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Diffstat (limited to 'Documentation/mm/vmemmap_dedup.rst')
| -rw-r--r-- | Documentation/mm/vmemmap_dedup.rst | 28 |
1 files changed, 5 insertions, 23 deletions
diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst index a4b12ff906c4..b4a55b6569fa 100644 --- a/Documentation/mm/vmemmap_dedup.rst +++ b/Documentation/mm/vmemmap_dedup.rst @@ -1,3 +1,4 @@ + .. SPDX-License-Identifier: GPL-2.0 ========================================= @@ -10,14 +11,14 @@ HugeTLB This section is to explain how HugeTLB Vmemmap Optimization (HVO) works. The ``struct page`` structures are used to describe a physical page frame. By -default, there is a one-to-one mapping from a page frame to it's corresponding +default, there is a one-to-one mapping from a page frame to its corresponding ``struct page``. HugeTLB pages consist of multiple base page size pages and is supported by many architectures. See Documentation/admin-guide/mm/hugetlbpage.rst for more details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB are currently supported. Since the base page size on x86 is 4KB, a 2MB HugeTLB page -consists of 512 base pages and a 1GB HugeTLB page consists of 4096 base pages. +consists of 512 base pages and a 1GB HugeTLB page consists of 262144 base pages. For each base page, there is a corresponding ``struct page``. Within the HugeTLB subsystem, only the first 4 ``struct page`` are used to @@ -179,27 +180,7 @@ this correctly. There is only **one** head ``struct page``, the tail ``struct page`` with ``PG_head`` are fake head ``struct page``. We need an approach to distinguish between those two different types of ``struct page`` so that ``compound_head()`` can return the real head ``struct page`` when the -parameter is the tail ``struct page`` but with ``PG_head``. The following code -snippet describes how to distinguish between real and fake head ``struct page``. - -.. code-block:: c - - if (test_bit(PG_head, &page->flags)) { - unsigned long head = READ_ONCE(page[1].compound_head); - - if (head & 1) { - if (head == (unsigned long)page + 1) - /* head struct page */ - else - /* tail struct page */ - } else { - /* head struct page */ - } - } - -We can safely access the field of the **page[1]** with ``PG_head`` because the -page is a compound page composed with at least two contiguous pages. -The implementation refers to ``page_fixed_fake_head()``. +parameter is the tail ``struct page`` but with ``PG_head``. Device DAX ========== @@ -210,6 +191,7 @@ the device (altmap). The following page sizes are supported in DAX: PAGE_SIZE (4K on x86_64), PMD_SIZE (2M on x86_64) and PUD_SIZE (1G on x86_64). +For powerpc equivalent details see Documentation/arch/powerpc/vmemmap_dedup.rst The differences with HugeTLB are relatively minor. |