Re: [RFC PATCH v2 2/5] fs: add RWF_ENCODED for reading/writing compressed data

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-10-21, Darrick J. Wong <darrick.wong@oracle.com> wrote:
> On Tue, Oct 22, 2019 at 05:38:31AM +1100, Aleksa Sarai wrote:
> > On 2019-10-21, Darrick J. Wong <darrick.wong@oracle.com> wrote:
> > > On Tue, Oct 15, 2019 at 11:42:40AM -0700, Omar Sandoval wrote:
> > > > From: Omar Sandoval <osandov@fb.com>
> > > > 
> > > > Btrfs supports transparent compression: data written by the user can be
> > > > compressed when written to disk and decompressed when read back.
> > > > However, we'd like to add an interface to write pre-compressed data
> > > > directly to the filesystem, and the matching interface to read
> > > > compressed data without decompressing it. This adds support for
> > > > so-called "encoded I/O" via preadv2() and pwritev2().
> > > > 
> > > > A new RWF_ENCODED flags indicates that a read or write is "encoded". If
> > > > this flag is set, iov[0].iov_base points to a struct encoded_iov which
> > > > is used for metadata: namely, the compression algorithm, unencoded
> > > > (i.e., decompressed) length, and what subrange of the unencoded data
> > > > should be used (needed for truncated or hole-punched extents and when
> > > > reading in the middle of an extent). For reads, the filesystem returns
> > > > this information; for writes, the caller provides it to the filesystem.
> > > > iov[0].iov_len must be set to sizeof(struct encoded_iov), which can be
> > > > used to extend the interface in the future. The remaining iovecs contain
> > > > the encoded extent.
> > > > 
> > > > Filesystems must indicate that they support encoded writes by setting
> > > > FMODE_ENCODED_IO in ->file_open().
> > > > 
> > > > Signed-off-by: Omar Sandoval <osandov@fb.com>
> > > > ---
> > > >  include/linux/fs.h      | 14 +++++++
> > > >  include/uapi/linux/fs.h | 26 ++++++++++++-
> > > >  mm/filemap.c            | 82 ++++++++++++++++++++++++++++++++++-------
> > > >  3 files changed, 108 insertions(+), 14 deletions(-)
> > > > 
> > > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > > index e0d909d35763..54681f21e05e 100644
> > > > --- a/include/linux/fs.h
> > > > +++ b/include/linux/fs.h
> > > > @@ -175,6 +175,9 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
> > > >  /* File does not contribute to nr_files count */
> > > >  #define FMODE_NOACCOUNT		((__force fmode_t)0x20000000)
> > > >  
> > > > +/* File supports encoded IO */
> > > > +#define FMODE_ENCODED_IO	((__force fmode_t)0x40000000)
> > > > +
> > > >  /*
> > > >   * Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector
> > > >   * that indicates that they should check the contents of the iovec are
> > > > @@ -314,6 +317,7 @@ enum rw_hint {
> > > >  #define IOCB_SYNC		(1 << 5)
> > > >  #define IOCB_WRITE		(1 << 6)
> > > >  #define IOCB_NOWAIT		(1 << 7)
> > > > +#define IOCB_ENCODED		(1 << 8)
> > > >  
> > > >  struct kiocb {
