On 12.03.21 23:45, Matthew Wilcox wrote:
On Fri, Mar 12, 2021 at 04:41:55PM +0100, Peter Weber wrote:
Thank you Matthew!
Am 2021-03-12 16:15, schrieb Matthew Wilcox:
The wikipedia diagram is wrong. Anonymous memory is not handled by the
page cache.
Is it roughly right to say, that the virtual memory uses page tables to
handle anonymous memory?
You'll have to distinguish between private and shared anonymous memory.
"private anonymous memory" -- mmap(MAP_ANONYMOUS | MAP_PRIVATE) -- is
usually completely managed using the process page tables. The only way
to get such pages mapped into another process is via fork(), whereby
page table are copied and COW (Copy On Write) applies.
Once dereferenced from all page tables, there are no other references
anymore; memory is handed back to the buddy as free memory, from where
it can be reused for other purposes. Of course, there are case where
there might be other references being taken (i.e., GUP, direct I/O ...),
but let's ignore that for now.
The essence for "private anonymous memory" is, that there is no other
way to get access to that memory besides the page tables. If you
MADV_DONTNEED virtual memory regions to zap the page table entries and
later reaccess the same virtual memory locations, always fresh memory
will be populated.
"shared anonymous memory" -- mmap(MAP_ANONYMOUS | MAP_SHARED) -- is a
little different and involves the page cache (it's pretty much shmem
without a user-visible fd). The only way to get such pages mapped into
another process is similarly via fork() (AFAIU), whereby page table are
copied and COW does _not_ apply.
Once dereferenced from all page tables, there is still a reference to
the page from the page cache. Only once evicted from the page cache, the
memory can be freed up.
The essence for "shared anonymous memory" is, that there are ways to get
access to that memory besides the page tables. If you MADV_DONTNEED such
virtual memory locations to zap the page table entries and later
reaccess the same virtual memory location again, the previous page will
be re-instantiated from the page cache. To evict the pages from the page
cache you would need MADV_REMOVE.
--
Thanks,
David / dhildenb
