Re: [PATCH v2 8/9] vfs: copy_file_range() can do a pagecache copy with splice

["Darrick J. Wong" <darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

On Tue, Sep 15, 2015 at 11:58:04AM -0400, Anna Schumaker wrote:
> On 09/14/2015 11:32 PM, Darrick J. Wong wrote:
> > On Fri, Sep 11, 2015 at 04:30:21PM -0400, Anna Schumaker wrote:
> >> The NFS server will need some kind offallback for filesystems that don't
> >> have any kind of copy acceleration, and it should be generally useful to
> >> have an in-kernel copy to avoid lots of switches between kernel and user
> >> space.
> >>
> >> I make this configurable by adding two new flags.  Users who only want a
> >> reflink can pass COPY_FR_REFLINK, and users who want a full data copy can
> >> pass COPY_FR_COPY.  The default (flags=0) means to first attempt a
> >> reflink, but use the pagecache if that fails.
> >>
> >> I moved the rw_verify_area() calls into the fallback code since some
> >> filesystems can handle reflinking a large range.
> >>
> >> Signed-off-by: Anna Schumaker <Anna.Schumaker-ZwjVKphTwtPQT0dZR+AlfA@public.gmane.org>
> >> ---
> >> v2:
> >> - Rename COPY_REFLINK -> COPY_FR_REFLINK
> >> - Introduce COPY_FR_COPY flag
> >> - Flags == 0 is really COPY_FR_COPY|COPY_FR_REFLINK
> >> - Drop check for invalid flags
> >> - Move call to do_splice_direct() into a new function
> >> - Move rw_verify_area() checks into the new fallback function
> >> ---
> >>  fs/read_write.c           | 56 ++++++++++++++++++++++++++++-------------------
> >>  include/linux/copy.h      |  6 +++++
> >>  include/uapi/linux/Kbuild |  1 +
> >>  include/uapi/linux/copy.h |  7 ++++++
> >>  4 files changed, 48 insertions(+), 22 deletions(-)
> >>  create mode 100644 include/linux/copy.h
> >>  create mode 100644 include/uapi/linux/copy.h
> >>
> >> diff --git a/fs/read_write.c b/fs/read_write.c
> >> index 363bd3e..ba24884 100644
> >> --- a/fs/read_write.c
> >> +++ b/fs/read_write.c
> >> @@ -7,6 +7,7 @@
> >>  #include <linux/slab.h> 
> >>  #include <linux/stat.h>
> >>  #include <linux/fcntl.h>
> >> +#include <linux/copy.h>
> >>  #include <linux/file.h>
> >>  #include <linux/uio.h>
> >>  #include <linux/fsnotify.h>
> >> @@ -1329,6 +1330,29 @@ COMPAT_SYSCALL_DEFINE4(sendfile64, int, out_fd, int, in_fd,
> >>  }
> >>  #endif
> >>  
> >> +static ssize_t vfs_copy_file_pagecache(struct file *file_in, loff_t pos_in,
> >> +				       struct file *file_out, loff_t pos_out,
> >> +				       size_t len)
> >> +{
> >> +	ssize_t ret;
> >> +
> >> +	ret = rw_verify_area(READ, file_in, &pos_in, len);
> >> +	if (ret >= 0) {
> >> +		len = ret;
> >> +		ret = rw_verify_area(WRITE, file_out, &pos_out, len);
> >> +		if (ret >= 0)
> >> +			len = ret;
> >> +	}
> >> +	if (ret < 0)
> >> +		return ret;
> >> +
> >> +	file_start_write(file_out);
> >> +	ret = do_splice_direct(file_in, &pos_in, file_out, &pos_out, len, 0);
> >> +	file_end_write(file_out);
> >> +
> >> +	return ret;
> >> +}
> >> +
> >>  /*
> >>   * copy_file_range() differs from regular file read and write in that it
> >>   * specifically allows return partial success.  When it does so is up to
> >> @@ -1338,34 +1362,17 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
> >>  			    struct file *file_out, loff_t pos_out,
> >>  			    size_t len, int flags)
> >>  {
> >> -	struct inode *inode_in;
> >> -	struct inode *inode_out;
> >>  	ssize_t ret;
> >>  
> >> -	if (flags)
> >> -		return -EINVAL;
> >> -
> >> -	/* copy_file_range allows full ssize_t len, ignoring MAX_RW_COUNT  */
> >> -	ret = rw_verify_area(READ, file_in, &pos_in, len);
> >> -	if (ret >= 0)
> >> -		ret = rw_verify_area(WRITE, file_out, &pos_out, len);
> >> -	if (ret < 0)
