svn commit: r368494 - in stable/12/sys: fs/ext2fs ufs/ufs

[Kirk McKusick]

Author: mckusick
Date: Wed Dec  9 22:37:23 2020
New Revision: 368494
URL: https://svnweb.freebsd.org/changeset/base/368494

Log:
  MFC of 368396 and 368425.
  
  Document BA_CLRBUF flag.
  
  Sponsored by: Netflix

Modified:
  stable/12/sys/fs/ext2fs/ext2_extern.h
  stable/12/sys/ufs/ufs/ufs_extern.h
Directory Properties:
  stable/12/   (props changed)

Modified: stable/12/sys/fs/ext2fs/ext2_extern.h
==============================================================================
--- stable/12/sys/fs/ext2fs/ext2_extern.h	Wed Dec  9 20:38:26 2020	(r368493)
+++ stable/12/sys/fs/ext2fs/ext2_extern.h	Wed Dec  9 22:37:23 2020	(r368494)
@@ -135,6 +135,13 @@ void	ext2_gd_csum_set(struct m_ext2fs *);
 
 /* Flags to low-level allocation routines.
  * The low 16-bits are reserved for IO_ flags from vnode.h.
+ *
+ * The BA_CLRBUF flag specifies that the existing content of the block
+ * will not be completely overwritten by the caller, so buffers for new
+ * blocks must be cleared and buffers for existing blocks must be read.
+ * When BA_CLRBUF is not set the buffer will be completely overwritten
+ * and there is no reason to clear them or to spend I/O fetching existing
+ * data. The BA_CLRBUF flag is handled in the ext2_balloc() functions.
  */
 #define	BA_CLRBUF	0x00010000	/* Clear invalid areas of buffer. */
 #define	BA_SEQMASK	0x7F000000	/* Bits holding seq heuristic. */

Modified: stable/12/sys/ufs/ufs/ufs_extern.h
==============================================================================
--- stable/12/sys/ufs/ufs/ufs_extern.h	Wed Dec  9 20:38:26 2020	(r368493)
+++ stable/12/sys/ufs/ufs/ufs_extern.h	Wed Dec  9 22:37:23 2020	(r368494)
@@ -119,6 +119,13 @@ void	softdep_revert_rmdir(struct inode *, struct inode
  *
  * Note: The general vfs code typically limits the sequential heuristic
  * count to 127.  See sequential_heuristic() in kern/vfs_vnops.c
+ *
+ * The BA_CLRBUF flag specifies that the existing content of the block
+ * will not be completely overwritten by the caller, so buffers for new
+ * blocks must be cleared and buffers for existing blocks must be read.
+ * When BA_CLRBUF is not set the buffer will be completely overwritten
+ * and there is no reason to clear them or to spend I/O fetching existing
+ * data. The BA_CLRBUF flag is handled in the UFS_BALLOC() functions.
  */
 #define	BA_CLRBUF	0x00010000	/* Clear invalid areas of buffer. */
 #define	BA_METAONLY	0x00020000	/* Return indirect block buffer. */