On Thu, Nov 29, 2012 at 07:13:02PM -0700, Linus Torvalds wrote: > On Thu, Nov 29, 2012 at 5:16 PM, Chris Mason <chris.mason@xxxxxxxxxxxx> wrote: > > > > I searched through filemap.c for the magic i_size check that would let > > us get away with ignoring i_blkbits in get_blocks, but its just not > > there. The whole fallback-to-buffered scheme seems to rely on > > get_blocks checking for i_size. I really hope I'm just missing > > something. > > So generic_write_checks() limits the size to i_size at for writes (and > for "isblk"). Great, that's what I was missing. > > Sure, then it will do the buffered part after that, but that should > all be fine anyway, since by then we use the normal page cache. > > For reads, generic_file_aio_read() will check pos < size, but doesn't > seem to actually limit the size of the iovec. I couldn't explain that either. > > I'm not sure why it doesn't just do "iov_shorten()". > > Anyway, having looked at actually passing in the block size to > get_block(), I can say that is a horrible idea. There are tons of > get_block functions (for various filesystems), and *none* of them > really want the block size, because they tend to work on block > indexes. And if they do want the block size, they'll just get it from > the inode or sb, since they are filesystems and it's all stable. > > So the *only* of the places that would want the block size is > fs/block_dev.c. And the callers really already seem to do the i_size > check, although they sometimes do it badly. And since there are fewer > callers than there are get_block() implementations, I think we should > just fix the callers and be done with it. > > Those generic_file_aio_read/write() functions in fs/direct-io.c really > just seem to be badly written. The fact that they may depend on the > i_size check in get_blocks() is sad, but I think we should fix it and > just remove the check for block devices. That's going to simplify so > much.. > > I updated the 'block-dev' branch to have that simpler fs/block_dev.c > model instead. I'll look at the iovec shortening later. It's a > non-fast-forward thing, look out! > > (I actually think we should just add the max-offset check to > rw_copy_check_uvector(). That one already does the MAX_RW_COUNT thing, > and we could make it do a max_offset check as well). This is definitely easier, and I can't see any reason not to do it. I'm used to get_block being expensive and so it didn't even cross my mind. We can benchmark things just to make sure. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
