[PATCH RFC v5 11/11] docs: ext4.rst: Document encoding and case-insensitive

[Gabriel Krisman Bertazi <krisman@collabora.com>]

From: Gabriel Krisman Bertazi <krisman@collabora.co.uk>

Introduces the encoding-awareness and case-insensitive features on ext4
for system administrators.  Explain the minimum of design decisions that
are important for sysadmins wanting to enable this feature.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
---
 Documentation/admin-guide/ext4.rst | 41 ++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/Documentation/admin-guide/ext4.rst b/Documentation/admin-guide/ext4.rst
index e506d3dae510..4e08d0309f1e 100644
--- a/Documentation/admin-guide/ext4.rst
+++ b/Documentation/admin-guide/ext4.rst
@@ -91,10 +91,51 @@ Currently Available
 * large block (up to pagesize) support
 * efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
   the ordering)
+* Encoding aware file names
+* Case insensitive file name lookups
 
 [1] Filesystems with a block size of 1k may see a limit imposed by the
 directory hash tree having a maximum depth of two.
 
+Encoding-aware file names and case-insensitive lookups
+======================================================
+
+Ext4 optionally supports filesystem-wide charset knowledge when handling
+file names, which allows the user to perform file system lookups using
+charset equivalent versions of the same file name, and optionally ensure
+that no invalid names are held by the filesystem.  charset encoding
+awareness is also essential for performing case-insensitive lookups,
+because it is what defines the casefold operation.
+
+The case-insensitive file name lookup feature is supported in a smaller
+granularity, on a per-directory basis, allowing the user to mix
+case-insensitive and case-sensitive directories in the same filesystem.
+It is enabled by flipping a file attribute on an empty directory.  For
+the reason stated above, the filesystem must have encoding enabled to
+use this feature.
+
+Both encoding-awareness and case-awareness are name-preserving on the
+disk, meaning that the file name provided by userspace is a
+byte-per-byte match to what is actually written in the disk.  The
+Unicode normalization format used by the kernel is thus an internal
+representation, and not exposed to the userspace nor to the disk, with
+the important exception of disk hashes, used on large directories with
+DX feature.  On DX directories, the hash must be calculated using the
+normalized version of the filename, meaning that the normalization
+format used actually has an impact on where the directory entry is
+stored.
+
+When we change from viewing filenames as opaque byte sequences to seeing
+them as encoded strings we need to address what happens when a program
+tries to create a file with an invalid name.  The Unicode subsystem
+within the kernel leaves the decision of what to do in this case to the
+filesystem, which select its preferred behavior by enabling/disabling
+the strict mode.  When Ext4 encounters one of those strings and the
+filesystem did not require strict mode, it falls back to considering the
+entire string as an opaque byte sequence, which still allows the user to
+operate on that file but the case-insensitive and equivalent sequence
+lookups won't work.
+
 Options
 =======
 
-- 
2.20.1