On Thu, Mar 11, 2021 at 06:15:15PM +0100, David Hildenbrand wrote: > 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?
I see madvise_dontneed_free() calls zap_page_range(). > > > > > MADV_DONTNEED only invalidates private copies in the pagecache. It's > essentially useless for any kind of shared mappings. Since it's about zapping page tables, then I don't understand why it won't work for shmem.. -- Peter Xu
