Re: [patch] hugetlb: fix i_blocks accounting

[Adam Litke <agl@us.ibm.com>]

On Tue, 2007-10-23 at 17:34 -0700, Ken Chen wrote:
> On 10/23/07, Adam Litke <agl@us.ibm.com> wrote:
> > On Sat, 2007-10-20 at 11:18 -0700, Ken Chen wrote:
> > > For administrative purpose, we want to query actual block usage for
> > > hugetlbfs file via fstat.  Currently, hugetlbfs always return 0.  Fix
> > > that up since kernel already has all the information to track it
> > > properly.
> >
> > Hey Ken.  You might want to wait on this for another minute or two.  I
> > will be sending out patches later today to fix up hugetlbfs quotas.
> > Right now the code does not handle private mappings correctly (ie.  it
> > does not call get_quota() for COW pages and it never calls put_quota()
> > for any private page).  Because of this, your i_blocks number will be
> > wrong most of the time.
> 
> Adam, speaking of hugetlb file system quota, there is another bug in
> there for shared mapping as well.

Yep ;)  I already have a fix for that too in this series.  Coming right
up.

> At the time of mmap (MMAP_SHARED), kernel only check page reservation
> against available hugetlb page pool.  FS quota is not checked at all.
> Now we over commit the fs quota for shared mapping, but still let the
> mmap to succeed.  At later point in the page fault path, app will
> eventually die with SIGBUS due to lack of fs quota.  This behavior
> broke a few apps for us. The bad part is there is no easy recovery
> path once a SIGBUS is raised.
> 
> I tried with MAP_POPULATE, but unfortunately it doesn't propagate
> error code back up to user space on mmap; same thing with mlockall
> that also ignores error code returned from make_pages_present().
> Using mlock is at best half baked because VM_LOCKED maybe already set
> via other means.
> 
> So this fs quota thing really needs some love and attention.

Yep, now that we are actually starting to use it...

-- 
Adam Litke - (agl at us.ibm.com)
IBM Linux Technology Center

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