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Subject[PATCH 0/8] avoid allocation in show_numa_map()
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Recently a concern was raised[1] that performing an allocation while holding a
reference on a tasks mm could lead to a stalemate in the oom killer. The
concern was specific to the goings-on in /proc. Hugh Dickins stated the issue
thusly:

...imagine what happens if the system is out of memory, and the mm
we're looking at is selected for killing by the OOM killer: while we
wait in __get_free_page for more memory, no memory is freed from the
selected mm because it cannot reach exit_mmap while we hold that
reference.

The primary goal of this series is to eliminate repeated allocation/free cycles
currently happening in show_numa_maps() while we hold a reference to an mm.

The strategy is to perform the allocation once when /proc/pid/numa_maps is
opened, before a reference on the target tasks mm is taken.

Unfortunately, show_numa_maps() is implemented in mm/mempolicy.c while the
primary procfs implementation lives in fs/proc/task_mmu.c. This makes
clean cooperation between show_numa_maps() and the other seq_file operations
(start(), stop(), etc) difficult.


Patches 1-5 convert show_numa_maps() to use the generic walk_page_range()
functionality instead of the mempolicy.c specific page table walking logic.
Also, get_vma_policy() is exported. This makes the show_numa_maps()
implementation independent of mempolicy.c.

Patch 6 moves show_numa_maps() and supporting routines over to
fs/proc/task_mmu.c.

Finally, patches 7 and 8 provide minor cleanup and eliminates the troublesome
allocation.


Please note that moving show_numa_maps() into fs/proc/task_mmu.c essentially
reverts 1a75a6c825 and 48fce3429d. Also, please see the discussion at [2]. My
main justifications for moving the code back into task_mmu.c is:

- Having the show() operation "miles away" from the corresponding
seq_file iteration operations is a maintenance burden.

- The need to export ad hoc info like struct proc_maps_private is
eliminated.


These patches are based on v2.6.39-rc5.


Please note that this series is VERY LIGHTLY TESTED. I have been using
CONFIG_NUMA_EMU=y thus far as I will not have access to a real NUMA system for
another week or two.


--
steve


[1] lkml.org/lkml/2011/4/25/496
[2] marc.info/?t=113149255100001&r=1&w=2

Stephen Wilson (8):
mm: export get_vma_policy()
mm: use walk_page_range() instead of custom page table walking code
mm: remove MPOL_MF_STATS
mm: make gather_stats() type-safe and remove forward declaration
mm: remove check_huge_range()
mm: proc: move show_numa_map() to fs/proc/task_mmu.c
proc: make struct proc_maps_private truly private
proc: allocate storage for numa_maps statistics once


fs/proc/internal.h | 8 ++
fs/proc/task_mmu.c | 190 ++++++++++++++++++++++++++++++++++++++++++++-
include/linux/mempolicy.h | 3 +
include/linux/proc_fs.h | 8 --
mm/mempolicy.c | 164 +--------------------------------------
5 files changed, 200 insertions(+), 173 deletions(-)



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