Re: [PATCH 2/2] btrfs: add ioctl for directly writing compressed data

[Johannes Thumshirn <jthumshirn@suse.de>]

On 05/09/2019 14:16, Johannes Thumshirn wrote:
> On 05/09/2019 04:10, Dave Chinner wrote:
>> On Wed, Sep 04, 2019 at 12:13:26PM -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.
>>
>> So why can't you do this with pwritev2()? Heaps of flags, and
>> use a second iovec to hold the decompressed size of the previous
>> iovec. i.e.
>>
>> 	iov[0].iov_base = compressed_data;
>> 	iov[0].iov_len = compressed_size;
>> 	iov[1].iov_base = NULL;
>> 	iov[1].iov_len = uncompressed_size;
>> 	pwritev2(fd, iov, 2, offset, RWF_COMPRESSED_ZLIB);
>>
>> And you don't need to reinvent pwritev() with some whacky ioctl that
>> is bound to be completely screwed up is ways not noticed until
>> someone else tries to use it...
>>
>> I'd also suggest atht if we are going to be able to write compressed
>> data directly, then we should be able to read them as well directly
>> via preadv2()....
> 
> 
> While I'm with you on this from a design PoV, one question remains:
> What to do with the file systems that do not support compression?
> 
> Currently there's only a kernel global check for known RWF_* flags in
> kiocb_set_rw_flags().
> 
> So we need a way for the individual file systems to opt into the new
> RWF_COMPRESSED_* flags and fail early if they're not supported, that
> will cause a lot of code churn if we cannot do it in the vfs layer.
> 
> From the 52 ->write_iter callbacks in fs/ 32 are not using
> generic_file_write_iter(). So we'd have to patch 33 functions (+/- 1-2
> because my grep | wc fu isn't the best).
> 

This (from Anthony Iliopoulos) should be sufficient:

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 58a18ed11546..86f7ff0387d7 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -3299,7 +3299,7 @@ static loff_t btrfs_file_llseek(struct file *file,

 static int btrfs_file_open(struct inode *inode, struct file *filp)
 {
-       filp->f_mode |= FMODE_NOWAIT;
+       filp->f_mode |= (FMODE_NOWAIT|FMODE_CAN_COMPRESS);
        return generic_file_open(inode, filp);
 }

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 997a530ff4e9..1b59e795f448 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3357,6 +3357,11 @@ static inline int kiocb_set_rw_flags(struct kiocb
                ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
        if (flags & RWF_APPEND)
                ki->ki_flags |= IOCB_APPEND;
+       if (flags & RWF_COMPRESSED) {
+               if (!(ki->ki_filp->fmode & FMODE_CAN_COMPRESS))
+                       return -EOPNOTSUPP;
+               ki->ki_flags |= IOCB_COMPRESSED;
+       }
        return 0;
 }


-- 
Johannes Thumshirn                            SUSE Labs Filesystems
jthumshirn@suse.de                                +49 911 74053 689
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