Home > Articles > Operating Systems, Server > Linux/UNIX/Open Source

Page Table Management

  • Print
  • + Share This
This chapter is from the book

3.8 Translation Lookaside Buffer (TLB)

Initially, when the processor needs to map a virtual address to a physical address, it must traverse the full page directory searching for the PTE of interest. This would normally imply that each assembly instruction that references memory actually requires several separate memory references for the page table traversal [Tan01]. To avoid this considerable overhead, architectures take advantage of the fact that most processes exhibit a locality of reference, or, in other words, large numbers of memory references tend to be for a small number of pages. They take advantage of this reference locality by providing a Translation Lookaside Buffer (TLB), which is a small associative memory that caches virtual to physical page table resolutions.

Linux assumes that most architectures support some type of TLB, although the architecture-independent code does not care how it works. Instead, architecture-dependent hooks are dispersed throughout the VM code at points where it is known that some hardware with a TLB would need to perform a TLB-related operation. For example, when the page tables have been updated, such as after a page fault has completed, the processor may need to update the TLB for that virtual address mapping.

Not all architectures require these type of operations, but, because some do, the hooks have to exist. If the architecture does not require the operation to be performed, the function for that TLB operation will be a null operation that is optimized out at compile time.

A quite large list of TLB API hooks, most of which are declared in <asm/pgtable.h>, are listed in Tables 3.2 and 3.3, and the APIs are quite well documented in the kernel source by Documentation/cachetlb.txt [Mi100]. It is possible to have just one TLB flush function, but, because both TLB flushes and TLB refills are very expensive operations, unnecessary TLB flushes should be avoided if at all possible. For example, when context switching, Linux will avoid loading new page tables using Lazy TLB Flushing, discussed further in Section 4.3.

Table 3.2. Translation Lookaside Buffer Flush API

void flush_tlb_all(void)

This flushes the entire TLB on all processors running in the system, which makes it the most expensive TLB flush operation. After it completes, all modifications to the page tables will be visible globally. This is required after the kernel page tables, which are global in nature, have been modified, such as after vfree() (See Chapter 7) completes or after the PKMap is flushed (see Chapter 9).

void flush_tlb_mm(struct mm_struct *mm)

This flushes all TLB entries related to the userspace portion (i.e., below PAGE_OFFSET) for the requested mm context. In some architectures, such as MIPS, this will need to be performed for all processors, but usually it is confined to the local processor. This is only called when an operation has been performed that affects the entire address space, such as after all the address mapping has been duplicated with dup_mmap() for fork or after all memory mappings have been deleted with exit_mmap().

void flush_tlb_range(struct mm_struct *mm, unsigned long start, unsigned long end)

As the name indicates, this flushes all entries within the requested user space range for the mm context. This is used after a new region has been moved or changed as during mremap(), which moves regions, or mprotect(), which changes the permissions. The function is also indirectly used during unmapping a region with munmap(), which calls tlb_finish_mmu(), which tries to use flush_tlb_range() intelligently. This API is provided for architectures that can remove ranges of TLB entries quickly rather than iterating with flush_tlb_page().

Table 3.3. Translation Lookaside Buffer Flush API (cont.)

void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)

Predictably, this API is responsible for flushing a single page from the TLB. The two most common uses of it are for flushing the TLB after a page has been faulted in or has been paged out.

void flush_tlb_pgtables(struct mm_struct *mm, unsigned long start, unsigned long end)

This API is called when the page tables are being torn down and freed. Some platforms cache the lowest level of the page table, i.e., the actual page frame storing entries, which needs to be flushed when the pages are being deleted. This is called when a region is being unmapped and the page directory entries are being reclaimed.

void update_mmu_cache(struct vm_area_struct *vma, unsigned long addr, pte_t pte)

This API is only called after a page fault completes. It tells the architecture-dependent code that a new translation now exists at pte for the virtual address addr. Each architecture decides how this information should be used. For example, Sparc64 uses the information to decide if the local CPU needs to flush its data cache or does it need to send an Inter Processor Interrupt (IPI) to a remote processor.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020