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Message-ID: <20130318214646.GB1441@localhost.localdomain>
Date: Mon, 18 Mar 2013 17:46:46 -0400
From: Andrew Shewmaker <agshew@...il.com>
To: akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
alan@...rguk.ukuu.org.uk, simon.jeons@...il.com,
ric.masonn@...il.com
Subject: [PATCH v6 2/2] mm: replace hardcoded 3% with admin_reserve_pages knob
Add an admin_reserve_kbytes knob to allow admins to change the
hardcoded memory reserve to something other than 3%, which
may be multiple gigabytes on large memory systems. Only about
8MB is necessary to enable recovery in the default mode, and
only a few hundred MB are required even when overcommit is
disabled.
This affects OVERCOMMIT_GUESS and OVERCOMMIT_NEVER.
admin_reserve_kbytes is initialized to min(3% free pages, 8MB)
Background
1. user reserve
__vm_enough_memory reserves a hardcoded 3% of the current process size
for other applications when overcommit is disabled. This was done so
that a user could recover if they launched a memory hogging process.
Without the reserve, a user would easily run into a message such as:
bash: fork: Cannot allocate memory
2. admin reserve
Additionally, a hardcoded 3% of free memory is reserved for root in
both overcommit 'guess' and 'never' modes. This was intended to prevent
a scenario where root-cant-log-in and perform recovery operations.
Note that this reserve shrinks, and doesn't guarantee a useful reserve.
Motivation
The two hardcoded memory reserves should be updated to account for
current memory sizes.
Also, the admin reserve would be more useful if it didn't shrink too much.
When the current code was originally written, 1GB was considered
"enterprise". Now the 3% reserve can grow to multiple GB on large
memory systems, and it only needs to be a few hundred MB at most to
enable a user or admin to recover a system with an unwanted memory
hogging process.
I've found that reducing these reserves is especially beneficial
for a specific type of application load:
* single application system
* one or few processes (e.g. one per core)
* allocating all available memory
* not initializing every page immediately
* long running
I've run scientific clusters with this sort of load. A long running job
sometimes failed many hours (weeks of CPU time) into a calculation.
They weren't initializing all of their memory immediately, and they
weren't using calloc, so I put systems into overcommit 'never' mode.
These clusters run diskless and have no swap.
However, with the current reserves, a user wishing to allocate as much
memory as possible to one process may be prevented from using, for
example, almost 2GB out of 32GB.
The effect is less, but still significant when a user starts a job
with one process per core. I have repeatedly seen a set of processes
requesting the same amount of memory fail because one of them could
not allocate the amount of memory a user would expect to be able to
allocate. For example, Message Passing Interfce (MPI) processes, one
per core. And it is similar for other parallel programming frameworks.
Changing this reserve code will make the overcommit never mode more
useful by allowing applications to allocate nearly all of the
available memory.
Also, the new admin_reserve_kbytes will be safer than the current
behavior since the hardcoded 3% of available memory reserve can shrink
to something useless in the case where applications have grabbed all
available memory.
Risks
* "bash: fork: Cannot allocate memory"
The downside of the first patch-- which creates a tunable user reserve
that is only used in overcommit 'never' mode--is that an admin can set
it so low that a user may not be able to kill their process, even if
they already have a shell prompt.
Of course, a user can get in the same predicament with the current 3%
reserve--they just have to launch processes until 3% becomes negligible.
* root-cant-log-in problem
The second patch, adding the tunable rootuser_reserve_pages, allows
the admin to shoot themselves in the foot by setting it too small.
They can easily get the system into a state where root-can't-log-in.
However, the new admin_reserve_kbytes will be safer than the current
behavior since the hardcoded 3% of available memory reserve can shrink
to something useless in the case where applications have grabbed all
available memory.
Alternatives
* Memory cgroups provide a more flexible way to limit application memory.
Not everyone wants to set up cgroups or deal with their overhead.
