Re: [RFC PATCH 5/5] Btrfs: add ioctl for directly writing compressed data

[Omar Sandoval <osandov@osandov.com>]

On Tue, Aug 27, 2019 at 07:57:41AM -0400, Josef Bacik wrote:
> On Tue, Aug 27, 2019 at 09:26:21AM +0300, Nikolay Borisov wrote:
> > 
> > 
> > On 27.08.19 г. 0:36 ч., Josef Bacik wrote:
> > > On Thu, Aug 15, 2019 at 02:04:06PM -0700, Omar Sandoval wrote:
> > >> From: Omar Sandoval <osandov@fb.com>
> > >>
> > >> This adds an API for writing compressed data directly to the filesystem.
> > >> The use case that I have in mind is send/receive: currently, when
> > >> sending data from one compressed filesystem to another, the sending side
> > >> decompresses the data and the receiving side recompresses it before
> > >> writing it out. This is wasteful and can be avoided if we can just send
> > >> and write compressed extents. The send part will be implemented in a
> > >> separate series, as this ioctl can stand alone.
> > >>
> > >> The interface is essentially pwrite(2) with some extra information:
> > >>
> > >> - The input buffer contains the compressed data.
> > >> - Both the compressed and decompressed sizes of the data are given.
> > >> - The compression type (zlib, lzo, or zstd) is given.
> > >>
> > >> A more detailed description of the interface, including restrictions and
> > >> edge cases, is included in include/uapi/linux/btrfs.h.
> > >>
> > >> The implementation is similar to direct I/O: we have to flush any
> > >> ordered extents, invalidate the page cache, and do the io
> > >> tree/delalloc/extent map/ordered extent dance. From there, we can reuse
> > >> the compression code with a minor modification to distinguish the new
> > >> ioctl from writeback.
> > >>
> > > 
> > > I've looked at this a few times, the locking and space reservation stuff look
> > > right.  What about encrypted send/recieve?  Are we going to want to use this to
> > > just blind copy encrypted data without having to decrypt/re-encrypt?  Should
> > > this be taken into consideration for this interface?  I'll think more about it,
> > > but I can't really see any better option than this.  Thanks,
> > 
> > The main problem is we don't have encryption implemented. And one of the
> > larger aspects of the encryption support is going to be how we are
> > storing the encryption keys. E.g. should they be part of the send
> > format? Or are we going to limit send/receive based on whether the
> > source/dest have transferred encryption keys out of line?
> > 
> 
> Subvolume encryption will be coming soon, but I'm less worried about the
> mechanics of how that will be used and more worried about making this interface
> work for that eventual future.  I assume we'll want to be able to just blind
> copy the encrypted data instead of decrypting into the send stream and then
> re-encrypting on the other side.  Which means we'll have two uses for this
> interface, and I want to make sure we're happy with it before it gets merged.
> Thanks,
> 
> Josef

Right, I think the only way to do this would be to blindly send
encrypted data, and leave the key management to a higher layer.

Looking at the ioctl definition:

struct btrfs_ioctl_compressed_pwrite_args {
        __u64 offset;           /* in */
        __u32 orig_len;         /* in */
        __u32 compressed_len;   /* in */
        __u32 compress_type;    /* in */
        __u32 reserved[9];
        void __user *buf;       /* in */
} __attribute__ ((__packed__));

I think there are enough reserved fields in there for, e.g., encryption
type, any key management-related things we might need to stuff in, etc.
But the naming would be pretty bad if we extended it this way. Maybe
compressed write -> raw write, orig_len -> num_bytes, compressed_len ->
disk_num_bytes?

struct btrfs_ioctl_raw_pwrite_args {
        __u64 offset;           /* in */
        __u32 num_bytes;        /* in */
        __u32 disk_num_bytes;   /* in */
        __u32 compress_type;    /* in */
        __u32 reserved[9];
        void __user *buf;       /* in */
} __attribute__ ((__packed__));

Besides the naming, I don't think anything else would need to change for
now. And if we decide that we don't want encrypted send/receive, then
fine, this naming is still okay.