NAME

       buf — kernel buffer I/O scheme used in FreeBSD VM system

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 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 VM Buffer abstraction is mostly concentrated in /usr/src/sys/kern/vfs_bio.c.

       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  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 *block* aligned.  When you have a VM buffer representing
       some b_offset and b_size, the actual start of the buffer is (b_data + (b_offset  &  PAGE_MASK))  and  not
       just  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.  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 size is 4096).

       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 within the VM buffer.  If a buffer dirty operation creates a  '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 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 of it completely.

       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 kernel typically unmaps VM
       buffers the moment they are no longer needed but often keeps the 'struct buf' structure instantiated  and
       even  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[]  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 file system devices to remap underlying pages in order to
       deal with, for example, re-mapping a file fragment into a file block.

       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 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.   The
       kernel  uses an instantiated VM buffer (i.e., struct buf) to place-mark pages in this special state.  The
       buffer is typically flagged B_DELWRI.  When a device no longer needs a buffer it typically  flags  it  as
       B_RELBUF.   Due  to  the  underlying  pages being marked clean, the 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 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 regards to this situation ???)

       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  buffer  cache  from
       being freed.  This can complicate the life of the paging system.

HISTORY

       The  buf manual page was originally written by Matthew Dillon and first appeared in FreeBSD 3.1, December
       1998.

Debian                                          December 22, 1998                                         BUF(9)