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Adjust some comments to prevent future confusion on the implementation.

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Also add a reference to the buf(9) manual page.
This commit is contained in:
Matthew Dillon 1998-12-22 18:57:30 +00:00
parent e84c798977
commit e6ee8e16e0
1 changed files with 26 additions and 6 deletions
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32
sys/kern/vfs_bio.c
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@ -11,7 +11,7 @@
* 2. Absolutely no warranty of function or purpose is made by the author
* John S. Dyson.
*
* $Id: vfs_bio.c,v 1.187 1998/12/14 21:17:37 dillon Exp $
* $Id: vfs_bio.c,v 1.188 1998/12/22 14:43:58 luoqi Exp $
*/
/*
@ -23,6 +23,8 @@
* Author: John S. Dyson
* Significant help during the development and debugging phases
* had been provided by David Greenman, also of the FreeBSD core team.
*
* see man buf(9) for more info.
*/
#define VMIO
@ -610,11 +612,13 @@ brelse(struct buf * bp)
* but the VM object is kept around. The B_NOCACHE flag is used to
* invalidate the pages in the VM object.
*
* If the buffer is a partially filled NFS buffer, keep it
* since invalidating it now will lose informatio. The valid
* flags in the vm_pages have only DEV_BSIZE resolution but
* the b_validoff, b_validend fields have byte resolution.
* This can avoid unnecessary re-reads of the buffer.
* The b_{validoff,validend,dirtyoff,dirtyend} values are relative
* to b_offset and currently have byte granularity, whereas the
* valid flags in the vm_pages have only DEV_BSIZE resolution.
* The byte resolution fields are used to avoid unnecessary re-reads
* of the buffer but the code really needs to be genericized so
* other filesystem modules can take advantage of these fields.
*
* XXX this seems to cause performance problems.
*/
if ((bp->b_flags & B_VMIO)
@ -640,6 +644,22 @@ brelse(struct buf * bp)
vp = bp->b_vp;
/*
* Get the base offset and length of the buffer. Note that
* for block sizes that are less then PAGE_SIZE, the b_data
* base of the buffer does not represent exactly b_offset and
* neither b_offset nor b_size are necessarily page aligned.
* Instead, the starting position of b_offset is:
*
* b_data + (b_offset & PAGE_MASK)
*
* block sizes less then DEV_BSIZE (usually 512) are not
* supported due to the page granularity bits (m->valid,
* m->dirty, etc...).
*
* See man buf(9) for more information
*/
resid = bp->b_bufsize;
foff = bp->b_offset;