On Mon, Mar 04, 2024 at 03:19:42PM +0000, Matthew Wilcox wrote: > On Thu, Feb 29, 2024 at 11:34:33AM -0700, Yu Zhao wrote: > > ZONE_MOVABLE becomes the first policy zone, followed by two new policy > > zones: > > 1. ZONE_NOSPLIT, which contains pages that are movable (inherited from > > ZONE_MOVABLE) and restricted to a minimum order to be > > anti-fragmentation. The latter means that they cannot be split down > > below that order, while they are free or in use. > > 2. ZONE_NOMERGE, which contains pages that are movable and restricted > > to an exact order. The latter means that not only is split > > prohibited (inherited from ZONE_NOSPLIT) but also merge (see the > > reason in Chapter Three), while they are free or in use. > > These two zones end up solving a problem for memdescs. So I'm in favour! > I added Option 5 to https://kernelnewbies.org/MatthewWilcox/BuddyAllocator I realised that we don't even need a doubly-linked-list for ZONE_NOMERGE (would ZONE_FIXEDSIZE be a better name?) We only need a oubly linked list to make removal from the middle of the list an O(1) operation, and we only remove from the middle of a list when merging. So we can simply keep a stack of free "pages" and we have 60 bits to point to the next memdesc, so we can easily cover all memory that can exist in a 64-bit machine in ZONE_NOMERGE. ZONE_NOSPLIT would be limited to the first 1PB of memory (assuming it has a minimum size of 2MB -- with 29 bits to refer to each of next & prev, 29 + 21 = 50 bits of address space).
