GNU C Library master sources branch master updated. glibc-2.22-234-gca6be16

[carlos at sourceware dot org]

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http://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=ca6be1655bd357bf6ac8857fba9b9dce928edbdc

commit ca6be1655bd357bf6ac8857fba9b9dce928edbdc
Author: Carlos O'Donell <carlos@systemhalted.org>
Date:   Mon Sep 14 15:32:47 2015 -0400

    Use ALIGN_DOWN in systrim.
    
    While doing code review I converted another bespoke round down, and
    corrected a comment.
    
    The comment spoke about keeping at least one page allocated even
    during systrim, which is not correct. The code does nothing to keep
    a page allocated. The code does attempt to keep PAD padding as
    documented in comments and MINSIZE as required by design.
    
    Historically in 2002 when Ulrich wrote the code (fa8d436c) the math
    was inlined into one statement which did reserve an extra page:
    extra = ((top_size - pad - MINSIZE + (pagesz-1)) / pagesz - 1) * pagesz;
    There is no reason given for this extra page.
    
    In 2010 Anton Branchard's change (b9b42ee0) from division
    to shifts removed the extra page by dropping the "+ (pagesiz-1), which
    mean we might have attempted to return -0 via MORECORE. The fix by Will
    Newton in 2014 added a check for extra being zero (51a7380b).
    
    From first principles I see no reason why we should keep an extra
    page of memory from being trimmed back to the OS. The only sensible
    interface is to honour PAD padding as the function is documented,
    with the caveat the MINSIZE is maintained for the top chunk.
    
    Given that we've been using this code for 5+ years with no extra
    page allocated is sufficient evidence that the comment should be changed
    to match the code that I'm touching.
    
    Tested on x86_64 and i686, no regressions.

diff --git a/ChangeLog b/ChangeLog
index 33d0e1b..261f944 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,7 @@
 2015-09-14  Carlos O'Donell  <carlos@redhat.com>
 
+	* malloc/malloc.c (systrim): Use ALIGN_DOWN.
+
 	* Makefile ($(objpfx)check-local-headers.out): Redirect stdin from
 	/dev/null.
 
diff --git a/malloc/malloc.c b/malloc/malloc.c
index 452f036..0eca9ce 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -241,7 +241,7 @@
 /* For MIN, MAX, powerof2.  */
 #include <sys/param.h>
 
-/* For ALIGN_UP.  */
+/* For ALIGN_UP et. al.  */
 #include <libc-internal.h>
 
 
@@ -2767,8 +2767,8 @@ systrim (size_t pad, mstate av)
   if (top_area <= pad)
     return 0;
 
-  /* Release in pagesize units, keeping at least one page */
-  extra = (top_area - pad) & ~(pagesize - 1);
+  /* Release in pagesize units and round down to the nearest page.  */
+  extra = ALIGN_DOWN(top_area - pad, pagesize);
 
   if (extra == 0)
     return 0;

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog       |    2 ++
 malloc/malloc.c |    6 +++---
 2 files changed, 5 insertions(+), 3 deletions(-)


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