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Diffstat (limited to 'Documentation/nommu-mmap.txt')
| -rw-r--r-- | Documentation/nommu-mmap.txt | 26 |
1 files changed, 26 insertions, 0 deletions
diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt index b565e8279d1..8e1ddec2c78 100644 --- a/Documentation/nommu-mmap.txt +++ b/Documentation/nommu-mmap.txt @@ -119,6 +119,32 @@ FURTHER NOTES ON NO-MMU MMAP granule but will only discard the excess if appropriately configured as this has an effect on fragmentation. + (*) The memory allocated by a request for an anonymous mapping will normally + be cleared by the kernel before being returned in accordance with the + Linux man pages (ver 2.22 or later). + + In the MMU case this can be achieved with reasonable performance as + regions are backed by virtual pages, with the contents only being mapped + to cleared physical pages when a write happens on that specific page + (prior to which, the pages are effectively mapped to the global zero page + from which reads can take place). This spreads out the time it takes to + initialize the contents of a page - depending on the write-usage of the + mapping. + + In the no-MMU case, however, anonymous mappings are backed by physical + pages, and the entire map is cleared at allocation time. This can cause + significant delays during a userspace malloc() as the C library does an + anonymous mapping and the kernel then does a memset for the entire map. + + However, for memory that isn't required to be precleared - such as that + returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to + indicate to the kernel that it shouldn't bother clearing the memory before + returning it. Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled + to permit this, otherwise the flag will be ignored. + + uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this + to allocate the brk and stack region. + (*) A list of all the private copy and anonymous mappings on the system is visible through /proc/maps in no-MMU mode. |