* We could create a fourth overcommit mode which provides smaller reserves.
The size of useful reserves may be drastically different depending
on the whether the system is embedded or enterprise.
* Force users to initialize all of their memory or use calloc.
Some users don't want/expect the system to overcommit when they malloc.
Overcommit 'never' mode is for this scenario, and it should work well.
The new user and admin reserve tunables are simple to use, with low
overhead compared to cgroups. The patches preserve current behavior where
3% of memory is less than 128MB, except that the admin reserve doesn't
shrink to an unusable size under pressure. The code allows admins to tune
for embedded and enterprise usage.
FAQ
* How is the root-cant-login problem addressed?
What happens if admin_reserve_pages is set to 0?
Root is free to shoot themselves in the foot by setting
admin_reserve_kbytes too low.
On x86_64, the minimum useful reserve is:
8MB for overcommit 'guess'
128MB for overcommit 'never'
admin_reserve_pages defaults to min(3% free memory, 8MB)
So, anyone switching to 'never' mode needs to adjust
admin_reserve_pages.
* How do you calculate a minimum useful reserve?
A user or the admin needs enough memory to login and perform
recovery operations, which includes, at a minimum:
sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
For overcommit 'guess', we can sum resident set sizes (RSS).
On x86_64 this is about 8MB.
For overcommit 'never', we can take the max of their virtual sizes (VSZ)
and add the sum of their RSS.
On x86_64 this is about 128MB.
* What happens if user_reserve_pages is set to 0?
Note, this only affects overcomitt 'never' mode.
Then a user will be able to allocate all available memory minus
admin_reserve_kbytes.
However, they will easily see a message such as:
"bash: fork: Cannot allocate memory"
And they won't be able to recover/kill their application.
The admin should be able to recover the system if
admin_reserve_kbytes is set appropriately.
* What's the difference between overcommit 'guess' and 'never'?
"Guess" allows an allocation if there are enough free + reclaimable
pages. It has a hardcoded 3% of free pages reserved for root.
"Never" allows an allocation if there is enough swap + a configurable
percentage (default is 50) of physical RAM. It has a hardcoded 3% of
free pages reserved for root, like "Guess" mode. It also has a
hardcoded 3% of the current process size reserved for additional
applications.
* Why is overcommit 'guess' not suitable even when an app eventually
writes to every page? It takes free pages, file pages, available
swap pages, reclaimable slab pages into consideration. In other words,
these are all pages available, then why isn't overcommit suitable?
Because it only looks at the present state of the system. It
does not take into account the memory that other applications have
malloced, but haven't initialized yet. It overcommits the system.
Test Summary
There was little change in behavior in the default overcommit 'guess'
mode with swap enabled before and after the patch. This was expected.
Systems run most predictably (i.e. no oom kills) in overcommit 'never'
mode with swap enabled. This also allowed the most memory to be allocated
to a user application.
Overcommit 'guess' mode without swap is a bad idea. It is easy to
crash the system. None of the other tested combinations crashed.
This matches my experience on the Roadrunner supercomputer.
Without the tunable user reserve, a system in overcommit 'never' mode
and without swap does not allow the admin to recover, although the
admin can.
With the new tunable reserves, a system in overcommit 'never' mode
and without swap can be configured to:
1. maximize user-allocatable memory, running close to the edge of
recoverability
2. maximize recoverability, sacrificing allocatable memory to
ensure that a user cannot take down a system
Test Description
Fedora 18 VM - 4 x86_64 cores, 5725MB RAM, 4GB Swap
System is booted into multiuser console mode, with unnecessary services
turned off. Caches were dropped before each test.
