From 0ee931c4e31a5efb134c76440405e9219f896e33 Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Wed, 13 Sep 2017 16:28:29 -0700 Subject: mm: treewide: remove GFP_TEMPORARY allocation flag GFP_TEMPORARY was introduced by commit e12ba74d8ff3 ("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 Signed-off-by: Stephen Rothwell Acked-by: Mel Gorman Acked-by: Vlastimil Babka Cc: Matthew Wilcox Cc: Neil Brown Cc: "Theodore Ts'o" Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/gfp.h | 2 -- include/trace/events/mmflags.h | 1 - 2 files changed, 3 deletions(-) (limited to 'include') diff --git a/include/linux/gfp.h b/include/linux/gfp.h index bcfb9f7c46f5..f780718b7391 100644 --- a/include/linux/gfp.h +++ b/include/linux/gfp.h @@ -288,8 +288,6 @@ struct vm_area_struct; #define GFP_NOWAIT (__GFP_KSWAPD_RECLAIM) #define GFP_NOIO (__GFP_RECLAIM) #define GFP_NOFS (__GFP_RECLAIM | __GFP_IO) -#define GFP_TEMPORARY (__GFP_RECLAIM | __GFP_IO | __GFP_FS | \ - __GFP_RECLAIMABLE) #define GFP_USER (__GFP_RECLAIM | __GFP_IO | __GFP_FS | __GFP_HARDWALL) #define GFP_DMA __GFP_DMA #define GFP_DMA32 __GFP_DMA32 diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h index 4c2e4737d7bc..fec6291a6703 100644 --- a/include/trace/events/mmflags.h +++ b/include/trace/events/mmflags.h @@ -18,7 +18,6 @@ {(unsigned long)GFP_HIGHUSER_MOVABLE, "GFP_HIGHUSER_MOVABLE"},\ {(unsigned long)GFP_HIGHUSER, "GFP_HIGHUSER"}, \ {(unsigned long)GFP_USER, "GFP_USER"}, \ - {(unsigned long)GFP_TEMPORARY, "GFP_TEMPORARY"}, \ {(unsigned long)GFP_KERNEL_ACCOUNT, "GFP_KERNEL_ACCOUNT"}, \ {(unsigned long)GFP_KERNEL, "GFP_KERNEL"}, \ {(unsigned long)GFP_NOFS, "GFP_NOFS"}, \ -- cgit v1.2.3