docs: sysfs-block: document stable_writes

/sys/block/<disk>/queue/stable_writes is completely undocumented.
Document it.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20211209003833.6396-6-ebiggers@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/Documentation/ABI/stable/sysfs-block b/Documentation/ABI/stable/sysfs-block
index de3b86a3dfa55ab666500d03b7e268f40bf19fe0..288626e8cb5323ffc7049e1b0fe3e24ac21bd2eb 100644 (file)
--- a/Documentation/ABI/stable/sysfs-block
+++ b/Documentation/ABI/stable/sysfs-block
@@ -516,6 +516,23 @@ Description:
                scheduler module, if it isn't already present in the system.
 
 
+What:          /sys/block/<disk>/queue/stable_writes
+Date:          September 2020
+Contact:       linux-block@vger.kernel.org
+Description:
+               [RW] This file will contain '1' if memory must not be modified
+               while it is being used in a write request to this device.  When
+               this is the case and the kernel is performing writeback of a
+               page, the kernel will wait for writeback to complete before
+               allowing the page to be modified again, rather than allowing
+               immediate modification as is normally the case.  This
+               restriction arises when the device accesses the memory multiple
+               times where the same data must be seen every time -- for
+               example, once to calculate a checksum and once to actually write
+               the data.  If no such restriction exists, this file will contain
+               '0'.  This file is writable for testing purposes.
+
+
 What:          /sys/block/<disk>/queue/throttle_sample_time
 Date:          March 2017
 Contact:       linux-block@vger.kernel.org