Hogs are user memtester processes that attempt to allocate all free memory
as reported by /proc/meminfo
In overcommit 'never' mode, memory_ratio=100
Test Results
3.9.0-rc1-mm1
Overcommit | Swap | Hogs | MB Got/Wanted | OOMs | User Recovery | Admin Recovery
---------- ---- ---- ------------- ---- ------------- --------------
guess yes 1 5432/5432 no yes yes
guess yes 4 5444/5444 1 yes yes
guess no 1 5302/5449 no yes yes
guess no 4 - crash no no
never yes 1 5460/5460 1 yes yes
never yes 4 5460/5460 1 yes yes
never no 1 5218/5432 no no yes
never no 4 5203/5448 no no yes
3.9.0-rc1-mm1-tunablereserves
User and Admin Recovery show their respective reserves, if applicable.
Overcommit | Swap | Hogs | MB Got/Wanted | OOMs | User Recovery | Admin Recovery
---------- ---- ---- ------------- ---- ------------- --------------
guess yes 1 5419/5419 no - yes 8MB yes
guess yes 4 5436/5436 1 - yes 8MB yes
guess no 1 5440/5440 * - yes 8MB yes
guess no 4 - crash - no 8MB no
* process would successfully mlock, then the oom killer would pick it
never yes 1 5446/5446 no 10MB yes 20MB yes
never yes 4 5456/5456 no 10MB yes 20MB yes
never no 1 5387/5429 no 128MB no 8MB barely
never no 1 5323/5428 no 226MB barely 8MB barely
never no 1 5323/5428 no 226MB barely 8MB barely
never no 1 5359/5448 no 10MB no 10MB barely
never no 1 5323/5428 no 0MB no 10MB barely
never no 1 5332/5428 no 0MB no 50MB yes
never no 1 5293/5429 no 0MB no 90MB yes
never no 1 5001/5427 no 230MB yes 338MB yes
never no 4* 4998/5424 no 230MB yes 338MB yes
* more memtesters were launched, able to allocate approximately another 100MB
Signed-off-by: Andrew Shewmaker <agshew@...il.com>
---
Patch Changelog
v6:
* Rebased onto v3.9-rc1-mmotm-2013-03-07-15-45
* Replace user_reserve_pages with user_reserve_kbytes
* Replace admin_reserve_pages with admin_reserve_kbytes
* Increase verbosity of patch changelog
* Add background, motivation, risks, alternatives, and testing
* Add Alan Cox's example of sparse arrays to the
documentation of the 'always' overcommit mode
* Add note in overcommit_memory documentation that
user_reserve_kbytes affects 'never' mode
* Improve wording of user_reserve_kbytes documentation
* Clearly document risk of root-cant-log-in
in admin_reserve_kbytes documentation
v5:
* Change nontunable k in min(3% process size, k) into
user_reserve_pages knob
* user_reserve_pages defaults to min(3% free pages, 128MB)
previous k=8MB wasn't enough for OVERCOMMIT_NEVER mode
and 128MB worked when I tested it
* 128MB from taking max VSZ of sshd, login, bash, and top
and adding the RSS of each
v5 discussion:
* Request for more complete changelog with detailed motivation,
problems, alternatives, and discussion. -Andrew Morton
* How is the root-cant-login problem addressed?
* What happens if user_reserve_pages is set to 0?
* What happens if admin_reserve_pages is set to 0?
* Clearly describe risks in documentation
-Andrew Morton
As long as admin_reserve_pages is set to at least 8MB for
OVERCOMMIT_GUESS or above 128MB for OVERCOMMIT_NEVER, I was able to
log in as root and kill processes. The root-cant-log-in problem
cannot be hit if user_reserve_pages is set to 0 because that
reserve only exists in OVERCOMMIT_NEVER mode.
* Exported interfaces which deal in "pages" are considered harmful.
PAGE_SIZE can vary by a factor of 16 depending upon config (ie:
architecture). The risk is that a setup script which works nicely on
4k x86_64 will waste memory when executed on a 64k PAGE_SIZE powerpc
box. A smart programmer will recognize this and will adapt the setting
using getpagesize(2), but if we define these things in "bytes" rather
than "pages" then dumb programmers can use it too.
