current_process_memory_usage: Better document what happens with inacc…

…urate commit charge calculation on older Linux kernels.

include/llfio/revision.hpp

@@ -1,4 +1,4 @@
 // Note the second line of this file must ALWAYS be the git SHA, third line ALWAYS the git SHA update time
-#define LLFIO_PREVIOUS_COMMIT_REF    46f0760e7ed2b80c46acd91bacc130049620bee6
-#define LLFIO_PREVIOUS_COMMIT_DATE   "2021-08-21 13:31:38 +00:00"
-#define LLFIO_PREVIOUS_COMMIT_UNIQUE 46f0760e
+#define LLFIO_PREVIOUS_COMMIT_REF    8cec4ff8fcd601b774c0ed882fe869e73d23d136
+#define LLFIO_PREVIOUS_COMMIT_DATE   "2021-08-30 14:53:35 +00:00"
+#define LLFIO_PREVIOUS_COMMIT_UNIQUE 8cec4ff8

include/llfio/v2.0/detail/impl/posix/utils.ipp

@@ -333,7 +333,7 @@ namespace utils
 
       (values are in pages)
 
-      /proc/[pid]/smaps_rollup:
+      /proc/[pid]/smaps_rollup (Linux 3.16 onwards only):
 
       total_address_space_in_use = ??? MISSING
       total_address_space_paged_in = ??? MISSING
@@ -363,7 +363,16 @@ namespace utils
         if(want & process_memory_usage::want::private_committed)
         {
           std::vector<char> smaps_rollup(256), maps(65536);
-          OUTCOME_TRY(fill_buffer(smaps_rollup, "/proc/self/smaps_rollup"));
+          auto r = fill_buffer(smaps_rollup, "/proc/self/smaps_rollup");
+          if(!r)
+          {
+            if(r.error() == errc::no_such_file_or_directory)
+            {
+              // Linux kernel is too old
+              return errc::operation_not_supported;
+            }
+            return std::move(r).error();
+          }
           OUTCOME_TRY(fill_buffer(maps, "/proc/self/maps"));
           uint64_t lazyfree = 0;
           {

include/llfio/v2.0/utils.hpp

@@ -236,6 +236,14 @@ namespace utils
   cannot distinguish between regions with the accounted
   flag enabled or disabled. By default, this fast path is enabled.
 
+  \note `/proc/pid/smaps_rollup` was added in Linux kernel 3.16, so the default specifying
+  `process_memory_usage::want::private_committed_inaccurate` will always fail on Linux
+  kernels preceding that with an error code comparing equal to `errc::operation_not_supported`.
+  As one would assume users would prefer this operation to fail on older kernels rather than
+  silently go slowly in complex memory spaces, it is left opt-in to request
+  the accurate implementation which works on older Linux kernels. Or, just don't request
+  `private_committed` at all, and pretend `private_paged_in` means the same thing.
+
   \note Mac OS provides no way of reading how much memory a process has committed.
   We therefore supply as `private_committed` the same value as `private_paged_in`.
   */