Re: ENOSPC returned during writepages

[Mingming Cao <cmm@us.ibm.com>]

在 2008-08-20三的 17:42 -0600，Andreas Dilger写道：
> On Aug 20, 2008  16:22 -0700, Mingming Cao wrote:
> > ext4: fall back to non delalloc mode if filesystem is almost full
> > From: Mingming Cao <cmm@us.ibm.com>
> > 
> > In the case of filesystem is close to full (free blocks is below 
> > the watermark NRCPUS *4) and there is not enough to reserve blocks for
> > delayed allocation, instead of return user back with ENOSPC error, with
> > this patch, it tries to fall back to non delayed allocation mode.
> 
> I don't think that making a low watermark of only 4 blocks is enough,
> because each of the per-CPU counters could be off by as much as FBC_BATCH.
> I think dropping delalloc support earlier is safer, something like
> (FBC_BATCH * NR_CPUS).
> 
Okay, make sense.

> > +static int ext4_write_begin_nondelalloc(struct file *file,
> > +				struct address_space *mapping,
> > +				loff_t pos, unsigned len, unsigned flags,
> > +				struct page **pagep, void **fsdata)
> > +{
> > +	struct inode *inode = mapping->host;
> > +
> > +	/* turn off delalloc for this inode*/
> > +	ext4_set_aops(inode, 0);
> > +
> > +	return mapping->a_ops->write_begin(file, mapping, pos, len,
> > +					   flags, pagep, fsdata);
> > +}
> 
> Hmm, I don't understand this - isn't delalloc already off here, because
> this is "ext4_write_begin_nondelalloc()"?
> 

This function probably should be called
ext4_wb_fall_back_to_nondelalloc().  it is called when we detect ENOSPC
and trying to fall back to non delalloc.

This function eventually will call nondelalloc write_begin function
ext4_write_begin().

> > +void ext4_set_aops(struct inode *inode, int delalloc)
> >  {
> > +	if (test_opt(inode->i_sb, DELALLOC)) {
> > +		if (ext4_has_free_blocks(EXT4_SB(inode->i_sb),
> > +			 EXT4_MIN_FREE_BLOCKS) > EXT4_MIN_FREE_BLOCKS)
> > +			delalloc = 0;
> > +
> > +		if (delalloc) {
> > +			inode->i_mapping->a_ops = &ext4_da_aops;
> > +			return;
> > +		} else
> > +			printk(KERN_INFO "filesystem is close to full, "
> > +				"delayed allocation is turned off for "
> > +				" inode %lu\n", inode->i_ino);
> > +	}
> 
> Also, if you are doing this by changing the aops on the inode, isn't
> it possible that a large write starts outside the EXT4_MIN_FREE_BLOCKS
> boundary and then still runs out of space without changing the aops?
> 

> Instead it is maybe better to do the check at the start of
> ext4_da_write_begin() and if it fails then call the non-delalloc
> write_begin from there?
> 

Yeah that's better.

But I realize  a problem. Actually now I think we can't fall back to
nondelalloc mode if the inode has any dirty pages in the page cache, as
those pages need delalloc aops ->ext4_da_writepages() to handle delayed
allocation writeout..


> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
> 

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