Date:	Fri, 14 Sep 2012 13:31:47 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Michael Kerrisk <mtk.manpages@...il.com>
Cc:	linux-mm@...ck.org, cgroups@...r.kernel.org,
	Mel Gorman <mel@....ul.ie>,
	Glauber Costa <glommer@...allels.com>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch] memcg: trivial fixes for Documentation/cgroups/memory.txt

On Tue 11-09-12 13:20:33, Michael Kerrisk wrote:
> While reading through Documentation/cgroups/memory.txt, I found a number
> of minor wordos and typos. The patch below is a conservative
> handling of some of these: it provides just a number of "obviously
> correct" fixes to the English that improve the readability
> of the document somewhat. Obviously some more significant
> fixes need to be made to the document, but some of those 
> may not be in the "obvious correct" category.
> 
> Please apply.
> 
> Signed-off-by: Michael Kerrisk <mtk.manpages@...il.com>

I am normally not that happy about pure typo fixes but as this is a
documentation they make sense here.

Acked-by: Michal Hocko <mhocko@...e.cz>

Andrew, could you take the patch, please? The original patch is here:
https://lkml.org/lkml/2012/9/11/167

Thanks!

> diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
> index 4372e6b..c07f7b4 100644
> --- a/Documentation/cgroups/memory.txt
> +++ b/Documentation/cgroups/memory.txt
> @@ -18,16 +18,16 @@ from the rest of the system. The article on LWN [12] mentions some probable
>  uses of the memory controller. The memory controller can be used to
>  
>  a. Isolate an application or a group of applications
> -   Memory hungry applications can be isolated and limited to a smaller
> +   Memory-hungry applications can be isolated and limited to a smaller
>     amount of memory.
> -b. Create a cgroup with limited amount of memory, this can be used
> +b. Create a cgroup with a limited amount of memory; this can be used
>     as a good alternative to booting with mem=XXXX.
>  c. Virtualization solutions can control the amount of memory they want
>     to assign to a virtual machine instance.
>  d. A CD/DVD burner could control the amount of memory used by the
>     rest of the system to ensure that burning does not fail due to lack
>     of available memory.
> -e. There are several other use cases, find one or use the controller just
> +e. There are several other use cases; find one or use the controller just
>     for fun (to learn and hack on the VM subsystem).
>  
>  Current Status: linux-2.6.34-mmotm(development version of 2010/April)
> @@ -38,12 +38,12 @@ Features:
>   - optionally, memory+swap usage can be accounted and limited.
>   - hierarchical accounting
>   - soft limit
> - - moving(recharging) account at moving a task is selectable.
> + - moving (recharging) account at moving a task is selectable.
>   - usage threshold notifier
>   - oom-killer disable knob and oom-notifier
>   - Root cgroup has no limit controls.
>  
> - Kernel memory support is work in progress, and the current version provides
> + Kernel memory support is a work in progress, and the current version provides
>   basically functionality. (See Section 2.7)
>  
>  Brief summary of control files.
> @@ -144,9 +144,9 @@ Figure 1 shows the important aspects of the controller
>  3. Each page has a pointer to the page_cgroup, which in turn knows the
>     cgroup it belongs to
>  
> -The accounting is done as follows: mem_cgroup_charge() is invoked to setup
> +The accounting is done as follows: mem_cgroup_charge() is invoked to set up
>  the necessary data structures and check if the cgroup that is being charged
> -is over its limit. If it is then reclaim is invoked on the cgroup.
> +is over its limit. If it is, then reclaim is invoked on the cgroup.
>  More details can be found in the reclaim section of this document.
>  If everything goes well, a page meta-data-structure called page_cgroup is
>  updated. page_cgroup has its own LRU on cgroup.
> @@ -163,13 +163,13 @@ for earlier. A file page will be accounted for as Page Cache when it's
>  inserted into inode (radix-tree). While it's mapped into the page tables of
>  processes, duplicate accounting is carefully avoided.
>  
> -A RSS page is unaccounted when it's fully unmapped. A PageCache page is
> +An RSS page is unaccounted when it's fully unmapped. A PageCache page is
>  unaccounted when it's removed from radix-tree. Even if RSS pages are fully
>  unmapped (by kswapd), they may exist as SwapCache in the system until they
> -are really freed. Such SwapCaches also also accounted.
> +are really freed. Such SwapCaches are also accounted.
>  A swapped-in page is not accounted until it's mapped.
>  
> -Note: The kernel does swapin-readahead and read multiple swaps at once.
> +Note: The kernel does swapin-readahead and reads multiple swaps at once.
>  This means swapped-in pages may contain pages for other tasks than a task
>  causing page fault. So, we avoid accounting at swap-in I/O.
>  
> @@ -209,7 +209,7 @@ memsw.limit_in_bytes.
>  Example: Assume a system with 4G of swap. A task which allocates 6G of memory
>  (by mistake) under 2G memory limitation will use all swap.
>  In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
> -By using memsw limit, you can avoid system OOM which can be caused by swap
