sf_buf_page9freebsd

Usage

Overview

The sf_buf interface, historically the sendfile(2) buffer interface, allows kernel subsystems to manage temporary kernel address space mappings for physical memory pages. On systems with a direct memory map region (allowing all physical pages to be visible in the kernel address space at all times), the struct sf_buf will point to an address in the direct map region; on systems without a direct memory map region, the struct sf_buf will manage a temporary kernel address space mapping valid for the lifetime of the struct sf_buf.

Call sf_buf_alloc() to allocate a struct sf_buf for a physical memory page. sf_buf_alloc() is not responsible for arranging for the page to be present in physical memory; the caller should already have arranged for the page to be wired, i.e., by calling vm_page_wire(9). Several flags may be passed to sf_buf_alloc():

SFB_CATCH

Cause sf_buf_alloc() to abort and return NULL if a signal is received waiting for a struct sf_buf to become available.

SFB_NOWAIT

Cause sf_buf_alloc() to return NULL rather than sleeping if a struct sf_buf is not immediately available.

SFB_CPUPRIVATE

Cause sf_buf_alloc() to only arrange that the temporary mapping be valid on the current CPU, avoiding unnecessary TLB shootdowns for mappings that will only be accessed on a single CPU at a time. The caller must ensure that accesses to the virtual address occur only on the CPU from which sf_buf_alloc() was invoked, perhaps by using sched_pin().

Call sf_buf_kva() to return a kernel mapped address for the page.

Call sf_buf_page() to return a pointer to the page originally passed into sf_buf_alloc().

Call sf_buf_free() to release the struct sf_buf reference. The caller is responsible for releasing any wiring they have previously acquired on the physical page; sf_buf_free() releases only the temporary kernel address space mapping, not the page itself.

Uses of this interface include managing mappings of borrowed pages from user memory, such as in zero-copy socket I/O, or pages of memory from the buffer cache referenced by mbuf external storage for sendfile(2).