[haiku-commits] haiku: hrev43267 - src/system/kernel/vm

From: mmlr@xxxxxxxx
To: haiku-commits@xxxxxxxxxxxxx
Date: Mon, 14 Nov 2011 19:13:10 +0100 (CET)
Revision:    hrev43267
Commit:      c12f51264ba5a43bd1ba6408d96ee8b5da9e822f
URL:         http://cgit.haiku-os.org/haiku/commit/?id=c12f512
Author:      Michael Lotz <mmlr@xxxxxxxx>
Date:        Mon Nov 14 18:07:37 2011 UTC

Ensure the sanity of the stats returned, make the TODO a Note.

* Ensure that we don't underflow the used_pages count and that used
  + cached pages don't overflow max_pages. As there is no locking the
  values may change while we read them so that such situations could
  arise.
* Make the TODO about the missing locking into a Note explaining the
  above, as it is not really worth adding locking here. The stats are
  only informational.

............................................................................

 src/system/kernel/vm/vm_page.cpp |   20 +++++++++++++++-----
 1 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/src/system/kernel/vm/vm_page.cpp b/src/system/kernel/vm/vm_page.cpp
index 559aace..6a6dbb1 100644
--- a/src/system/kernel/vm/vm_page.cpp
+++ b/src/system/kernel/vm/vm_page.cpp
@@ -4053,9 +4053,11 @@ vm_page_num_unused_pages(void)
 void
 vm_page_get_stats(system_info *info)
 {
-       // TODO: there's no locking protecting any of the queues or counters 
here,
-       // so we run the risk of getting bogus values when evaluating them at
-       // throughout this function...
+       // Note: there's no locking protecting any of the queues or counters 
here,
+       // so we run the risk of getting bogus values when evaluating them
+       // throughout this function. As these stats are for informational 
purposes
+       // only, it is not really worth introducing such locking. Therefore we 
just
+       // ensure that we don't under- or overflow any of the values.
 
        // The pages used for the block cache buffers. Those should not be 
counted
        // as used but as cached pages.
@@ -4079,8 +4081,16 @@ vm_page_get_stats(system_info *info)
        //      active + inactive + unused + wired + modified + cached + free + 
clear
        // So taking out the cached (including modified non-temporary), free and
        // clear ones leaves us with all used pages.
-       info->used_pages = info->max_pages - info->cached_pages
-               - sFreePageQueue.Count() - sClearPageQueue.Count();
+       int32 subtractPages = info->cached_pages + sFreePageQueue.Count()
+               + sClearPageQueue.Count();
+       info->used_pages = subtractPages > info->max_pages
+               ? 0 : info->max_pages - subtractPages;
+
+       if (info->used_pages + info->cached_pages > info->max_pages) {
+               // Something was shuffled around while we were summing up the 
counts.
+               // Make the values sane, preferring the worse case of more used 
pages.
+               info->cached_pages = info->max_pages - info->used_pages;
+       }
 
        info->page_faults = vm_num_page_faults();
        info->ignored_pages = sIgnoredPages;
[haiku-commits] haiku: hrev43267 - src/system/kernel/vm

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