> +By using the memsw limit, you can avoid system OOM which can be caused by swap
>  shortage.
>  
>  * why 'memory+swap' rather than swap.
> @@ -217,7 +217,7 @@ The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
>  to move account from memory to swap...there is no change in usage of
>  memory+swap. In other words, when we want to limit the usage of swap without
>  affecting global LRU, memory+swap limit is better than just limiting swap from
> -OS point of view.
> +an OS point of view.
>  
>  * What happens when a cgroup hits memory.memsw.limit_in_bytes
>  When a cgroup hits memory.memsw.limit_in_bytes, it's useless to do swap-out
> @@ -236,7 +236,7 @@ an OOM routine is invoked to select and kill the bulkiest task in the
>  cgroup. (See 10. OOM Control below.)
>  
>  The reclaim algorithm has not been modified for cgroups, except that
> -pages that are selected for reclaiming come from the per cgroup LRU
> +pages that are selected for reclaiming come from the per-cgroup LRU
>  list.
>  
>  NOTE: Reclaim does not work for the root cgroup, since we cannot set any
> @@ -316,7 +316,7 @@ We can check the usage:
>  # cat /sys/fs/cgroup/memory/0/memory.usage_in_bytes
>  1216512
>  
> -A successful write to this file does not guarantee a successful set of
> +A successful write to this file does not guarantee a successful setting of
>  this limit to the value written into the file. This can be due to a
>  number of factors, such as rounding up to page boundaries or the total
>  availability of memory on the system. The user is required to re-read
> @@ -350,7 +350,7 @@ Trying usual test under memory controller is always helpful.
>  4.1 Troubleshooting
>  
>  Sometimes a user might find that the application under a cgroup is
> -terminated by OOM killer. There are several causes for this:
> +terminated by the OOM killer. There are several causes for this:
>  
>  1. The cgroup limit is too low (just too low to do anything useful)
>  2. The user is using anonymous memory and swap is turned off or too low
> @@ -358,7 +358,7 @@ terminated by OOM killer. There are several causes for this:
>  A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
>  some of the pages cached in the cgroup (page cache pages).
>  
> -To know what happens, disable OOM_Kill by 10. OOM Control(see below) and
> +To know what happens, disabling OOM_Kill as per "10. OOM Control" (below) and
>  seeing what happens will be helpful.
>  
>  4.2 Task migration
> @@ -399,10 +399,10 @@ About use_hierarchy, see Section 6.
>  
>    Almost all pages tracked by this memory cgroup will be unmapped and freed.
>    Some pages cannot be freed because they are locked or in-use. Such pages are
> -  moved to parent(if use_hierarchy==1) or root (if use_hierarchy==0) and this
> +  moved to parent (if use_hierarchy==1) or root (if use_hierarchy==0) and this
>    cgroup will be empty.
>  
> -  Typical use case of this interface is that calling this before rmdir().
> +  The typical use case for this interface is before calling rmdir().
>    Because rmdir() moves all pages to parent, some out-of-use page caches can be
>    moved to the parent. If you want to avoid that, force_empty will be useful.
>  
> @@ -486,7 +486,7 @@ You can reset failcnt by writing 0 to failcnt file.
>  
>  For efficiency, as other kernel components, memory cgroup uses some optimization
>  to avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
> -method and doesn't show 'exact' value of memory(and swap) usage, it's an fuzz
> +method and doesn't show 'exact' value of memory (and swap) usage, it's a fuzz
>  value for efficient access. (Of course, when necessary, it's synchronized.)
>  If you want to know more exact memory usage, you should use RSS+CACHE(+SWAP)
>  value in memory.stat(see 5.2).
> @@ -496,8 +496,8 @@ value in memory.stat(see 5.2).
>  This is similar to numa_maps but operates on a per-memcg basis.  This is
>  useful for providing visibility into the numa locality information within
>  an memcg since the pages are allowed to be allocated from any physical
> -node.  One of the usecases is evaluating application performance by
> -combining this information with the application's cpu allocation.
> +node.  One of the use cases is evaluating application performance by
> +combining this information with the application's CPU allocation.
>  
>  We export "total", "file", "anon" and "unevictable" pages per-node for
>  each memcg.  The ouput format of memory.numa_stat is:
> @@ -561,10 +561,10 @@ are pushed back to their soft limits. If the soft limit of each control
>  group is very high, they are pushed back as much as possible to make
>  sure that one control group does not starve the others of memory.
>  
> -Please note that soft limits is a best effort feature, it comes with
> +Please note that soft limits is a best-effort feature; it comes with
>  no guarantees, but it does its best to make sure that when memory is
>  heavily contended for, memory is allocated based on the soft limit
> -hints/setup. Currently soft limit based reclaim is setup such that
> +hints/setup. Currently soft limit based reclaim is set up such that
>  it gets invoked from balance_pgdat (kswapd).
>  
>  7.1 Interface
