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NAME    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     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

     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'm 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    [Toc]    [Back]

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

FreeBSD 5.2.1		       December 22, 1998		 FreeBSD 5.2.1
[ Back ]
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