[Previously mshare patches. After discussion on mailing list and at LSF/MM, mshare implementation has been reworked to eliminate the mshare API and use mmap instead with a new flag. This eliminates the need for msharefs. Alignment and size restrictions were changed to PMD] Memory pages shared between processes require a page table entry (PTE) for each process. Each of these PTE consumes consume some of the memory and as long as number of mappings being maintained is small enough, this space consumed by page tables is not objectionable. When very few memory pages are shared between processes, the number of page table entries (PTEs) to maintain is mostly constrained by the number of pages of memory on the system. As the number of shared pages and the number of times pages are shared goes up, amount of memory consumed by page tables starts to become significant. This issue does not apply to threads. Any number of threads can share the same pages inside a process while sharing the same PTEs. Extending this same model to sharing pages across processes can eliminate this issue for sharing across processes as well. Some of the field deployments commonly see memory pages shared across 1000s of processes. On x86_64, each page requires a PTE that is only 8 bytes long which is very small compared to the 4K page size. When 2000 processes map the same page in their address space, each one of them requires 8 bytes for its PTE and together that adds up to 8K of memory just to hold the PTEs for one 4K page. On a database server with 300GB SGA, a system crash was seen with out-of-memory condition when 1500+ clients tried to share this SGA even though the system had 512GB of memory. On this server, in the worst case scenario of all 1500 processes mapping every page from SGA would have required 878GB+ for just the PTEs. If these PTEs could be shared, amount of memory saved is very significant. This patch series adds a new flag to mmap() call - MAP_SHARED_PT. This map can be specified along with MAP_SHARED by a process to hint to kernel that it wishes to share page table entries for this file mapping mmap region with other processes. Any other process that mmaps the same file with MAP_SHARED_PT flag can then share the same page table entries. Besides specifying MAP_SHARED_PT flag, the processes must map the files at a PMD aligned address with a size that is a multiple of PMD size and at the same virtual addresses. This last requirement of same virtual addresses can possibly be relaxed if that is the consensus. When mmap() is called with MAP_SHARED_PT flag, a new host mm struct is created to hold the shared page tables. Host mm struct is not attached to a process. Start and size of host mm are set to the start and size of the mmap region and a VMA covering this range is also added to host mm struct. Existing page table entries from the process that creates the mapping are copied over to the host mm struct. All processes mapping this shared region are considered guest processes. When a guest process mmap's the shared region, a vm flag VM_SHARED_PT is added to the VMAs in guest process. Upon a page fault, VMA is checked for the presence of VM_SHARED_PT flag. If the flag is found, its corresponding PMD is updated with the PMD from host mm struct so the PMD will point to the page tables in host mm struct. vm_mm pointer of the VMA is also updated to point to host mm struct for the duration of fault handling to ensure fault handling happens in the context of host mm struct. When a new PTE is created, it is created in the host mm struct page tables and the PMD in guest mm points to the same PTEs. This is a basic working implementation. It will need to go through more testing and refinements. Some notes and questions: - PMD size alignment and size requirement is currently hard coded in. Is there a need or desire to make this more flexible and work with other alignments/sizes? PMD size allows for adapting this infrastructure to form the basis for hugetlbfs page table sharing as well. More work ill be needed to make that happen. - Is there a reason to allow a userspace app to query this size and alignment requirement for MAP_SHARED_PT in some way? - Shared PTEs means mprotect() call made by one process affects all processes sharing the same mapping and that behavior will need to be documented clearly. Effect of mprotect call being different for processes using shared page tables is the primary reason to require an explicit opt-in from userspace processes to share page tables. With a transparent sharing derived from MAP_SHARED alone, changed effect of mprotect can break significant number of userspace apps. One could work around that by unsharing whenever mprotect changes modes on shared mapping but that introduces complexity and the capability to execute a single mprotect to change modes across 1000's of processes sharing a mapped database is a feature explicitly asked for by database folks. This capability has significant performance advantage when compared to mechanism of sending messages to every process using shared mapping to call mprotect and change modes in each process, or using traps on permissions mismatch in each process. - This implementation does not allow unmapping page table shared mappings partially. Should that be supported in future? - Patch 2 will be broken up into smaller patches after RFC. It likely is easier to review the proposal with all code in one patch for now. Khalid Aziz (2): mm/ptshare: Add vm flag for shared PTE mm/ptshare: Create a new mm to share pagetables include/linux/fs.h | 1 + include/linux/mm.h | 8 + include/trace/events/mmflags.h | 3 +- include/uapi/asm-generic/mman-common.h | 1 + mm/Makefile | 2 +- mm/internal.h | 21 ++ mm/memory.c | 52 ++++- mm/mmap.c | 87 ++++++++ mm/ptshare.c | 262 +++++++++++++++++++++++++ 9 files changed, 433 insertions(+), 4 deletions(-) create mode 100644 mm/ptshare.c -- 2.34.1
