> +     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 :))

>        * We also want to make sure the amount of IO required to do
>        * a random read is reasonably small, so we limit the size of
> -      * a compressed extent to 128k.
> +      * a compressed extent (default of 128k).

Just drop the value so that this comment doesn't need to be updated
again.

-        * a compressed extent to 128k.
+        * a compressed extent.

> +     {Opt_compr_extent_size, "compressed_extent_size=%d"},

It's even more important to make the exposed option self-documenting
than it was to get the fs_info member right.

> +                     if ((intarg == 128) || (intarg == 512)) {
> +                             info->compressed_extent_size = intarg;
> +                             printk(KERN_INFO "btrfs: compressed extent size 
> %d\n",
> +                                    info->compressed_extent_size);
> +                     } else {
> +                             printk(KERN_INFO "btrfs: "
> +                                     "Invalid compressed extent size,"
> +                                     " using default.\n");

I'd print the default value when it's used and would include a unit in
both.

> +     if (btrfs_test_opt(root, COMPR_EXTENT_SIZE))
> +             seq_printf(seq, ",compressed_extent_size=%d",
> +                        (unsigned long long)info->compressed_extent_size);

The (ull) cast doesn't match the %d format and wouldn't be needed if it
was printed with %u.

- z
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