-Andrew Morton
v4:
* Rebased onto v3.8-mmotm-2013-03-01-15-50
* No longer assumes 4kb pages
* Code duplicated for nommu
v4 discussion:
* "Please add changelog, otherwise it's for other guys to review."
-Simon Jeons
Sorry, I'll be sure to include one in the future. And it
looks like I do need a v5 ... I think this needs to
be tunable like the admin reserve. The user_reserve_pages default
certainly needs to be higher since this reserve is only for
OVERCOMMIT_NEVER mode and 8MB is too little to allow
the user to recover. I was thinking of OVERCOMMIT_GUESS
mode when I chose that size.
v3:
* New patch summary because it wasn't unique
New is "mm: limit growth of 3% hardcoded other user reserve"
Old was "mm: tuning hardcoded reserve memory"
* "bash: fork: Cannot allocate memory"
First patch limits growth of other user reserve to
min(3% process size, k) as Alan Cox suggested.
I chose k=2000 pages to allow recovery with sshd or login, bash,
and top or kill. Of course, memory will still be exhausted eventually.
I had simply removed the reserve previously, but that caused forks
to fail easily. This allows a user to recover similar to the
simple 3% reserve, but allows a single process to allocate more
memory.
* root-cant-log-in
Add an admin_reserve_pages knob to allow admins of large memory
systems running with overcommit disabled to change the hardcoded
memory reserve to something other than 3%.
admin_reserve_pages is initialized to min(3% free pages, k) similar
to what Alan suggested for the other user reserve.
k=2000 pages should allow the admin to spawn new sshd, bash, and top
to recover if necessary. This reserve doesn't shrink like the other
user reserve.
v2:
* Rebased onto v3.8-mmotm-2013-02-19-17-20
* Motivation:
On scientific clusters, systems are generally dedicated to one user.
Also, overcommit is sometimes disabled in order to prevent a long
running job from suddenly failing days or weeks into a calculation.
In this case, a user wishing to allocate as much memory as possible
to one process may be prevented from using, for example, around 7GB
out of 128GB.
The effect is less, but still significant when a user starts a job
with one process per core. I have repeatedly seen a set of processes
requesting the same amount of memory fail because one of them could
not allocate the amount of memory a user would expect to be able to
allocate.
* The first patch only affects OVERCOMMIT_NEVER mode, entirely removing
the 3% reserve for other user processes.
* The second patch affects both OVERCOMMIT_GUESS and OVERCOMMIT_NEVER
modes, replacing an additional hardcoded 3% reserve for the root user
with a tunable knob, rootuser_reserve_pages.
* rootuser_reserve_pages defaults to 1000
v2 discussion:
* Both these patches had the same title. Please avoid this.
Documentation/SubmittingPatches section 15 has all the details.
Sorry. This will be fixed.
* Documentation/vm/overcommit-accounting says that OVERCOMMIT_ALWAYS is
"Appropriate for some scientific applications", but doesn't say why.
You're running a scientific cluster but you're using OVERCOMMIT_NEVER,
I think? Is the documentation wrong?
"Classic example is code using sparse arrays and just relying on the
virtual memory consisting almost entirely of zero pages." -Alan Cox
My users would run jobs that appeared to initialize correctly. However,
they wouldn't write to every page they malloced (and they wouldn't use
calloc), so I saw jobs failing well into a computation once the
simulation tried to access a page and the kernel couldn't give it to them.
I think Roadrunner (http://en.wikipedia.org/wiki/IBM_Roadrunner) was
the first cluster I put into OVERCOMMIT_NEVER mode. Jobs with
infeasible memory requirements fail early and the OOM killer
gets triggered much less often than in guess mode. More often than not
the OOM killer seemed to kill the wrong thing causing a subtle brokenness.
Disabling overcommit worked so well during the stabilization and
early user phases that we did the same with other clusters.
