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Resolve The Bounce Buffer Kernel Problem



It’s worth reading these troubleshooting ideas if you see an error message in the crash buffer kernel on your PC. The fault buffer is in sufficient memory for the device to copy and write data. It is then copied to the desired user page in high memory. Small pages of memory are allocated and used as buffer pages for direct memory access and from the device.



This section provides information on the application and use of the crash buffer patch in the Linux 2.4 kernel. Bounce Buffer Patch, written by Jens Axboe, allows device drivers that support DMA (Direct Memory Access) I / O for high address physical memory to avoid buffer failures.

This document provides a brief overview of memory and addressing in the Linux kernel, as well as information on why and how to use the buffer patch.

9.6 What’s New In 2.6


Storage Pools

In version 2.4, the high memory manager was the only supported subsystem.Emergency pools from pages. In version 2.6, storage pools are implemented as sharedThe concept when it is necessary to reserve the minimum amount of “material” for storagetight. “Material” in this case can be any type of object, for example. B. Pages inthe case of a high memory manager or, more commonly, a managed objectthrough the manifold. Pools are initialized with mempool_create () A number of arguments are required. This is the minimum number of objectswhich should bereserved ( min_nr ), distribution function forObject type ( alloc_fn () ), free function ( free_fn () )and optional personal data transferred to designated and free features.

The storage pool API has two common functions, dedicated and free, which are called mempool_alloc_slab () and mempool_free_slab () . IfWhen shared functions are used, private data is the disk cache that objects have.must be appointed and released.

The high memory manager creates two page pools. Of courseThe page pool is for general use, and the second page pool is for use with ISA.Devices to be assigned to ZONE_DMA . Assignment function page_pool_alloc () and the parameters passed for private data indicateGFP indicators for use. Free function – page_pool_free () . thenStorage pools replace the existing emergency pool code in 2.4.

The storage pool API works to allocate or free objects from the storage pool. mempool_alloc () and mempool_free () are provided. memory sizePools are destroyed using mempool_destroy () .

Allocate high memory pages

In version 2.4, the Page virtual field was used to saveThe page address in the pkmap_count table. because ofThe number of structure pages available on a system with a large amount of memory,This is a very large penalty for a relatively small amount.Pages to be assigned to ZONE_NORMAL . 2.6 there is always this pkmap_count but it is handled differently.

bounce buffer kernel

Version 2.6 creates a hash table named page_address_htable . TheseThe table is hashed and based on the page structure addressThe list is used to search for struct page_address_slot . Thesestruct has two fields of interest: page struct and virtualThe address. When the kernel needs to find the virtual address used by the cardThe page was found by iterating over this hash segment. As a siteThe actual distribution of the bottom store is basically 2.4, excluding the present Page virtual is no longer needed.

Do I / O

Last big change – struct bio is now usedinstead of struct buffer_head when executed in ode-output.Working with Bio structures is outside the scope of this book. However,The main reason for implementing Bio structures isso I / O can be done in blocks of any size from the base valueSupported device. In 2.4, all I / O had to be split into page blocks.regardless of the baud rate of the main unit.

Figure 9.1: Call Diagram: kmap ()
void * kmap (struct page * page)
Takes a structure page from high memory and assigns it to low memory.The returned address is the virtual address of the mapping
void * kmap_nonblock (struct page * page)
This is the same as kmap () , except that it does not block whenNo slots available, NULL is returned instead. This is not the same as kmap_atomic () which uses specially reserved locations
void * kmap_atomic (structure page * page, enumeration of km_type types)
 There is a map places for atomic use by interrupts.(see section 9.3). Your usestrongly discouraged and callers using this feature may not be able to sleep or schedule. TheseThe function atomically allocates a page from high memory for a specific purpose.

Table 9.1: High Memory Mapping API
Figure 9.2: Call diagram: kunmap ()
void kunmap (struct page * page)
 Cancels the page with insufficient memory structure and frees the page table.Entrance mapping
void kunmap_atomic (void * kvaddr, type enumeration km_type)
Free the page that was atomically allocated

Table 9.2: High Memory Unmapping API
Figure 9.3: Call Diagram: create_bounce ()
Figure 9.4: Calling diagram: bounce_end_io_read / write ()
Figure 9.5: Pergrabbing pages from an emergencyPools

Reallocate RAM Using The Nopage Method

  • scullp does not free device memory while it is present. assigned. This is a policy more than a requirement and it is different behavior of skulls and similar devices when opened to truncate to length 0 Write. Refusing to share a connected Scullp device allows Process to overwrite regions that have been actively allocated by another process so you can test us See how device and storage processes interact. To prevent the release of the assigned device, The driver should count the number of active assignments. For this, the vmas field in the device structure is used.

  • Memory allocation is performed only for Scullp The order parameter (defined when loading the module): 0 . The parameter defines how _ _Get_free_pages is called (see section 8.3). Limit zero order (which forces pages to be assigned individually rather than in large groups) dictated by internals _ _get_free_pages, assignment Function used by Scullp. To maximize your mapping performance, The Linux kernel maintains a free list for each allocation order and only The counter of transitions to the first page of the cluster is increased by get_free_pages and is decremented by free_pages. Mmap method disabled for a Scullp device if the display order is greater than Zero because nopage is more about individual pages than Groups of pages. Skullp just doesn’t know how to do it right Maintain conversion counts for pages that are part of a higher order map. (Come back in section 8.3.1 if you need Scullp update and memory allocation order Value.)

Although it is rare is necessary, It is interesting to see how the driver can display the kernel virtual address, andusing user space. mmap. The real kernel virtual address is the address used by function , for example vmalloc – i.e. kernel-side assigned virtual address Tables. The code for this section is taken from scullv. A module that works like Scullp, but allocates its memory across vmalloc.

Most of Scullv’s implementation is similar to the one we just saw for scullp, except that the order parameter, which controls memory allocation, does not need to be checked. Cause vmalloc assigns its pages individually because



What is low memory and high memory in operating system?

High memory is a segment of memory that can be accessed by user-space programs. It can’t deal with low memory. Low memory is a segment of memory that the Linux kernel can directly access. If the kernel needs access to high memory, it must first allocate it to its own address space.

What is DMA mapping?