RVM: Recoverable Virtual Memory, Release 1.3: RVM Segment Loader

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## 5. RVM Segment Loader

Designed and programmed by David C. Steere

The RVM segment loader provides a mechanism that allows a load map to be created for a segment. Regions can then be mapped to specific virtual memory addresses or to virtual memory allocated by RVM in a single library call. There are only two library functions: `rvm_create_segment` and `rvm_load_segment`.

To use the segment loader, a segment header must be created by `rvm_create_segment`. The header, which occupies the first page of the segment, the load map for regions defined within the segment, and a version identifier so that `rvm_load_segment` can recognize suitable segments. All region lengths must be integral multiple of the page size of the machine, which is available as the macro `RVM_PAGE_SIZE`. Virtual memory addresses must be page-aligned.

The load map is prepared in a vector of descriptors for the desired regions (type `rvm_region_def_t`). The descriptors specify just two parameters: the region length and the virtual memory address where the region is to be loaded. The offsets of the regions in the segment are assigned by `rvm_create_segment`. If pointers are included in the data, a fixed virtual memory address must be specified so that the loader will place the region there at each loading. If there are no pointers, the segment loader can be instructed to allocate space for the region by specifying an address of zero. The segment loader will make virtual memory addressable as necessary when loading.

The virtual memory address for the first region should be specified far enough beyond the applications break point so that the program can grow during development. Otherwise, there will have to be a way to modify the pointers or rebuild the structures.

The segment header establishes the load map for only that segment. If multiple segments are used, each must have its own header, and `rvm_load_segment` must be called for each.

The segment loader is used by the RDS allocator (see Chapter of RDS) to automate loading of a persistent heap region and a statically allocated region. A utility program is available to create the segment header and initialize the heap so that it is easily loaded.

The header file giving all necessary definitions for the RVM segment loader is included in Appendix of C Declarations.

NAME

### of C Declarations

rvm_create_segment - create segment header

SYNOPSIS

###

    #include "rvm_segment.h

    typedef struct
        {
        rvm_offset_t        offset;
        rvm_length_t        length;
        char                *vmaddr;
        }
    rvm_region_def_t;

    rvm_return_t rvm_create_segment (DevName, DevLength, options,
                                     nregions, regions)

    char             *DevName;  /* name of heap segment */
    rvm_offset_t     DevLength; /* length of heap raw partition */
    rvm_options_t    *options;  /* optional pointer to an options record */
    unsigned long    nregions;  /* number of region descriptors */
    rvm_region_def_t *region_defs; /* pointer to array of region descriptors */

DESCRIPTION

###

`rvm_create_segment` is used to initialize the segment header with the region load map. The name of the file or partition for the segment is specified by `DevName`, and the length of the partition is specified in `DevLength`. If the segment is represented by a file, `DevLength` should be zero. Also, the file must exist and be at least one page long.

The parameter `region_defs` must point to an array of `rvm_region_def_t` descriptors, with one descriptor for each region to be loaded by `rvm_load_segment`. The descriptor array must be allocated and deallocated by application, and must be as long as the number of descriptors specified by `nregions`. The maximum number of regions that can be described in the header is limited by the page size of the machine, which is available as the macro `RVM_PAGE_SIZE`. If the number of regions requested by the parameter `nregions` causes the header to exceed page size, the error `RVM_ENO_MEMORY` will be returned.

Each `rvm_region_def_t` descriptor requires the regions virtual memory address and its length. The virtual memory address where the region will be loaded must be page-aligned. It can be specified as zero and, if so, will cause the loader to allocate virtual memory for the region. Regions containing pointers must specify an address to guarantee that they are loaded in the same place each time. The region length must be an integral multiple of the page size, or the error code `RVM_ERANGE` will be returned.

Regions are not allowed to overlap in virtual memory. If this condition is not met, the error code `RVM_EVM_OVERLAP` will be returned.

`rvm_create_segment` assigns space in the segment to the header and regions contiguously, beginning with the header at byte zero and with length equal to the page size. The regions are allocated next, in the order of their occurrence in the `region_defs` array. The offset is returned in the `offset` field of each descriptor.

The virtual memory specified in the descriptors does not have to be addressable when calling `rvm_create_segment`. Also, `rvm_create_segment` does not map or alter the applications data in the segment; only the first page of the segment for the header is affected.

