Prevent wasted memory in guard regions

diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c
index 2a5ac22117141a1a0b7d04e46fcba458feec80e0..e8dfbbb8deb15c864b994aa2e4e6c531048ba5a8 100644 (file)
--- a/nptl/allocatestack.c
+++ b/nptl/allocatestack.c
@@ -656,6 +656,15 @@ allocate_stack (const struct pthread_attr *attr, struct pthread **pdp,
 
              return errno;
            }
+         /* We've marked this guard region unwritable, but it's
+            possible it already became resident, the most common case
+            being transparent hugepages; if stack + guard (+ adjacent
+            mmap regions) were more than 2MB, the kernel might have
+            filled in a full hugepage when we touched the thread
+            descriptor above.  This can waste a lot of memory; mark
+            the guard unused. This shouldn't fail, but if it does,
+            we can't really do anything about it so ignore it. */
+         madvise (guard, guardsize, MADV_DONTNEED);
 
          pd->guardsize = guardsize;
        }