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authorDan Williams <dan.j.williams@intel.com>2008-04-07 15:35:01 -0700
committerGreg Kroah-Hartman <gregkh@suse.de>2008-04-19 19:10:29 -0700
commit2424b5dd062cbe3e0578ae7b11a1b360ad22f451 (patch)
tree75b6d67412969074237a371947280b419c5d43db /Documentation/filesystems/sysfs.txt
parentb844eba292b477cda14582bfc6f535deed57a82d (diff)
sysfs: refill attribute buffer when reading from offset 0
Requiring userspace to close and re-open sysfs attributes has been the policy since before 2.6.12. It allows userspace to get a consistent snapshot of kernel state and consume it with incremental reads and seeks. Now, if the file position is zero the kernel assumes userspace wants to see the new value. The application for this change is to allow a userspace RAID metadata handler to check the state of an array without causing any memory allocations. Thus not causing writeback to a raid array that might be blocked waiting for userspace to take action. Cc: Neil Brown <neilb@suse.de> Acked-by: Tejun Heo <htejun@gmail.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'Documentation/filesystems/sysfs.txt')
-rw-r--r--Documentation/filesystems/sysfs.txt9
1 files changed, 7 insertions, 2 deletions
diff --git a/Documentation/filesystems/sysfs.txt b/Documentation/filesystems/sysfs.txt
index 4598ef7b622b..7f27b8f840d0 100644
--- a/Documentation/filesystems/sysfs.txt
+++ b/Documentation/filesystems/sysfs.txt
@@ -176,8 +176,10 @@ implementations:
Recall that an attribute should only be exporting one value, or an
array of similar values, so this shouldn't be that expensive.
- This allows userspace to do partial reads and seeks arbitrarily over
- the entire file at will.
+ This allows userspace to do partial reads and forward seeks
+ arbitrarily over the entire file at will. If userspace seeks back to
+ zero or does a pread(2) with an offset of '0' the show() method will
+ be called again, rearmed, to fill the buffer.
- On write(2), sysfs expects the entire buffer to be passed during the
first write. Sysfs then passes the entire buffer to the store()
@@ -192,6 +194,9 @@ implementations:
Other notes:
+- Writing causes the show() method to be rearmed regardless of current
+ file position.
+
- The buffer will always be PAGE_SIZE bytes in length. On i386, this
is 4096.