linux/drivers/misc/cxl
Michal Hocko 0ee931c4e3 mm: treewide: remove GFP_TEMPORARY allocation flag
GFP_TEMPORARY was introduced by commit e12ba74d8f ("Group short-lived
and reclaimable kernel allocations") along with __GFP_RECLAIMABLE.  It's
primary motivation was to allow users to tell that an allocation is
short lived and so the allocator can try to place such allocations close
together and prevent long term fragmentation.  As much as this sounds
like a reasonable semantic it becomes much less clear when to use the
highlevel GFP_TEMPORARY allocation flag.  How long is temporary? Can the
context holding that memory sleep? Can it take locks? It seems there is
no good answer for those questions.

The current implementation of GFP_TEMPORARY is basically GFP_KERNEL |
__GFP_RECLAIMABLE which in itself is tricky because basically none of
the existing caller provide a way to reclaim the allocated memory.  So
this is rather misleading and hard to evaluate for any benefits.

I have checked some random users and none of them has added the flag
with a specific justification.  I suspect most of them just copied from
other existing users and others just thought it might be a good idea to
use without any measuring.  This suggests that GFP_TEMPORARY just
motivates for cargo cult usage without any reasoning.

I believe that our gfp flags are quite complex already and especially
those with highlevel semantic should be clearly defined to prevent from
confusion and abuse.  Therefore I propose dropping GFP_TEMPORARY and
replace all existing users to simply use GFP_KERNEL.  Please note that
SLAB users with shrinkers will still get __GFP_RECLAIMABLE heuristic and
so they will be placed properly for memory fragmentation prevention.

I can see reasons we might want some gfp flag to reflect shorterm
allocations but I propose starting from a clear semantic definition and
only then add users with proper justification.

This was been brought up before LSF this year by Matthew [1] and it
turned out that GFP_TEMPORARY really doesn't have a clear semantic.  It
seems to be a heuristic without any measured advantage for most (if not
all) its current users.  The follow up discussion has revealed that
opinions on what might be temporary allocation differ a lot between
developers.  So rather than trying to tweak existing users into a
semantic which they haven't expected I propose to simply remove the flag
and start from scratch if we really need a semantic for short term
allocations.

[1] http://lkml.kernel.org/r/20170118054945.GD18349@bombadil.infradead.org

[akpm@linux-foundation.org: fix typo]
[akpm@linux-foundation.org: coding-style fixes]
[sfr@canb.auug.org.au: drm/i915: fix up]
  Link: http://lkml.kernel.org/r/20170816144703.378d4f4d@canb.auug.org.au
Link: http://lkml.kernel.org/r/20170728091904.14627-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Neil Brown <neilb@suse.de>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-09-13 18:53:16 -07:00
..
Kconfig cxl: Export library to support IBM XSL 2017-07-03 23:07:03 +10:00
Makefile cxl: Export library to support IBM XSL 2017-07-03 23:07:03 +10:00
api.c cxl: Fix driver use count 2017-08-31 14:26:49 +10:00
base.c cxl: Add support for interrupts on the Mellanox CX4 2016-07-14 20:27:08 +10:00
context.c cxl: Fixes for Coherent Accelerator Interface Architecture 2.0 2017-06-23 16:26:23 +10:00
cxl.h cxl: Export library to support IBM XSL 2017-07-03 23:07:03 +10:00
cxllib.c cxl: Export library to support IBM XSL 2017-07-03 23:07:03 +10:00
debugfs.c cxl: Add psl9 specific code 2017-04-13 23:34:31 +10:00
fault.c powerpc/mm/cxl: Add barrier when setting mm cpumask 2017-08-31 14:26:39 +10:00
file.c cxl: Fix driver use count 2017-08-31 14:26:49 +10:00
flash.c cxl: Unlock on error in probe 2017-06-06 19:23:52 +10:00
guest.c cxl: Add psl9 specific code 2017-04-13 23:34:31 +10:00
hcalls.c cxl: Remove unused values in bare-metal environment. 2017-04-13 23:34:28 +10:00
hcalls.h cxl: Add guest-specific code 2016-03-09 23:36:52 +11:00
irq.c cxl: Add psl9 specific code 2017-04-13 23:34:31 +10:00
main.c cxl: Fixes for Coherent Accelerator Interface Architecture 2.0 2017-06-23 16:26:23 +10:00
native.c cxl: Export library to support IBM XSL 2017-07-03 23:07:03 +10:00
of.c cxl: replace loop with for_each_child_of_node(), remove unneeded of_node_put() 2016-10-04 16:19:23 +11:00
pci.c mm: treewide: remove GFP_TEMPORARY allocation flag 2017-09-13 18:53:16 -07:00
phb.c cxl: Fix error handling in _cxl_pci_associate_default_context() 2016-11-18 22:41:08 +11:00
sysfs.c cxl: Prevent adapter reset if an active context exists 2016-10-19 20:35:39 +11:00
trace.c cxl: Add tracepoints 2015-01-22 17:31:51 +11:00
trace.h cxl: Add psl9 specific code 2017-04-13 23:34:31 +10:00
vphb.c cxl: fix nested locking hang during EEH hotplug 2017-02-21 21:32:52 +11:00