> > > >  	struct file		*ki_filp;
> > > > @@ -3088,6 +3092,11 @@ extern int sb_min_blocksize(struct super_block *, int);
> > > >  extern int generic_file_mmap(struct file *, struct vm_area_struct *);
> > > >  extern int generic_file_readonly_mmap(struct file *, struct vm_area_struct *);
> > > >  extern ssize_t generic_write_checks(struct kiocb *, struct iov_iter *);
> > > > +struct encoded_iov;
> > > > +extern int generic_encoded_write_checks(struct kiocb *, struct encoded_iov *);
> > > > +extern ssize_t check_encoded_read(struct kiocb *, struct iov_iter *);
> > > > +extern int import_encoded_write(struct kiocb *, struct encoded_iov *,
> > > > +				struct iov_iter *);
> > > >  extern int generic_remap_checks(struct file *file_in, loff_t pos_in,
> > > >  				struct file *file_out, loff_t pos_out,
> > > >  				loff_t *count, unsigned int remap_flags);
> > > > @@ -3403,6 +3412,11 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
> > > >  			return -EOPNOTSUPP;
> > > >  		ki->ki_flags |= IOCB_NOWAIT;
> > > >  	}
> > > > +	if (flags & RWF_ENCODED) {
> > > > +		if (!(ki->ki_filp->f_mode & FMODE_ENCODED_IO))
> > > > +			return -EOPNOTSUPP;
> > > > +		ki->ki_flags |= IOCB_ENCODED;
> > > > +	}
> > > >  	if (flags & RWF_HIPRI)
> > > >  		ki->ki_flags |= IOCB_HIPRI;
> > > >  	if (flags & RWF_DSYNC)
> > > > diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> > > > index 379a612f8f1d..ed92a8a257cb 100644
> > > > --- a/include/uapi/linux/fs.h
> > > > +++ b/include/uapi/linux/fs.h
> > > > @@ -284,6 +284,27 @@ struct fsxattr {
> > > >  
> > > >  typedef int __bitwise __kernel_rwf_t;
> > > >  
> > > > +enum {
> > > > +	ENCODED_IOV_COMPRESSION_NONE,
> > > > +	ENCODED_IOV_COMPRESSION_ZLIB,
> > > > +	ENCODED_IOV_COMPRESSION_LZO,
> > > > +	ENCODED_IOV_COMPRESSION_ZSTD,
> > > > +	ENCODED_IOV_COMPRESSION_TYPES = ENCODED_IOV_COMPRESSION_ZSTD,
> > > > +};
> > > > +
> > > > +enum {
> > > > +	ENCODED_IOV_ENCRYPTION_NONE,
> > > > +	ENCODED_IOV_ENCRYPTION_TYPES = ENCODED_IOV_ENCRYPTION_NONE,
> > > > +};
> > > > +
> > > > +struct encoded_iov {
> > > > +	__u64 len;
> > > > +	__u64 unencoded_len;
> > > > +	__u64 unencoded_offset;
> > > > +	__u32 compression;
> > > > +	__u32 encryption;
> > > 
> > > Can we add some must-be-zero padding space at the end here for whomever
> > > comes along next wanting to add more encoding info?
> > 
> > I would suggest to copy the extension design of copy_struct_from_user().
> > Adding must-be-zero padding is a less-ideal solution to the extension
> > problem than length-based extension.
> 
> Come to think of it, you /do/ have to specify iov_len so... yeah, do
> that instead; we can always extend the structure later.
> 
> > Also (I might be wrong) but shouldn't the __u64s be __aligned_u64 (as
> > with syscall structure arguments)?
> 
> <shrug> No idea, that's the first I've heard of that type and it doesn't
> seem to be used by the fs code.  Why would we care about alignment for
> an incore structure?

