diff options
| -rw-r--r-- | Documentation/vm/remap_file_pages.txt | 28 | ||||
| -rw-r--r-- | mm/fremap.c | 4 |
2 files changed, 32 insertions, 0 deletions
diff --git a/Documentation/vm/remap_file_pages.txt b/Documentation/vm/remap_file_pages.txt new file mode 100644 index 000000000000..560e4363a55d --- /dev/null +++ b/Documentation/vm/remap_file_pages.txt @@ -0,0 +1,28 @@ +The remap_file_pages() system call is used to create a nonlinear mapping, +that is, a mapping in which the pages of the file are mapped into a +nonsequential order in memory. The advantage of using remap_file_pages() +over using repeated calls to mmap(2) is that the former approach does not +require the kernel to create additional VMA (Virtual Memory Area) data +structures. + +Supporting of nonlinear mapping requires significant amount of non-trivial +code in kernel virtual memory subsystem including hot paths. Also to get +nonlinear mapping work kernel need a way to distinguish normal page table +entries from entries with file offset (pte_file). Kernel reserves flag in +PTE for this purpose. PTE flags are scarce resource especially on some CPU +architectures. It would be nice to free up the flag for other usage. + +Fortunately, there are not many users of remap_file_pages() in the wild. +It's only known that one enterprise RDBMS implementation uses the syscall +on 32-bit systems to map files bigger than can linearly fit into 32-bit +virtual address space. This use-case is not critical anymore since 64-bit +systems are widely available. + +The plan is to deprecate the syscall and replace it with an emulation. +The emulation will create new VMAs instead of nonlinear mappings. It's +going to work slower for rare users of remap_file_pages() but ABI is +preserved. + +One side effect of emulation (apart from performance) is that user can hit +vm.max_map_count limit more easily due to additional VMAs. See comment for +DEFAULT_MAX_MAP_COUNT for more details on the limit. diff --git a/mm/fremap.c b/mm/fremap.c index 2c5646f11f41..72b8fa361433 100644 --- a/mm/fremap.c +++ b/mm/fremap.c @@ -149,6 +149,10 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, int has_write_lock = 0; vm_flags_t vm_flags = 0; + pr_warn_once("%s (%d) uses deprecated remap_file_pages() syscall. " + "See Documentation/vm/remap_file_pages.txt.\n", + current->comm, current->pid); + if (prot) return err; /* |