On Thu, Mar 11, 2021 at 1:35 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 11.03.21 22:26, Peter Xu wrote: > > On Thu, Mar 11, 2021 at 07:14:02PM +0100, David Hildenbrand wrote: > >> I was wondering, is there any mechanism that reclaims basically empty page > >> tables in a running process? > > > > Would munmap() count? :) > > Haha, no -- also not mmap(FIXED) or mremap(FIXED) ;) > > As so often lately, the use case is sparse memory mappings where we > > a) may want to reuse the area later. > b) don't want to hold the mmap lock in write while optimizing > c) don't want to create a lot of individual mappings that we might not > be able to merge again. Will the below work for you? 1. acquire write mmap lock 2. unlink vmas from the list and rbtree (so the vmas won't be visible to any concurrent readers/writers) 3. downgrade write lock to read lock 4. zap page tables and free page tables 5. upgrade to write lock 6. relink vmas back to list and rbtree Actually the current implementation of munmap() does the first 5 steps. But there is no rwsem upgrade function, so you may have to release the lock then reacquire the write lock. > > > -- > Thanks, > > David / dhildenb > >
