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? -- Peter Xu
