diff options
Diffstat (limited to 'share/man/man9/buf.9')
| -rw-r--r-- | share/man/man9/buf.9 | 8 |
1 files changed, 4 insertions, 4 deletions
diff --git a/share/man/man9/buf.9 b/share/man/man9/buf.9 index 9d8b643..1a2a68a 100644 --- a/share/man/man9/buf.9 +++ b/share/man/man9/buf.9 @@ -40,7 +40,7 @@ .Sh DESCRIPTION The kernel implements a KVM abstraction of the buffer cache which allows it to map potentially disparate vm_page's into contiguous KVM for use by -(mainly filesystem) devices and device I/O. This abstraction supports +(mainly file system) devices and device I/O. This abstraction supports block sizes from DEV_BSIZE (usually 512) to upwards of several pages or more. It also supports a relatively primitive byte-granular valid range and dirty range currently hardcoded for use by NFS. The code implementing the @@ -49,7 +49,7 @@ VM Buffer abstraction is mostly concentrated in .Pp One of the most important things to remember when dealing with buffer pointers (struct buf) is that the underlying pages are mapped directly from the buffer -cache. No data copying occurs in the scheme proper, though some filesystems +cache. No data copying occurs in the scheme proper, though some file systems such as UFS do have to copy a little when dealing with file fragments. The second most important thing to remember is that due to the underlying page mapping, the b_data base pointer in a buf is always *page* aligned, not @@ -83,14 +83,14 @@ system typically unmaps it from KVM and replaces the page in the b_pages[] array with a place-marker called bogus_page. The place-marker forces any kernel subsystems referencing the associated struct buf to re-lookup the associated page. I believe the place-marker hack is used to allow sophisticated devices -such as filesystem devices to remap underlying pages in order to deal with, +such as file system devices to remap underlying pages in order to deal with, for example, re-mapping a file fragment into a file block. .Pp VM buffers are used to track I/O operations within the kernel. Unfortunately, the I/O implementation is also somewhat of a hack because the kernel wants to clear the dirty bit on the underlying pages the moment it queues the I/O to the VFS device, not when the physical I/O is actually initiated. This -can create confusion within filesystem devices that use delayed-writes because +can create confusion within file system devices that use delayed-writes because you wind up with pages marked clean that are actually still dirty. If not treated carefully, these pages could be thrown away! Indeed, a number of serious bugs related to this hack were not fixed until the 2.2.8/3.0 release. |
