Re: [PATCH 0/4] hugetlbfs: optionally reserve all fs pages at mount time

[David Rientjes]

On Tue, 3 Mar 2015, Mike Kravetz wrote:

> hugetlbfs allocates huge pages from the global pool as needed.  Even if
> the global pool contains a sufficient number pages for the filesystem
> size at mount time, those global pages could be grabbed for some other
> use.  As a result, filesystem huge page allocations may fail due to lack
> of pages.
> 
> Applications such as a database want to use huge pages for performance
> reasons.  hugetlbfs filesystem semantics with ownership and modes work
> well to manage access to a pool of huge pages.  However, the application
> would like some reasonable assurance that allocations will not fail due
> to a lack of huge pages.  At application startup time, the application
> would like to configure itself to use a specific number of huge pages.
> Before starting, the application will can check to make sure that enough
> huge pages exist in the system global pools.  What the application wants
> is exclusive use of a subpool of huge pages. 
> 
> Add a new hugetlbfs mount option 'reserved' to specify that the number
> of pages associated with the size of the filesystem will be reserved.  If
> there are insufficient pages, the mount will fail.  The reservation is
> maintained for the duration of the filesystem so that as pages are
> allocated and free'ed a sufficient number of pages remains reserved.
> 

This functionality is somewhat limited because it's not possible to 
reserve a subset of the size for a single mount point, it's either all or 
nothing.  It shouldn't be too difficult to just add a reserved=<value> 
option where <value> is <= size.  If it's done that way, you should be 
able to omit size= entirely for unlimited hugepages but always ensure that 
a low watermark of hugepages are reserved for the database.

> Comments from RFC addressed/incorporated
> 
> Mike Kravetz (4):
>   hugetlbfs: add reserved mount fields to subpool structure
>   hugetlbfs: coordinate global and subpool reserve accounting
>   hugetlbfs: accept subpool reserved option and setup accordingly
>   hugetlbfs: document reserved mount option
> 
>  Documentation/vm/hugetlbpage.txt | 18 ++++++++------
>  fs/hugetlbfs/inode.c             | 15 ++++++++++--
>  include/linux/hugetlb.h          |  7 ++++++
>  mm/hugetlb.c                     | 53 +++++++++++++++++++++++++++++++++-------
>  4 files changed, 75 insertions(+), 18 deletions(-)
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