BUF(9)                 FreeBSD Kernel Developer's Manual                BUF(9)

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
     sys/kern/vfs_bio.c in the FreeBSD source tree.

     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 FreeBSD 2.2.8 / FreeBSD 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.

FreeBSD 15.1-STABLE-HBSD       December 22, 1998                        BUF(9)