When passing u64s from userspace, it's generally considered a good idea
to use __aligned_u64 -- the main reason is that 32-bit userspace on a
64-bit kernel will use different structure alignment for 64-bit fields.

This means you'd need to implement a bunch of COMPAT_SYSCALL-like
handling for that case. It's much simpler to use __aligned_u64 (and on
the plus side I don't think you need to add any fields to ensure the
padding is zero).

> > 
> > > (And maybe a manpage and some basic testing, to reiterate Dave...)
> > > 
> > > --D
> > > 
> > > > +};
> > > > +
> > > >  /* high priority request, poll if possible */
> > > >  #define RWF_HIPRI	((__force __kernel_rwf_t)0x00000001)
> > > >  
> > > > @@ -299,8 +320,11 @@ typedef int __bitwise __kernel_rwf_t;
> > > >  /* per-IO O_APPEND */
> > > >  #define RWF_APPEND	((__force __kernel_rwf_t)0x00000010)
> > > >  
> > > > +/* encoded (e.g., compressed or encrypted) IO */
> > > > +#define RWF_ENCODED	((__force __kernel_rwf_t)0x00000020)
> > > > +
> > > >  /* mask of flags supported by the kernel */
> > > >  #define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
> > > > -			 RWF_APPEND)
> > > > +			 RWF_APPEND | RWF_ENCODED)
> > > >  
> > > >  #endif /* _UAPI_LINUX_FS_H */
> > > > diff --git a/mm/filemap.c b/mm/filemap.c
> > > > index 1146fcfa3215..d2e6d9caf353 100644
> > > > --- a/mm/filemap.c
> > > > +++ b/mm/filemap.c
> > > > @@ -2948,24 +2948,15 @@ static int generic_write_check_limits(struct file *file, loff_t pos,
> > > >  	return 0;
> > > >  }
> > > >  
> > > > -/*
> > > > - * Performs necessary checks before doing a write
> > > > - *
> > > > - * Can adjust writing position or amount of bytes to write.
> > > > - * Returns appropriate error code that caller should return or
> > > > - * zero in case that write should be allowed.
> > > > - */
> > > > -inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
> > > > +static int generic_write_checks_common(struct kiocb *iocb, loff_t *count)
> > > >  {
> > > >  	struct file *file = iocb->ki_filp;
> > > >  	struct inode *inode = file->f_mapping->host;
> > > > -	loff_t count;
> > > > -	int ret;
> > > >  
> > > >  	if (IS_SWAPFILE(inode))
> > > >  		return -ETXTBSY;
> > > >  
> > > > -	if (!iov_iter_count(from))
> > > > +	if (!*count)
> > > >  		return 0;
> > > >  
> > > >  	/* FIXME: this is for backwards compatibility with 2.4 */
> > > > @@ -2975,8 +2966,21 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
> > > >  	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
> > > >  		return -EINVAL;
> > > >  
> > > > -	count = iov_iter_count(from);
> > > > -	ret = generic_write_check_limits(file, iocb->ki_pos, &count);
> > > > +	return generic_write_check_limits(iocb->ki_filp, iocb->ki_pos, count);
> > > > +}
> > > > +
> > > > +/*
> > > > + * Performs necessary checks before doing a write
> > > > + *
> > > > + * Can adjust writing position or amount of bytes to write.
> > > > + * Returns a negative errno or the new number of bytes to write.
> > > > + */
> > > > +inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
> > > > +{
> > > > +	loff_t count = iov_iter_count(from);
> > > > +	int ret;
> > > > +
> > > > +	ret = generic_write_checks_common(iocb, &count);
> > > >  	if (ret)
> > > >  		return ret;
> > > >  
> > > > @@ -2985,6 +2989,58 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
> > > >  }
> > > >  EXPORT_SYMBOL(generic_write_checks);
> > > >  
> > > > +int generic_encoded_write_checks(struct kiocb *iocb,
> > > > +				 struct encoded_iov *encoded)
> > > > +{
> > > > +	loff_t count = encoded->unencoded_len;
> > > > +	int ret;
> > > > +
> > > > +	ret = generic_write_checks_common(iocb, &count);
> > > > +	if (ret)
> > > > +		return ret;
> > > > +
> > > > +	if (count != encoded->unencoded_len) {
> > > > +		/*
> > > > +		 * The write got truncated by generic_write_checks_common(). We
> > > > +		 * can't do a partial encoded write.
> > > > +		 */
> > > > +		return -EFBIG;
> > > > +	}
> > > > +	return 0;
> > > > +}
> > > > +EXPORT_SYMBOL(generic_encoded_write_checks);
> > > > +
> > > > +ssize_t check_encoded_read(struct kiocb *iocb, struct iov_iter *iter)
> > > > +{
> > > > +	if (!(iocb->ki_filp->f_flags & O_ENCODED))
> > > > +		return -EPERM;
> > > > +	if (iov_iter_single_seg_count(iter) != sizeof(struct encoded_iov))
> > > > +		return -EINVAL;
> > > > +	return iov_iter_count(iter) - sizeof(struct encoded_iov);
> > > > +}
> > > > +EXPORT_SYMBOL(check_encoded_read);
> > > > +
> > > > +int import_encoded_write(struct kiocb *iocb, struct encoded_iov *encoded,
> > > > +			 struct iov_iter *from)
> > > > +{
> > > > +	if (!(iocb->ki_filp->f_flags & O_ENCODED))
> > > > +		return -EPERM;
> > > > +	if (iov_iter_single_seg_count(from) != sizeof(*encoded))
> > > > +		return -EINVAL;
> > > > +	if (copy_from_iter(encoded, sizeof(*encoded), from) != sizeof(*encoded))
> > > > +		return -EFAULT;
> > > > +	if (encoded->compression == ENCODED_IOV_COMPRESSION_NONE &&
> > > > +	    encoded->encryption == ENCODED_IOV_ENCRYPTION_NONE)
> > > > +		return -EINVAL;
> > > > +	if (encoded->compression > ENCODED_IOV_COMPRESSION_TYPES ||
> > > > +	    encoded->encryption > ENCODED_IOV_ENCRYPTION_TYPES)
> > > > +		return -EINVAL;
> > > > +	if (encoded->unencoded_offset >= encoded->unencoded_len)
> > > > +		return -EINVAL;
> > > > +	return 0;
> > > > +}
> > > > +EXPORT_SYMBOL(import_encoded_write);
> > > > +
> > > >  /*
> > > >   * Performs necessary checks before doing a clone.
> > > >   *

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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