* Do you mean OVERCOMMIT_NEVER is more suitable for scientific application
than OVERCOMMIT_GUESS and OVERCOMMIT_ALWAYS? Or should depend on
workload? Since your users would run jobs that wouldn't write to every
page they malloced, so why OVERCOMMIT_GUESS is not more suitable for you?
It depends on the workload. They eventually wrote to every page,
but not early in the life of the process, so they thought they
were fine until the simulation crashed.
* Why overcommit guess is not suitable even they eventually wrote to every
page? It takes free pages, file pages, available swap pages, reclaimable
slab pages into consideration. In other words, these are all pages
available, then why overcommit is not suitable?
Because the check only looks at the present state of the system. It
does not take into account the memory that other applications have
malloced, but haven't initialized yet. It overcommits the system.
* What's the root difference between overcommit guess and never?
"Guess" allows an allocation if there are enough free + reclaimable
pages. It has a hardcoded 3% of free pages reserved for root.
"Never" allows an allocation if there is enough swap + a configurable
percentage (default is 50) of physical RAM. It has a hardcoded 3% of
free pages reserved for root, like "Guess" mode. It also has a
hardcoded 3% of the current process size reserved for additional
applications.
* Should rootuser_reserve_pages be initialized more intelligently?
rootuser_reserve_pages should scale the initial value according to
the machine size in some fashion
I suspect the tunable should nowdays be something related to min(3%,
someconstant), at the time we did the 3% I think 1GB was an "enterprise
system" ;) -Alan Cox
* So what might be the downside for this change? root-cant-log-in?
With just the first patch that eliminates the other user reserve, root
still has a 3% of free pages reserve.
The second patch, adding the tunable rootuser_reserve_pages, allows
the admin to shoot themselves in the foot and easily get the system
into a state where they root-can't-log-in.
This new tunable will be safer than the current behavior since the
hardcoded 3% reserve shrinks to something useless in the case where
an application has grabbed all available memory with many processes.
* "bash: fork: Cannot allocate memory"
The downside of the first patch, which removes the "other" reserve
(sorry about the confusing duplicated subject line), is that a user
may not be able to kill their process, even if they have a shell prompt.
When testing, I did sometimes get into spot where I attempted to execute
kill, but got: "bash: fork: Cannot allocate memory". Of course, a
user can get in the same predicament with the current 3% reserve--they
just have to launch processes until 3% becomes negligible.
* Have you actually tested for this scenario and observed the effects?
"If there *are* observable risks and/or to preserve back-compatibility,
I guess we could create a fourth overcommit mode which provides the
headroom which you desire." -Andrew Morton
When I resubmit the second patch, I'll test both guess and never
overcommit modes to see what minimum initial values allow root to log in
and kill a user's memory hogging process.
* Should we be looking at removing root's 3% from OVERCOMMIT_GUESS
as well?
"The 3% reserve was added to the original code *because* users kept hitting
problems where they couldn't recover." -Alan Cox
* What is the minimum useful reserve?
"As an estimate of a useful rootuser_reserve_pages, the rss+share size of
sshd, bash, and top is about 16MB. Overcommit disabled mode would need
closer to 360MB for the same processes. On a 128GB box 3% is 3.8GB, so
the new tunable would still be a win." -Andrew Shewmaker
"Sorry for my silly, why you mean share size is not consist in rss size?"
-Ric Mason
"For some reason I had it in my head that RSS was just the memory
private to the process and that I needed to add memory shared for
libraries. So yeah, it looks like 8MB, or 2000 pages should be
enough of a reserve." -Andrew Shewmaker
v1:
* Based on 3.8
* Remove hardcoded 3% other user reserve in OVERCOMMIT_NEVER mode
* __vm_enough_memory reserves a hardcoded 3% of free memory for other processes
when overcommit is disabled. However, 3% is becoming excessive as memory sizes
increase
* Memory cgroups provide a more flexible way to limit application memory.
