has the *exact* same IO engine as mmap() would have. > > So here's my suggestion: > > - get rid of *all* the locking in aio_read/write and the splice paths > - get rid of all the stupid mmap games > > - instead, add them to the functions that actually use > "blkdev_get_block()" and "blkdev_get_blocks()" and nowhere else. > > That's a fairly limited number of functions: > blkdev_{read,write}page(), blkdev_direct_IO() and > blkdev_write_{begin,end}() > > Doesn't that sounds simpler? And more logical: it protects the actual > places that use the block size of the device. > > I dunno. Maybe there is some fundamental reason why the above is > broken, but it seems to be a much simpler approach. Sure, you need to > guarantee that the people who get the write-lock cannot possibly cause > IO while holding it, but since the only reason to get the write lock > would be to change the block size, that should be pretty simple, no? > > Yeah, yeah, I'm probably missing something fundamental, but the above > sounds like the simple approach to fixing things. Aiming for having > the block size read-lock be taken by the things that pass in the > block-size itself. > > It would be nice for things to be logical and straightforward. > > Linus The problem with this approach is that it is very easy to miss points where it is assumed that the block size doesn't change - and if you miss a point, it results in a hidden bug that has a little possibility of being found. For example, __block_write_full_page and __block_write_begin do if (!page_has_buffers(page)) { create_empty_buffers... } and then they do WARN_ON(bh->b_size != blocksize) err = get_block(inode, block, bh, 1) ... so if the buffers were left over from some previous call to create_empty_buffers with a different blocksize, that WARN_ON is trigged. And it's not only a harmless warning - now bh->b_size is left set to the old block size, but bh->b_blocknr is set to a number, that was calculated according to the new block size - and when you submit that buffer with submit_bh, it is written to the wrong place! Now, prove that there are no more bugs like this. Locking the whole read/write/mmap operations is crude, but at least it can be done without thorough review of all the memory management code. Mikulas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/