Re: ENOSPC returned during writepages

Mingming Cao
2008-08-20T23:58:49Z

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.