Re: Recvfile patch used for Samba.

[Jeff Layton]

On Tue, 23 Jul 2013 17:10:27 +1000
Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> On Mon, Jul 22, 2013 at 02:57:38PM -0700, Jeremy Allison wrote:
> > Hi Steve and Jeff (and others).
> > 
> > Here is a patch that Samba vendors have been using
> > to implement recvfile (copy directly from socket
> > to file). It can improve write performance on boxes
> > by a significant amount (10% or more).
> 
> Is that all? It's just moving the memcpy() into the kernel?
> 
> > I'm not qualified to evaluate this code, can someone
> > who is (hi there Steve and Jeff :-) take a look at
> > this and see if it's work shepherding into the kernel ?
> 
> It's pretty nasty.
> 
> > 
> > Cheers,
> > 
> > 	Jeremy.
> 
> > diff -urp linux-2.6.37-rc5.orig/fs/splice.c linux-2.6.37-rc5/fs/splice.c
> > --- linux-2.6.37-rc5.orig/fs/splice.c	2010-12-06 20:09:04.000000000 -0800
> > +++ linux-2.6.37-rc5/fs/splice.c	2010-12-07 16:16:48.000000000 -0800
> > @@ -31,6 +31,7 @@
> >  #include <linux/uio.h>
> >  #include <linux/security.h>
> >  #include <linux/gfp.h>
> > +#include <net/sock.h>
> >  
> >  /*
> >   * Attempt to steal a page from a pipe buffer. This should perhaps go into
> > @@ -1387,6 +1388,141 @@ static long do_splice(struct file *in, l
> >  	return -EINVAL;
> >  }
> >  
> > +static ssize_t do_splice_from_socket(struct file *file, struct socket *sock,
> > +				     loff_t __user *ppos, size_t count)
> > +{		
> > +	struct address_space *mapping = file->f_mapping;
> > +	struct inode	*inode = mapping->host;
> > +	loff_t pos;
> > +	int count_tmp;
> > +	int err = 0;
> > +	int cPagePtr = 0;		
> > +	int cPagesAllocated = 0;
> 
> camel case.
> 
> > +	struct recvfile_ctl_blk rv_cb[MAX_PAGES_PER_RECVFILE];
> 
> MAX_PAGES_PER_RECVFILE = 32, and the structure is 32 bytes on 64
> bit. So 1k of stack usage right there. Bad.
> 
> > +	struct kvec iov[MAX_PAGES_PER_RECVFILE];
> 
> And that's another 512 bytes of stack. Way too much.
> 
> > +	struct msghdr msg;
> 
> Another ~52 bytes of stack.
> 
> > +	long rcvtimeo;
> > +	int ret;
> > +
> > +	if(copy_from_user(&pos, ppos, sizeof(loff_t)))
> > +		return -EFAULT;
> > +
> > +	if(count > MAX_PAGES_PER_RECVFILE * PAGE_SIZE) {
> > +		printk("%s: count(%u) exceeds maxinum\n", __func__, count);
> > +		return -EINVAL;
> > +	}    
> > +	mutex_lock(&inode->i_mutex);
> 
> Locking on write needs to be passed to filesystems, not done by
> generic code. i.e. this isn't sufficient for XFS, and may also be
> problematic for btrfs.
> 
> > +
> > +	vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
> 
> That doesn't exist anymore - it was removed over a year ago....
> 
> > +
> > +	/* We can write back this queue in page reclaim */
> > +	current->backing_dev_info = mapping->backing_dev_info;
> > +
> > +	err = generic_write_checks(file, &pos, &count, S_ISBLK(inode->i_mode));
> > +	if (err != 0 || count == 0)
> > +		goto done;
> > +
> > +	file_remove_suid(file);
> > +	file_update_time(file);	
> 
> error checking?
> 
> > +
> > +	count_tmp = count;
> > +	do {
> > +		unsigned long bytes;	/* Bytes to write to page */
> > +		unsigned long offset;	/* Offset into pagecache page */
> > +		struct page *pageP;
> 
> camel case
> 
> > +		void *fsdata;
> > +
> > +		offset = (pos & (PAGE_CACHE_SIZE - 1));
> > +		bytes = PAGE_CACHE_SIZE - offset;
> > +		if (bytes > count_tmp)
> > +			bytes = count_tmp;
> > +		ret = mapping->a_ops->write_begin(file, mapping, pos, bytes,
> > +						  AOP_FLAG_UNINTERRUPTIBLE,
> > +						  &pageP, &fsdata);
> > +
> > +		if (unlikely(ret)) {
> > +			err = ret;
> > +			for(cPagePtr = 0; cPagePtr < cPagesAllocated; cPagePtr++) {
> > +				kunmap(rv_cb[cPagePtr].rv_page);
> > +				ret = mapping->a_ops->write_end(file, mapping,
> > +								rv_cb[cPagePtr].rv_pos,
> > +								rv_cb[cPagePtr].rv_count,
> > +								rv_cb[cPagePtr].rv_count,
> > +								rv_cb[cPagePtr].rv_page,
> > +								rv_cb[cPagePtr].rv_fsdata);
> > +			}
> > +			goto done;
> > +		}
> > +		rv_cb[cPagesAllocated].rv_page = pageP;
> > +		rv_cb[cPagesAllocated].rv_pos = pos;
> > +		rv_cb[cPagesAllocated].rv_count = bytes;
> > +		rv_cb[cPagesAllocated].rv_fsdata = fsdata;
> > +		iov[cPagesAllocated].iov_base = kmap(pageP) + offset;
> > +		iov[cPagesAllocated].iov_len = bytes;
> > +		cPagesAllocated++;
> > +		count_tmp -= bytes;
> > +		pos += bytes;
> > +	} while (count_tmp);
> 
> So, we are nesting up to 32 page locks here. That's bad. And we are
> nesting kmap() calls for all the pages individually - is that even
> safe to do?
> 

