On Wed, Feb 06, 2013 at 05:41:42PM -0600, Mitch Harder wrote:
> On Wed, Feb 6, 2013 at 12:46 PM, Zach Brown <z...@redhat.com> wrote:
> >> + unsigned compressed_extent_size;
> >
> > It kind of jumps out that this mentions neither that it's the max nor
> > that it's in KB. How about max_compressed_extent_kb?
> >
> >> + fs_info->compressed_extent_size = 128;
> >
> > I'd put a DEFAULT_MAX_EXTENTS up by the MAX_ definition instead of using
> > a raw 128 here.
> >
> >> + unsigned long max_compressed = root->fs_info->compressed_extent_size
> >> * 1024;
> >> + unsigned long max_uncompressed =
> >> root->fs_info->compressed_extent_size * 1024;
> >
> > (so max_compressed is in bytes)
> >
> >> nr_pages = (end >> PAGE_CACHE_SHIFT) - (start >> PAGE_CACHE_SHIFT) +
> >> 1;
> >> - nr_pages = min(nr_pages, (128 * 1024UL) / PAGE_CACHE_SIZE);
> >> + nr_pages = min(nr_pages, (max_compressed * 1024UL) /
> >> PAGE_CACHE_SIZE);
> >
> > (and now that expression adds another * 1024, allowing {128,512}MB
> > extents :))
> >
>
> Yuk! I'm surprised this never manifested as a problem during testing.
Answer for the curious:
cow_file_range_async()
1076 if (BTRFS_I(inode)->flags & BTRFS_INODE_NOCOMPRESS)
1077 cur_end = end;
1078 else
1079 cur_end = min(end, start + 512 * 1024 - 1);
in case of a compressed extent the end is at most 512K and the range
[start,end) processed in comprssed_file_range() calculates nr_pages from
that range.
So, 512 should be replaced wit max_compressed_extent_kb value. I've
experimented with higher values, it starts to break with 1024. The
errors seem to be around calculating block checksums and they just do
not fit into one page, roughly:
blocks = 1M / 4096 = 256
csum bytes = csum * blocks = 4 * 256 = 4096
leaf headers + csum bytes > PAGE_SIZE and slab debug reports corruption
max at 768 worked fine, the safe maximum is ~900.
david
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