[Okular-devel] [okular] [Bug 320932] New: Okular's cache wrongly counts RAM used for shm as free, including that used by its cache. System-wide hilarity may ensue.

[makosoft at googlemail.com]

https://bugs.kde.org/show_bug.cgi?id=320932

            Bug ID: 320932
           Summary: Okular's cache wrongly counts RAM used for shm as
                    free, including that used by its cache. System-wide
                    hilarity may ensue.
    Classification: Unclassified
           Product: okular
           Version: 0.16.3
          Platform: Gentoo Packages
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: makosoft at googlemail.com

Okular is designed to cache pixmaps of pages in RAM for faster viewing. In
order to prevent it from using too much RAM for its cache, it dynamically trims
the cache based on the amount of free RAM. However the code that calculates the
amount of free memory is subtly flawed, occassionally with spectacular
consequences. From DocumentPrivate::getFreeMemory in document.cpp:

    // read /proc/meminfo and sum up the contents of 'MemFree', 'Buffers'
    // and 'Cached' fields. consider swapped memory as used memory.

For various reasons, Linux counts SysV shared memory as "Cached" memory even
though unlike real cache it can't ever be discarded as a result of memory
pressure. So when there's lots of SysV shared memory in use okular
overestimates how much memory is free. This wouldn't be an issue, except that
at least on my system Okular's cache seems to itself be stored as SysV shared
memory and is therefore itself counted as free by Okular. Under the right
circumstances this can apparently help bring a system to its knees.

Reproducible: Didn't try

Steps to Reproduce:
Probably not trivially reproducable. I noticed there was a problem after about
a month of uptime with numerous copies of Okular open, heavy use of search
within multi-hundred-page documents, etc. Due to the way Okular decides to free
images from its cache, reproducing this fully probably also requires much more
RAM than swap and another shm-hogging application to confuse Okular initially.
Actual Results:  
About half of my available 12 gigabytes of RAM was consumed by shared memory,
of which Okular instances were responsible for about 2-3 GB. I noticed there
was a problem when the system started swapping to the point of unusability
despite half of the RAM superficially appearing to be free. The Linux OOM
killer helpfully failed to kill the offending apps, instead killing other
applications and helping to free up more memory for them.

Expected Results:  
Okular should have detected that the system was running low on memory and
dropped its caches.

Gentoo x86-64, Qt  4.8.4

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