[Bug malloc/26663] New: malloc() doesn't handle MALLOC_MMAP_THRESHOLD_=0 well

["toiwoton at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26663

            Bug ID: 26663
           Summary: malloc() doesn't handle MALLOC_MMAP_THRESHOLD_=0 well
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: malloc
          Assignee: unassigned at sourceware dot org
          Reporter: toiwoton at gmail dot com
  Target Milestone: ---

Using environment variable MALLOC_MMAP_THRESHOLD_=0 may cause malloc() to fail
very easily. A case in point is startup of systemd-cryptsetup, which fails when
lvm2 library can't malloc() something
(https://github.com/lvmteam/lvm2/issues/39). This is probably a bug in kernel,
there's plenty of RAM available and I suppose the memory shouldn't ever be too
fragmented for page sized items.

But perhaps malloc() should use a better allocation strategy when mmap()ing,
for example mmap() larger areas at once, use them as arenas and deal them to
malloc() users as needed.

Maybe also when mmap() specifically fails with EAGAIN it could be retried a few
times. It looks like sysmalloc() only checks for MAP_FAILED
(https://sourceware.org/git?p=glibc.git;a=blob;f=malloc/malloc.c;h=cd9933b4e580a58a694ebf34e76ac6fecee29c14;hb=HEAD#l2330)
but not errno. Perhaps also some unused memory could be released by
munmap()ping unused parts of arenas etc. before retrying.

-Topi

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