> >> -		return ret;
> >> +	if (flags == 0)
> >> +		flags = COPY_FR_COPY | COPY_FR_REFLINK;
> > 
> > This function must return -EINVAL if any of the undefined flags bits are
> > set.
> 
> Sure, I'll add that.
> 
> > 
> >>  
> >>  	if (!(file_in->f_mode & FMODE_READ) ||
> >>  	    !(file_out->f_mode & FMODE_WRITE) ||
> >>  	    (file_out->f_flags & O_APPEND) ||
> >> -	    !file_out->f_op || !file_out->f_op->copy_file_range)
> >> +	    !file_in->f_op)
> >>  		return -EBADF;
> >>  
> >> -	inode_in = file_inode(file_in);
> >> -	inode_out = file_inode(file_out);
> >> -
> >> -	/* make sure offsets don't wrap and the input is inside i_size */
> >> -	if (pos_in + len < pos_in || pos_out + len < pos_out ||
> >> -	    pos_in + len > i_size_read(inode_in))
> >> -		return -EINVAL;
> >> -
> >>  	if (len == 0)
> >>  		return 0;
> >>  
> >> @@ -1373,8 +1380,13 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
> >>  	if (ret)
> >>  		return ret;
> >>  
> >> -	ret = file_out->f_op->copy_file_range(file_in, pos_in, file_out, pos_out,
> >> -					      len, flags);
> >> +	ret = -EOPNOTSUPP;
> >> +	if (file_out->f_op->copy_file_range)
> >> +		ret = file_out->f_op->copy_file_range(file_in, pos_in, file_out,
> >> +						      pos_out, len, flags);
> >> +	if ((ret < 0) && (flags & COPY_FR_COPY))
> >> +		ret = vfs_copy_file_pagecache(file_in, pos_in, file_out,
> >> +					      pos_out, len);
> >>  	if (ret > 0) {
> >>  		fsnotify_access(file_in);
> >>  		add_rchar(current, ret);
> >> diff --git a/include/linux/copy.h b/include/linux/copy.h
> >> new file mode 100644
> >> index 0000000..fd54543
> >> --- /dev/null
> >> +++ b/include/linux/copy.h
> >> @@ -0,0 +1,6 @@
> >> +#ifndef _LINUX_COPY_H
> >> +#define _LINUX_COPY_H
> >> +
> >> +#include <uapi/linux/copy.h>
> >> +
> >> +#endif /* _LINUX_COPY_H */
> >> diff --git a/include/uapi/linux/Kbuild b/include/uapi/linux/Kbuild
> >> index 70ff1d9..d46830a 100644
> >> --- a/include/uapi/linux/Kbuild
> >> +++ b/include/uapi/linux/Kbuild
> >> @@ -90,6 +90,7 @@ header-y += coda_psdev.h
> >>  header-y += coff.h
> >>  header-y += connector.h
> >>  header-y += const.h
> >> +header-y += copy.h
> >>  header-y += cramfs_fs.h
> >>  header-y += cuda.h
> >>  header-y += cyclades.h
> >> diff --git a/include/uapi/linux/copy.h b/include/uapi/linux/copy.h
> >> new file mode 100644
> >> index 0000000..2da59a8
> >> --- /dev/null
> >> +++ b/include/uapi/linux/copy.h
> >> @@ -0,0 +1,7 @@
> >> +#ifndef _UAPI_LINUX_COPY_H
> >> +#define _UAPI_LINUX_COPY_H
> >> +
> >> +#define COPY_FR_COPY		(1 << 0)  /* Only do a pagecache copy.  */
> >> +#define COPY_FR_REFLINK		(1 << 1)  /* Only make a reflink.       */
> > 
> > Could I have a COPY_FR_DEDUPE flag too, please?
> > 
> > (I don't mind adding it myself when I get around to hooking up XFS, but I
> > was hoping to get it in during the first round).
> 
> I guess I can, but only iff everybody has agreed on using copy for dedupes
> instead of somethink like fallocate.

I don't see how fallocate could even handle userspace-directed deduplication
since the dedupe operation compares two file ranges and reflinks them if the
contents match whereas fallocate only operates on a single file range.

A single file range would work if userspace was telling the kernel to start a
lengthy search for all duplicates of a particular file's range, but ...
yuck.

(Granted, the btrfs extent_same ioctl lets userspace call dedupe on multiple
files simultaneously, but that's for another day.)

--D

> 
> Anna
> 
> > 
> > --D
> > 
> >> +
> >> +#endif /* _UAPI_LINUX_COPY_H */
> >> -- 
> >> 2.5.1
> >>
> >> --
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