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;
         /*