You can do it, but if you run out of kmap space, it'll hang and you'll
be screwed. Wouldn't take much either -- just a few of these calls
running in parallel would be enough to deadlock all of them.

> So, what happens when we've got 16 pages in, and the filesystem has
> allocated space for those 16 blocks, and we get ENOSPC on the 17th?
> Sure, you undo the state here, but what about the 16 blocks that the
> filesystem has allocated to this file? There's no notification to
> the filesystem that they need to be truncated away because the write
> failed....
> 
> > +
> > +	/* IOV is ready, receive the date from socket now */
> > +	msg.msg_name = NULL;
> > +	msg.msg_namelen = 0;
> > +	msg.msg_iov = (struct iovec *)&iov[0];
> > +	msg.msg_iovlen = cPagesAllocated ;
> > +	msg.msg_control = NULL;
> > +	msg.msg_controllen = 0;
> > +	msg.msg_flags = MSG_KERNSPACE;
> > +	rcvtimeo = sock->sk->sk_rcvtimeo;    
> > +	sock->sk->sk_rcvtimeo = 8 * HZ;
> 
> We can hold the inode and the pages locked for 8 seconds?
> 
> I'll stop there. This is fundamentally broken. It's an attempt to do
> a multi-page write operation without any of the supporting
> structures needed to handle the failure cases properly.  The nested
> page locking has "deadlock" written all over it, and the lack of
> partial failure handling shouts "data corruption" and "stale data
> exposure" to me. The fact it can block for up to 8 seconds waiting
> for network shenanigans to be completed while holding lots of locks
> is going to cause all sorts of problems under memory pressure.
> 
> Not to mention it means that all memory allocations in the msgrcv
> path need to be done with GFP_NOFS, because GFP_KERNEL allocations
> are almost guaranteed to deadlock on the locked pages this path
> already holds....
> 

Yep, pretty broken.

A better approach might be to hook this call up to the sk_write_space
callback on the socket. Have it block until there is something to
actually be read, and then do the loop over the pages one at a time at
that point until all of the ready data is read (non-blocking) into the
pages. When you run out of data, unlock everything and go back to
blocking until there is more or until you hit the "len" passed into the
syscall.

Locking and error handling with that approach would still be pretty
tricky, however...

-- 
Jeff Layton <jlayton@xxxxxxxxxx>
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