> @@ -592,7 +592,7 @@ page tables.
>  
>  8.1 Interface
>  
> -This feature is disabled by default. It can be enabled(and disabled again) by
> +This feature is disabled by default. It can be enabledi (and disabled again) by
>  writing to memory.move_charge_at_immigrate of the destination cgroup.
>  
>  If you want to enable it:
> @@ -601,8 +601,8 @@ If you want to enable it:
>  
>  Note: Each bits of move_charge_at_immigrate has its own meaning about what type
>        of charges should be moved. See 8.2 for details.
> -Note: Charges are moved only when you move mm->owner, IOW, a leader of a thread
> -      group.
> +Note: Charges are moved only when you move mm->owner, in other words,
> +      a leader of a thread group.
>  Note: If we cannot find enough space for the task in the destination cgroup, we
>        try to make space by reclaiming memory. Task migration may fail if we
>        cannot make enough space.
> @@ -612,25 +612,25 @@ And if you want disable it again:
>  
>  # echo 0 > memory.move_charge_at_immigrate
>  
> -8.2 Type of charges which can be move
> +8.2 Type of charges which can be moved
>  
> -Each bits of move_charge_at_immigrate has its own meaning about what type of
> -charges should be moved. But in any cases, it must be noted that an account of
> -a page or a swap can be moved only when it is charged to the task's current(old)
> -memory cgroup.
> +Each bit in move_charge_at_immigrate has its own meaning about what type of
> +charges should be moved. But in any case, it must be noted that an account of
> +a page or a swap can be moved only when it is charged to the task's current
> +(old) memory cgroup.
>  
>    bit | what type of charges would be moved ?
>   -----+------------------------------------------------------------------------
> -   0  | A charge of an anonymous page(or swap of it) used by the target task.
> -      | You must enable Swap Extension(see 2.4) to enable move of swap charges.
> +   0  | A charge of an anonymous page (or swap of it) used by the target task.
> +      | You must enable Swap Extension (see 2.4) to enable move of swap charges.
>   -----+------------------------------------------------------------------------
> -   1  | A charge of file pages(normal file, tmpfs file(e.g. ipc shared memory)
> +   1  | A charge of file pages (normal file, tmpfs file (e.g. ipc shared memory)
>        | and swaps of tmpfs file) mmapped by the target task. Unlike the case of
> -      | anonymous pages, file pages(and swaps) in the range mmapped by the task
> +      | anonymous pages, file pages (and swaps) in the range mmapped by the task
>        | will be moved even if the task hasn't done page fault, i.e. they might
>        | not be the task's "RSS", but other task's "RSS" that maps the same file.
> -      | And mapcount of the page is ignored(the page can be moved even if
> -      | page_mapcount(page) > 1). You must enable Swap Extension(see 2.4) to
> +      | And mapcount of the page is ignored (the page can be moved even if
> +      | page_mapcount(page) > 1). You must enable Swap Extension (see 2.4) to
>        | enable move of swap charges.
>  
>  8.3 TODO
> @@ -640,11 +640,11 @@ memory cgroup.
>  
>  9. Memory thresholds
>  
> -Memory cgroup implements memory thresholds using cgroups notification
> +Memory cgroup implements memory thresholds using the cgroups notification
>  API (see cgroups.txt). It allows to register multiple memory and memsw
>  thresholds and gets notifications when it crosses.
>  
> -To register a threshold application need:
> +To register a threshold, an application must:
>  - create an eventfd using eventfd(2);
>  - open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
>  - write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to
> @@ -659,24 +659,24 @@ It's applicable for root and non-root cgroup.
>  
>  memory.oom_control file is for OOM notification and other controls.
>  
> -Memory cgroup implements OOM notifier using cgroup notification
> +Memory cgroup implements OOM notifier using the cgroup notification
>  API (See cgroups.txt). It allows to register multiple OOM notification
>  delivery and gets notification when OOM happens.
>  
> -To register a notifier, application need:
> +To register a notifier, an application must:
>   - create an eventfd using eventfd(2)
>   - open memory.oom_control file
>   - write string like "<event_fd> <fd of memory.oom_control>" to
>     cgroup.event_control
>  
> -Application will be notified through eventfd when OOM happens.
> -OOM notification doesn't work for root cgroup.
> +The application will be notified through eventfd when OOM happens.
> +OOM notification doesn't work for the root cgroup.
>  
> -You can disable OOM-killer by writing "1" to memory.oom_control file, as:
> +You can disable the OOM-killer by writing "1" to memory.oom_control file, as:
>  
>  	#echo 1 > memory.oom_control
>  
> -This operation is only allowed to the top cgroup of sub-hierarchy.
> +This operation is only allowed to the top cgroup of a sub-hierarchy.
>  If OOM-killer is disabled, tasks under cgroup will hang/sleep
>  in memory cgroup's OOM-waitqueue when they request accountable memory.
>  

-- 
Michal Hocko
SUSE Labs
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