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UBC(9)                     Kernel Developer's Manual                    UBC(9)

     ubc - unified buffer cache

     #include <uvm/uvm.h>

     void *
     ubc_alloc(struct uvm_object *uobj, voff_t offset, vsize_t *lenp,
         int advice, int flags);

     ubc_release(void *va, int flags);

     ubc_uiomove(struct uvm_object *uobj, struct uio *uio, vsize_t todo,
         int advice, int flags);

     ubc_zerorange(struct uvm_bject *uobj, off_t off, size_t len, int flags);

     ubc_purge(struct uvm_object *uobj);

     ubc_alloc() creates a kernel mapping of uobj starting at offset offset.
     The desired length of the mapping is pointed to by lenp, but the actual
     mapping may be smaller than this.  lenp is updated to contain the actual
     length mapped.  advice is the access pattern hint, which must be one of

           UVM_ADV_NORMAL      No hint
           UVM_ADV_RANDOM      Random access hint
           UVM_ADV_SEQUENTIAL  Sequential access hint (from lower offset to
                               higher offset)

     The possible flags are

           UBC_READ            Mapping will be accessed for read.
           UBC_WRITE           Mapping will be accessed for write.
           UBC_FAULTBUSY       Fault in window's pages already during mapping
                               operation.  Makes sense only for write.

     Once the mapping is created, it must be accessed only by methods that can
     handle faults, such as uiomove(9) or kcopy(9).  Page faults on the
     mapping will result in the object's pager method being called to resolve
     the fault.

     ubc_release() frees the mapping at va for reuse.  The mapping may be
     cached to speed future accesses to the same region of the object.  The
     flags can be any of

           UBC_UNMAP           Do not cache mapping.

     ubc_uiomove() allocates an UBC memory window, performs I/O on it and
     unmaps the window.  The advice parameter takes the same values as the
     respective parameter in ubc_alloc() and the flags parameter takes the
     same arguments as ubc_alloc() and ubc_release().  Additionally, the flag
     UBC_PARTIALOK can be provided to indicate that it is acceptable to return
     if an error occurs mid-transfer.

     ubc_zerorange() sets a range of bytes in a UVM object to zero.  The flags
     parameter takes the same arguments as ubc_release().

     ubc_purge() disassociates all UBC structures from an empty UVM object,
     specified by uobj.

     The ubc subsystem is implemented within the file sys/uvm/uvm_bio.c.

     kcopy(9), pmap(9), uiomove(9), uvm(9), vnode(9), vnodeops(9)

     Chuck Silvers, "UBC: An Efficient Unified I/O and Memory Caching
     Subsystem for NetBSD", Proceedings of the FREENIX Track: 2000 USENIX
     Annual Technical Conference, USENIX Association,
     285-290, June 18-23, 2000.

     UBC first appeared in NetBSD 1.6.

     Chuck Silvers <chuq@chuq.com> designed and implemented the UBC part of
     UVM, which uses UVM pages to cache vnode data rather than the traditional
     buffer cache buffers.

NetBSD 8.0                       April 6, 2017                      NetBSD 8.0