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mdoc(7)'fy, fix broken FreeBSD release number (3.0.1 -> 3.1)
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2020-12-20 02:59:44 +00:00
svn path=/head/; revision=52662
@ -36,7 +36,9 @@
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.Os
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.Sh NAME
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.Nm BUF
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.Nd Kernel Buffer I/O scheme used in FreeBSD VM system
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.Nd Kernel Buffer I/O scheme used in
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.Fx
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VM system
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.Sh DESCRIPTION
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.Pp
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The kernel implements a KVM abstraction of the buffer cache which allows it
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@ -45,7 +47,8 @@ to map potentially disparate vm_page's into contiguous KVM for use by
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block sizes from DEV_BSIZE (usually 512) to upwards of several pages or more.
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It also supports a relatively primitive byte-granular valid range and dirty
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range currently hardcoded for use by NFS. The code implementing the
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VM Buffer abstraction is mostly concentrated in /usr/src/sys/kern/vfs_bio.c.
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VM Buffer abstraction is mostly concentrated in
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.Pa /usr/src/sys/kern/vfs_bio.c .
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.Pp
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One of the most important things to remember when dealing with buffer pointers
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(struct buf) is that the underlying pages are mapped directly from the buffer
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@ -93,7 +96,7 @@ to the VFS device, not when the physical I/O is actually initiated. This
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can create confusion within filesystem devices that use delayed-writes because
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you wind up with pages marked clean that are actually still dirty. If not
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treated carefully, these pages could be thrown away! Indeed, a number of
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serious bugs related to this hack were not fixed until the 2.2.8/3.0.0 release.
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serious bugs related to this hack were not fixed until the 2.2.8/3.0 release.
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The kernel uses an instantiated VM buffer (i.e. struct buf) to placemark pages
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in this special state. The buffer is typically flagged B_DELWRI. When a
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device no longer needs a buffer it typically flags it as B_RELBUF. Due to
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@ -117,8 +120,9 @@ system.
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.Sh HISTORY
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The
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.Nm
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manual page was originally written by Matthew Dillon and first appeared
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in
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.Fx 3.0.1 ,
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manual page was originally written by
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.An Matthew Dillon
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and first appeared in
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.Fx 3.1 ,
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December 1998.
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