Documentation/mm/highmem.rst

30 		+--------+ 0xffffffff
32 		+--------+ 0xc0000000
36 		+--------+ 0x00000000
39 time, but because we need virtual address space for other things - including
40 temporary maps to access the rest of the physical memory - the actual direct
54 * kmap_local_page(), kmap_local_folio() - These functions are used to create
64   These mappings are thread-local and CPU-local, meaning that the mapping
68   CPU-hotplug until the mapping is disposed.
81   On CONFIG_HIGHMEM=n kernels and for low memory pages they return the
125   Each call of kmap_atomic() in the kernel creates a non-preemptible section
152   long time but the bulk of high-memory mappings in the kernel are
153   short-lived and only used in one place. This means that the cost of
159   On 64-bit systems, calls to kmap_local_page(), kmap_atomic() and kmap() have
160   no real work to do because a 64-bit address space is more than sufficient to
168 Cost of Temporary Mappings
171 The cost of creating temporary mappings can be quite high.  The arch has to
187 of RAM into your 32-bit machine.  This has a number of consequences:
189 * Linux needs a page-frame structure for each page in the system and the
192 * you can have 896M/sizeof(struct page) page-frames at most; with struct
193   page being 32-bytes that would end up being something in the order of 112G
195   page-frames in that memory...
197 * PAE makes your page tables larger - which slows the system down as more
202 The general recommendation is that you don't use more than 8GiB on a 32-bit
203 machine - although more might work for you and your workload, you're pretty
204 much on your own - don't expect kernel developers to really care much if things
211 .. kernel-doc:: include/linux/highmem.h
212 .. kernel-doc:: mm/highmem.c
213 .. kernel-doc:: include/linux/highmem-internal.h