* I've found that reducing this reserve is beneficial in the case where a system
with overcommit disabled has one primary user that wants to allocate as much
memory as possible with a just few processes.
* An additional hardcoded 3% is reserved for root, both when overcommit is enabled
and when it is disabled. I've made it tunable in private patches, and I plan on
submitting some version of them, but I can't decide whether a ratio or a byte
count would be more acceptable. What would people prefer see?
---
Documentation/sysctl/vm.txt | 30 ++++++++++++++++++++++++++++++
kernel/sysctl.c | 8 ++++++++
mm/mmap.c | 30 ++++++++++++++++++++++++++----
mm/nommu.c | 30 ++++++++++++++++++++++++++----
4 files changed, 90 insertions(+), 8 deletions(-)
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 494402b..e10f7b8 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -18,6 +18,7 @@ files can be found in mm/swap.c.
Currently, these files are in /proc/sys/vm:
+- admin_reserve_kbytes
- block_dump
- compact_memory
- dirty_background_bytes
@@ -59,6 +60,35 @@ Currently, these files are in /proc/sys/vm:
==============================================================
+admin_reserve_kbytes
+
+The amount of free memory in the system that should be reserved for users
+with the capability cap_sys_admin.
+
+admin_reserve_kbytes defaults to min(3% of free pages, 8MB)
+
+That should provide enough for the admin to log in and kill a process,
+if necessary, under the default overcommit 'guess' mode.
+
+Systems running under overcommit 'never' should increase this to account
+for the full Virtual Memory Size of programs used to recover. Otherwise,
+root may not be able to log in to recover the system.
+
+How do you calculate a minimum useful reserve?
+
+sshd or login + bash (or some other shell) + top (or ps, kill, etc.)
+
+For overcommit 'guess', we can sum resident set sizes (RSS).
+On x86_64 this is about 8MB.
+
+For overcommit 'never', we can take the max of their virtual sizes (VSZ)
+and add the sum of their RSS.
+On x86_64 this is about 128MB.
+
+Changing this takes effect whenever an application requests memory.
+
+==============================================================
+
block_dump
block_dump enables block I/O debugging when set to a nonzero value. More
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 87df7d1..5914b79 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -98,6 +98,7 @@
extern int sysctl_overcommit_memory;
extern int sysctl_overcommit_ratio;
extern unsigned long sysctl_user_reserve_kbytes;
+extern unsigned long sysctl_admin_reserve_kbytes;
extern int max_threads;
extern int suid_dumpable;
#ifdef CONFIG_COREDUMP
@@ -1438,6 +1439,13 @@ static struct ctl_table vm_table[] = {
.mode = 0644,
.proc_handler = proc_doulongvec_minmax,
},
+ {
+ .procname = "admin_reserve_kbytes",
+ .data = &sysctl_admin_reserve_kbytes,
+ .maxlen = sizeof(sysctl_admin_reserve_kbytes),
+ .mode = 0644,
+ .proc_handler = proc_doulongvec_minmax,
+ },
{ }
};
diff --git a/mm/mmap.c b/mm/mmap.c
index 79c4f7a..acf7f97 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -86,6 +86,7 @@ int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS; /* heuristic ove
int sysctl_overcommit_ratio __read_mostly = 50; /* default is 50% */
int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
unsigned long sysctl_user_reserve_kbytes __read_mostly = 1UL << 17; /* 128MB */
+unsigned long sysctl_admin_reserve_kbytes __read_mostly = 1UL << 13; /* 8MB */
/*
* Make sure vm_committed_as in one cacheline and not cacheline shared with
* other variables. It can be updated by several CPUs frequently.
