linux-imx

.. _hugetlbfs_reserve:

​

=====================

Hugetlbfs Reservation

=====================

​

Overview

========

​

Huge pages as described at :ref:`hugetlbpage` are typically

preallocated for application use.  These huge pages are instantiated in a

task's address space at page fault time if the VMA indicates huge pages are

to be used.  If no huge page exists at page fault time, the task is sent

a SIGBUS and often dies an unhappy death.  Shortly after huge page support

was added, it was determined that it would be better to detect a shortage

of huge pages at mmap() time.  The idea is that if there were not enough

huge pages to cover the mapping, the mmap() would fail.  This was first

done with a simple check in the code at mmap() time to determine if there

were enough free huge pages to cover the mapping.  Like most things in the

kernel, the code has evolved over time.  However, the basic idea was to

'reserve' huge pages at mmap() time to ensure that huge pages would be

available for page faults in that mapping.  The description below attempts to

describe how huge page reserve processing is done in the v4.10 kernel.

​

​

Audience

========

This description is primarily targeted at kernel developers who are modifying

hugetlbfs code.

​

​

The Data Structures

===================

​

resv_huge_pages

    This is a global (per-hstate) count of reserved huge pages.  Reserved

    huge pages are only available to the task which reserved them.

    Therefore, the number of huge pages generally available is computed

    as (``free_huge_pages - resv_huge_pages``).

Reserve Map

    A reserve map is described by the structure::

​

        struct resv_map {

            struct kref refs;

            spinlock_t lock;

            struct list_head regions;

            long adds_in_progress;

            struct list_head region_cache;

            long region_cache_count;

        };

​

    There is one reserve map for each huge page mapping in the system.

    The regions list within the resv_map describes the regions within

    the mapping.  A region is described as::

​

        struct file_region {

            struct list_head link;

            long from;

            long to;

        };

​

    The 'from' and 'to' fields of the file region structure are huge page

    indices into the mapping.  Depending on the type of mapping, a