On 11.03.21 18:11, Peter Xu wrote:
On Thu, Mar 11, 2021 at 05:45:46PM +0100, David Hildenbrand wrote:
On 11.03.21 17:39, Dr. David Alan Gilbert wrote:
* David Hildenbrand (da...@redhat.com) wrote:
We can create shared anonymous memory via
"-object memory-backend-ram,share=on,..."
which is, for example, required by PVRDMA for mremap() to work.
Shared anonymous memory is weird, though. Instead of MADV_DONTNEED, we
have to use MADV_REMOVE. MADV_DONTNEED fails silently and does nothing.
OK, I wonder how stable these rules are; is it defined anywhere that
it's required?
I had a look at the Linux implementation: it's essentially shmem ... but we
don't have an fd exposed, so we cannot use fallocate() ... :)
MADV_REMOVE documents (man):
"In the initial implementation, only tmpfs(5) was supported MADV_REMOVE; but
since Linux 3.5, any filesystem which supports the fallocate(2)
FALLOC_FL_PUNCH_HOLE mode also supports MADV_REMOVE."
Hmm, I see that MADV_DONTNEED will still tear down all mappings even for
anonymous shmem.. what did I miss?
Where did you see that?
MADV_DONTNEED only invalidates private copies in the pagecache. It's
essentially useless for any kind of shared mappings.
(I am 99.9% sure that we can replace fallocate()+MADV_DONTNEED by
fallocate() for fd-based shared mappings, but that's a different story)
--
Thanks,
David / dhildenb
