buf.9: Sprinkle with mdoc macros

I did not bump the date here as the manual page looks more like a draft
and I'm not sure if it is actually up-to-date considering that it's
current Dd dates back to 1998.

MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D52770

share/man/man9/buf.9

@@ -36,44 +36,70 @@ 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 file system) devices and device I/O.
 This abstraction supports
-block sizes from DEV_BSIZE (usually 512) to upwards of several pages or more.
+block sizes from
+.Dv 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
 VM Buffer abstraction is mostly concentrated in
-.Pa /usr/src/sys/kern/vfs_bio.c .
+.Pa sys/kern/vfs_bio.c
+in the
+.Fx
+source tree.
 .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
+.Pq Vt struct buf
+is that the underlying pages are mapped directly from the buffer
 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
+mapping, the
-*block* aligned.
+.Va b_data
-When you have a VM buffer representing some b_offset and
+base pointer in a buf is always
-b_size, the actual start of the buffer is (b_data + (b_offset & PAGE_MASK))
+.Em page Ns -aligned ,
-and not just b_data.
+not
+.Em block Ns -aligned .
+When you have a VM buffer representing some
+.Va b_offset
+and
+.Va b_size ,
+the actual start of the buffer is
+.Ql b_data + (b_offset & PAGE_MASK)
+and not just
+.Ql b_data .
 Finally, the VM system's core buffer cache supports
-valid and dirty bits (m->valid, m->dirty) for pages in DEV_BSIZE chunks.
+valid and dirty bits
+.Pq Va m->valid , m->dirty
+for pages in
+.Dv DEV_BSIZE
+chunks.
 Thus
 a platform with a hardware page size of 4096 bytes has 8 valid and 8 dirty
 bits.
 These bits are generally set and cleared in groups based on the device
 block size of the device backing the page.
 Complete page's worth are often
-referred to using the VM_PAGE_BITS_ALL bitmask (i.e., 0xFF if the hardware page
+referred to using the
+.Dv VM_PAGE_BITS_ALL
+bitmask (i.e., 0xFF if the hardware page
 size is 4096).
 .Pp
 VM buffers also keep track of a byte-granular dirty range and valid range.
 This feature is normally only used by the NFS subsystem.
 I am not sure why it
-is used at all, actually, since we have DEV_BSIZE valid/dirty granularity
+is used at all, actually, since we have
+.Dv DEV_BSIZE
+valid/dirty granularity
 within the VM buffer.
-If a buffer dirty operation creates a 'hole',
+If a buffer dirty operation creates a
+.Dq hole ,
 the dirty range will extend to cover the hole.
 If a buffer validation
-operation creates a 'hole' the byte-granular valid range is left alone and
+operation creates a
+.Dq hole
+the byte-granular valid range is left alone and
 will not take into account the new extension.
 Thus the whole byte-granular
 abstraction is considered a bad hack and it would be nice if we could get rid
@@ -81,16 +107,24 @@ of it completely.
 .Pp
 A VM buffer is capable of mapping the underlying VM cache pages into KVM in
 order to allow the kernel to directly manipulate the data associated with
-the (vnode,b_offset,b_size).
+the
+.Pq Va vnode , b_offset , b_size .
 The kernel typically unmaps VM buffers the moment
-they are no longer needed but often keeps the 'struct buf' structure
+they are no longer needed but often keeps the
-instantiated and even bp->b_pages array instantiated despite having unmapped
+.Vt struct buf
+structure
+instantiated and even
+.Va bp->b_pages
+array instantiated despite having unmapped
 them from KVM.
 If a page making up a VM buffer is about to undergo I/O, the
-system typically unmaps it from KVM and replaces the page in the b_pages[]
+system typically unmaps it from KVM and replaces the page in the
+.Va 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
+subsystems referencing the associated
+.Vt struct buf
+to re-lookup the associated
 page.
 I believe the place-marker hack is used to allow sophisticated devices
 such as file system devices to remap underlying pages in order to deal with,
@@ -107,18 +141,29 @@ 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.
+serious bugs related to this hack were not fixed until the
-The kernel uses an instantiated VM buffer (i.e., struct buf) to place-mark pages
+.Fx 2.2.8 /
+.Fx 3.0
+release.
+The kernel uses an instantiated VM buffer (i.e.,
+.Vt struct buf )
+to place-mark pages
 in this special state.
-The buffer is typically flagged B_DELWRI.
+The buffer is typically flagged
+.Dv B_DELWRI .
 When a
-device no longer needs a buffer it typically flags it as B_RELBUF.
+device no longer needs a buffer it typically flags it as
+.Dv B_RELBUF .
 Due to
-the underlying pages being marked clean, the B_DELWRI|B_RELBUF combination must
+the underlying pages being marked clean, the
+.Ql B_DELWRI|B_RELBUF
+combination must
 be interpreted to mean that the buffer is still actually dirty and must be
 written to its backing store before it can actually be released.
 In the case
-where B_DELWRI is not set, the underlying dirty pages are still properly
+where
+.Dv B_DELWRI
+is not set, the underlying dirty pages are still properly
 marked as dirty and the buffer can be completely freed without losing that
 clean/dirty state information.
 (XXX do we have to check other flags in
@@ -128,7 +173,9 @@ The kernel reserves a portion of its KVM space to hold VM Buffer's data
 maps.
 Even though this is virtual space (since the buffers are mapped
 from the buffer cache), we cannot make it arbitrarily large because
-instantiated VM Buffers (struct buf's) prevent their underlying pages in the
+instantiated VM Buffers
+.Pq Vt struct buf Ap s
+prevent their underlying pages in the
 buffer cache from being freed.
 This can complicate the life of the paging
 system.