Since it is an initialization function, `rvm_create_segment` performs no internal synchronization, so if there is a possibility of concurrent access to the segment, the application must do appropriate serialization.

An optional RVM options descriptor can be passed to `rvm_create_segment` if no log has been previously declared. RVM must be initialized before `rvm_create_segment` is called.

DIAGNOSTICS

###

RVM_SUCCESS

  success

RVM_ERANGE

  a region length is not page sized

RVM_EVM_OVERLAP

  region mappings oberlap in virtual mapping

RVM_ENO_MEMORY

  heap exhausted, or header cannot be allocated

RVM_EOPTIONS

  invalid options record or pointer

RVM_ELOG

  no log file has been specified

RVM_EINIT

  RVM not initialized

SEE ALSO

###

`rvm_load_segment (3)`, `rvm_map (3)`, `rds_zap_heap (3)`, `rds_load_heap (3)`, `rdsinit (1)`

AUTHOR

###

David C. Steere

BUGS

###

Specification of region allocations with the `offset` field of the region descriptors is not allowed.

NAME

###

rvm_load_segment - automatically map regions of segment

SYNOPSIS

###

    #include "rvm_segment.h

    typedef struct
        {
        rvm_offset_t        offset;
        rvm_length_t        length;
        char                *vmaddr;
        }
    rvm_region_def_t;

    rvm_return_t rvm_load_segment (DevName, DevLength, options,
                                   nregions, regions)

    char             *DevName;  /* name of heap segment */
    rvm_offset_t     DevLength; /* length of heap raw partition */
    rvm_options_t    *options;  /* optional pointer to an options record */
    unsigned long    *nregions;  /* number of region descriptors  */
    rvm_region_def_t *regions[]; /* pointer to array of region descriptors  */

DESCRIPTION

###

`rvm_load_segment` is used to automatically map the regions of a segment prepared with `rvm_create_segment`. The name of the file or partition for the segment is specified by `DevName`, and the length of the partition is specified in `DevLength`. If the heap is in a file, `DevLength` should be zero.

`rvm_load_segment` will first to load the array of `rvm_region_def_t` descriptors contained in the segment header. If the segment header is not recognizable, the error `RVM_ESEGMENT_HDR` will be returned. A version identifier will also be checked, and if the version of the segment library that created the header does not match the version of the library linked with the application, the error `RVM_EVERSION_SKEW` will be returned. Otherwise, the address and length of the region descriptors array will be returned in regions and `nregions`, respectively. The header array must be deallocated by the application.

After the header is verified, each of the regions will be mapped via `rvm_map`. Allocation of virtual memory is done by `rvm_load_segment`, so the application must not attempt allocation, or the error `RVM_ERANGE` will result. Regions whose virtual memory address is non-zero will be automatically mapped to that address. Regions with zero virtual memory addresses will have space allocated by RVM, and the allocated address will be returned in the `vmaddr` field of the descriptor. After successful completion of `rvm_load_segment`, all data specified by the header is addressable to the application.

Regions are not allowed to overlap in either the segment or virtual memory. If this condition is not met, the error codes `RVM_EOVERLAP` or `RVM_EVM_OVERLAP` will be returned, as appropriate.

Since it is an initialization function, `rvm_load_segment` performs no internal synchronization, so if there is a possibility of concurrent access to the segment, the application must do appropriate serialization.

An optional RVM options descriptor can be passed to `rvm_load_segment` if no log has been previously declared. RVM must be initialized before `rvm_load_segment` is called.

DIAGNOSTICS

###

RVM_SUCCESS

  success

RVM_ESEGMENT_HDR

  segment header invalid

RVM_EVERSION_SKEW

  segment header built with different version of segment loader library

RVM_EOVERLAP

  segment regions overlap

RVM_EVM_OVERLAP

  segment regions overlap in virtual memory

RVM_EOPTIONS

  invalid options record or pointer

RVM_ENO_MEMORY

  heap exhausted, or buffer cannot be allocated

RVM_ELOG

  no log file has been specified or invalid file name

RVM_EINIT

  RVM not initialized

SEE ALSO

###

`rvm_create_segment (3)`, `rvm_map (3)`, `rds_load_heap (3)`

AUTHOR

###

David C. Steere

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