mm: document highmem_is_dirtyable sysctl

It seems that there are still people using 32b kernels which a lot of
memory and the IO tend to suck a lot for them by default.  Mostly
because writers are throttled too when the lowmem is used.  We have
highmem_is_dirtyable to work around that issue but it seems we never
bothered to document it.  Let's do it now, finally.

Link: http://lkml.kernel.org/r/20170626093200.18958-1-mhocko@kernel.orgSigned-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Alkis Georgopoulos <alkisg@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/vm.txt

@@ -240,6 +240,26 @@ fragmentation index is <= extfrag_threshold. The default value is 500.
 
 ==============================================================
 
+highmem_is_dirtyable
+
+Available only for systems with CONFIG_HIGHMEM enabled (32b systems).
+
+This parameter controls whether the high memory is considered for dirty
+writers throttling.  This is not the case by default which means that
+only the amount of memory directly visible/usable by the kernel can
+be dirtied. As a result, on systems with a large amount of memory and
+lowmem basically depleted writers might be throttled too early and
+streaming writes can get very slow.
+
+Changing the value to non zero would allow more memory to be dirtied
+and thus allow writers to write more data which can be flushed to the
+storage more effectively. Note this also comes with a risk of pre-mature
+OOM killer because some writers (e.g. direct block device writes) can
+only use the low memory and they can fill it up with dirty data without
+any throttling.
+
+==============================================================
+
 hugepages_treat_as_movable
 
 This parameter controls whether we can allocate hugepages from ZONE_MOVABLE