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From: Michal Hocko <mhocko@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Chen Liqin <liqin.linux@gmail.com>,
	Chris Metcalf <cmetcalf@mellanox.com>,
	"David S. Miller" <davem@davemloft.net>,
	Guan Xuetao <gxt@mprc.pku.edu.cn>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Helge Deller <deller@gmx.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Ingo Molnar <mingo@redhat.com>,
	"James E.J. Bottomley" <jejb@parisc-linux.org>,
	John Crispin <blogic@openwrt.org>,
	Lennox Wu <lennox.wu@gmail.com>, Ley Foon Tan <lftan@altera.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Matt Fleming <matt@codeblueprint.co.uk>,
	Michal Hocko <mhocko@suse.com>,
	Mikulas Patocka <mpatocka@redhat.com>,
	Rich Felker <dalias@libc.org>,
	Russell King <linux@arm.linux.org.uk>,
	Shaohua Li <shli@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vineet Gupta <vgupta@synopsys.com>,
	Will Deacon <will.deacon@arm.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>
Subject: [PATCH 0/19] get rid of superfluous __GFP_REPEAT
Date: Thu, 28 Apr 2016 15:23:46 +0200
Message-ID: <1461849846-27209-1-git-send-email-mhocko@kernel.org> (raw)

Hi,
this is the thrid version of the patchset previously sent [1]. I have
basically only rebased it on top of next-20160428 tree and dropped
"crypto: get rid of superfluous __GFP_REPEAT" which went through crypto
tree. I have added two more md patches as I couldn't resist more alloc
related cleanups at that area.

Motivation:
While working on something unrelated I've checked the current usage
of __GFP_REPEAT in the tree. It seems that a majority of the usage is
and always has been bogus because __GFP_REPEAT has always been about
costly high order allocations while we are using it for order-0 or very
small orders very often. It seems that a big pile of them is just a
copy&paste when a code has been adopted from one arch to another.

I think it makes some sense to get rid of them because they are just
making the semantic more unclear. Please note that GFP_REPEAT is
documented as
 * __GFP_REPEAT: Try hard to allocate the memory, but the allocation attempt
 *   _might_ fail.  This depends upon the particular VM implementation.
while !costly requests have basically nofail semantic. So one could
reasonably expect that order-0 request with __GFP_REPEAT will not loop
for ever. This is not implemented right now though.

I would like to move on with __GFP_REPEAT and define a better
semantic for it. One thought was to rename it to __GFP_BEST_EFFORT
which would behave consistently for all orders and guarantee that the
allocation would try as long as it seem feasible or fail eventually.
!costly request would then finally get a request context which neiter
fails too early (GFP_NORETRY) nor endlessly loops in the allocator for
ever (default behavior). Costly high order requests would keep the
current semantic.
We have discussed that at LSF/MM this year and another suggestion was
to introduce __GFP_TRYHARD instead which would be implicit for all
orders and users would opt out by ~__GFP_TRYHARD instead. I am not
sure which way is better right now but I plan to do the clean up first
before going further with semantic changes.

$ git grep __GFP_REPEAT next/master | wc -l
109
$ git grep __GFP_REPEAT | wc -l
35

So we are down to the third after this patch series. The remaining places
really seem to be relying on __GFP_REPEAT due to large allocation requests.
This still needs some double checking which I will do later after all the
simple ones are sorted out.

I am touching a lot of arch specific code here and I hope I got it right
but as a matter of fact I even didn't compile test for some archs as I
do not have cross compiler for them. Patches should be quite trivial to
review for stupid compile mistakes though. The tricky parts are usually
hidden by macro definitions and thats where I would appreciate help from
arch maintainers.

[1] http://lkml.kernel.org/r/1460372892-8157-1-git-send-email-mhocko@kernel.org

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Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-04-28 13:23 Michal Hocko [this message]
2016-04-28 13:23 ` [PATCH 01/20] tree wide: get rid of __GFP_REPEAT for order-0 allocations part I Michal Hocko
2016-04-28 13:23 ` [PATCH 02/20] x86: get rid of superfluous __GFP_REPEAT Michal Hocko
2016-04-28 13:23 ` [PATCH 03/20] x86/efi: " Michal Hocko
2016-04-28 13:23 ` [PATCH 04/20] arm: " Michal Hocko
2016-04-28 14:55   ` Russell King - ARM Linux
2016-04-28 15:08     ` Michal Hocko
2016-04-29  9:41       ` Michal Hocko
2016-04-28 13:23 ` [PATCH 05/20] arm64: " Michal Hocko
2016-04-28 13:23 ` [PATCH 06/20] arc: " Michal Hocko
2016-04-28 13:23 ` [PATCH 07/20] mips: " Michal Hocko
2016-04-28 13:23 ` [PATCH 08/20] nios2: " Michal Hocko
2016-04-28 13:23 ` [PATCH 09/20] parisc: " Michal Hocko
2016-04-28 13:23 ` [PATCH 10/20] score: " Michal Hocko
2016-04-28 13:23 ` [PATCH 11/20] powerpc: " Michal Hocko
2016-04-28 13:23 ` [PATCH 12/20] sparc: " Michal Hocko
2016-04-28 13:23 ` [PATCH 13/20] s390: " Michal Hocko
2016-04-28 13:24 ` [PATCH 14/20] sh: " Michal Hocko
2016-04-28 13:24 ` [PATCH 15/20] tile: " Michal Hocko
2016-04-28 16:21   ` Chris Metcalf
2016-04-28 13:24 ` [PATCH 16/20] unicore32: " Michal Hocko
2016-04-28 13:24 ` [PATCH 17/20] dm: get rid of superfluous gfp flags Michal Hocko
2016-04-29 18:54   ` Mike Snitzer
2016-05-02  7:31     ` Michal Hocko
2016-04-28 13:24 ` [PATCH 18/20] dm: clean up GFP_NIO usage Michal Hocko
2016-04-28 14:20   ` Mikulas Patocka
2016-04-28 14:41     ` Michal Hocko
2016-04-28 13:24 ` [PATCH 19/20] md: simplify free_params for kmalloc vs vmalloc fallback Michal Hocko
2016-04-28 14:51   ` [PATCH] " Michal Hocko
2016-04-28 15:04     ` Mikulas Patocka
2016-04-28 15:28       ` Michal Hocko
2016-04-28 15:40         ` Mikulas Patocka
2016-04-28 16:59           ` Michal Hocko
2016-04-28 15:37   ` [PATCH 19/20] " Mike Snitzer
2016-04-28 16:00     ` Michal Hocko
2016-04-28 13:24 ` [PATCH 20/20] jbd2: get rid of superfluous __GFP_REPEAT Michal Hocko
2016-05-12 16:53 ` [PATCH 0/19] " Michal Hocko
2016-05-12 20:13   ` Andrew Morton
2016-05-13  6:58     ` Michal Hocko
2016-05-30  9:14 Michal Hocko

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