[Bug malloc/20284] New: malloc: Corrupt arena avoidance causes unnecessary mmap fallbacks

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=20284

            Bug ID: 20284
           Summary: malloc: Corrupt arena avoidance causes unnecessary
                    mmap fallbacks
           Product: glibc
           Version: 2.24
            Status: NEW
          Severity: normal
          Priority: P2
         Component: malloc
          Assignee: fweimer at redhat dot com
          Reporter: fweimer at redhat dot com
  Target Milestone: ---
             Flags: security-

Josef Bacik of Facebook reports that âany sort of lock contention on the
arena's results in us falling back on mmap()'ing a new chunk.  This is because
we check to see if the uncontended arena we check is corrupt, and if it is we
loop through, and if we loop to the beginning we know we didn't find anything. 
Except if our initial arena isn't actually corrupt we'll still return NULL, so
we fall back on this mmap() thing more often, which really makes things
unstable.â

This was with a downstream backport of the arena corruption detection patches.

A previous discussion of this issue is here:

  https://sourceware.org/ml/libc-alpha/2015-09/msg00262.html

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