@@ -165,10 +166,10 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
free -= totalreserve_pages;
/*
- * Leave the last 3% for root
+ * Reserve some for root
*/
if (!cap_sys_admin)
- free -= free / 32;
+ free -= sysctl_admin_reserve_kbytes >> (PAGE_SHIFT - 10);
if (free > pages)
return 0;
@@ -179,10 +180,10 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
allowed = (totalram_pages - hugetlb_total_pages())
* sysctl_overcommit_ratio / 100;
/*
- * Leave the last 3% for root
+ * Reserve some for root
*/
if (!cap_sys_admin)
- allowed -= allowed / 32;
+ allowed -= sysctl_admin_reserve_kbytes >> (PAGE_SHIFT - 10);
allowed += total_swap_pages;
/*
@@ -3092,3 +3093,24 @@ int __meminit init_user_reserve(void)
return 0;
}
module_init(init_user_reserve)
+
+/*
+ * Initialise sysctl_admin_reserve_kbytes.
+ *
+ * The purpose of sysctl_admin_reserve_kbytes is to allow the sys admin
+ * to log in and kill a memory hogging process.
+ *
+ * Systems with more than 256MB will reserve 8MB, enough to recover
+ * with sshd, bash, and top in OVERCOMMIT_GUESS. Smaller systems will
+ * only reserve 3% of free pages by default.
+ */
+int __meminit init_admin_reserve(void)
+{
+ unsigned long free_kbytes;
+
+ free_kbytes = global_page_state(NR_FREE_PAGES) << (PAGE_SHIFT - 10);
+
+ sysctl_admin_reserve_kbytes = min(free_kbytes / 32, 1UL << 13);
+ return 0;
+}
+module_init(init_admin_reserve)
diff --git a/mm/nommu.c b/mm/nommu.c
index 724e460..e096ff2 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -65,6 +65,7 @@ int sysctl_overcommit_ratio = 50; /* default is 50% */
int sysctl_max_map_count = DEFAULT_MAX_MAP_COUNT;
int sysctl_nr_trim_pages = CONFIG_NOMMU_INITIAL_TRIM_EXCESS;
unsigned long sysctl_user_reserve_kbytes __read_mostly = 1UL << 17; /* 128MB */
+unsigned long sysctl_admin_reserve_kbytes __read_mostly = 1UL << 13; /* 8MB */
int heap_stack_gap = 0;
atomic_long_t mmap_pages_allocated;
@@ -1926,10 +1927,10 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
free -= totalreserve_pages;
/*
- * Leave the last 3% for root
+ * Reserve some for root
*/
if (!cap_sys_admin)
- free -= free / 32;
+ free -= sysctl_admin_reserve_kbytes >> (PAGE_SHIFT - 10);
if (free > pages)
return 0;
@@ -1939,10 +1940,10 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
allowed = totalram_pages * sysctl_overcommit_ratio / 100;
/*
- * Leave the last 3% for root
+ * Reserve some 3% for root
*/
if (!cap_sys_admin)
- allowed -= allowed / 32;
+ allowed -= sysctl_admin_reserve_kbytes >> (PAGE_SHIFT - 10);
allowed += total_swap_pages;
/*
@@ -2133,3 +2134,24 @@ int __meminit init_user_reserve(void)
return 0;
}
module_init(init_user_reserve)
+
+/*
+ * Initialise sysctl_admin_reserve_kbytes.
+ *
+ * The purpose of sysctl_admin_reserve_kbytes is to allow the sys admin
+ * to log in and kill a memory hogging process.
+ *
+ * Systems with more than 256MB will reserve 8MB, enough to recover
+ * with sshd, bash, and top in OVERCOMMIT_GUESS. Smaller systems will
+ * only reserve 3% of free pages by default.
+ */
+int __meminit init_admin_reserve(void)
+{
+ unsigned long free_kbytes;
+
+ free_kbytes = global_page_state(NR_FREE_PAGES) << (PAGE_SHIFT - 10);
+
+ sysctl_admin_reserve_kbytes = min(free_kbytes / 32, 1UL << 13);
+ return 0;
+}
+module_init(init_admin_reserve)
--
1.8.0.1
--
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