On 27.01.25 23:33, Andrew Morton wrote:
On Fri, 24 Jan 2025 15:54:34 -0800 Anthony Yznaga <anthony.yznaga@xxxxxxxxxx> wrote:Memory pages shared between processes require page table entries (PTEs) for each process. Each of these PTEs consume some of the memory and as long as the 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 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. ... API === mshare does not introduce a new API. It instead uses existing APIs to implement page table sharing. The steps to use this feature are: 1. Mount msharefs on /sys/fs/mshare - mount -t msharefs msharefs /sys/fs/mshare 2. mshare regions have alignment and size requirements. Start address for the region must be aligned to an address boundary and be a multiple of fixed size. This alignment and size requirement can be obtained by reading the file /sys/fs/mshare/mshare_info which returns a number in text format. mshare regions must be aligned to this boundary and be a multiple of this size. 3. For the process creating an mshare region: a. Create a file on /sys/fs/mshare, for example - fd = open("/sys/fs/mshare/shareme", O_RDWR|O_CREAT|O_EXCL, 0600); b. Establish the starting address and size of the region struct mshare_info minfo; minfo.start = TB(2); minfo.size = BUFFER_SIZE; ioctl(fd, MSHAREFS_SET_SIZE, &minfo)
>>>> c. Map some memory in the region
>> mcreate.size = BUFFER_SIZE;>> mcreate.offset = 0;struct mshare_create mcreate; mcreate.addr = TB(2);
mcreate.prot = PROT_READ | PROT_WRITE; mcreate.flags = MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED; mcreate.fd = -1; ioctl(fd, MSHAREFS_CREATE_MAPPING, &mcreate)I'm not really understanding why step a exists. It's basically an mmap() so why can't this be done within step d?
Conceptually, it's defining the content of the virtual file: by creating mappings/unmapping mappings/changing mappings. Some applications will require multiple different mappings in such a virtual file.
Processes mmap the resulting virtual file. -- Cheers, David / dhildenb
