Re: [PATCH] futex: bugfix for futex-key conflict when futex usehugepage

[Mel Gorman]

On Tue, May 07, 2013 at 05:24:57PM +0200, Thomas Gleixner wrote:
> 
> 
> On Tue, 7 May 2013, Mel Gorman wrote:
> 
> > On Tue, May 07, 2013 at 08:23:48PM +0800, Zhang Yi wrote:
> > > diff -uprN linux3.9-orig/kernel/futex.c linux3.9/kernel/futex.c
> > > --- linux3.9-orig/kernel/futex.c	2013-04-15 00:45:16.000000000 +0000
> > > +++ linux3.9/kernel/futex.c	2013-05-06 16:24:40.403525000 +0000
> > > @@ -215,6 +215,22 @@ static void drop_futex_key_refs(union fu
> > >  	}
> > >  }
> > > 
> > > +/*
> > > +* Get subpage index in compound page, and add it into futex_key.
> > > +*/
> > > +static void key_add_compound_idx(union futex_key *key,
> > > +				 struct page *head_page, struct page *page)
> > > +{
> > > +	int compound_idx;
> > > +
> > > +	if (compound_order(head_page) >= MAX_ORDER)
> > > +		compound_idx = page_to_pfn(page) - page_to_pfn(head_page);
> > > +	else
> > > +		compound_idx = page - head_page;
> > > +
> > > +	key->both.offset |= compound_idx << PAGE_SHIFT;
> > > +}
> > > +
> > 
> > This implicitely assumies it is dealing with a hugetlbfs page. Today, it
> > is the case that an inode-based futex with PageCompound is a hugetlbfs
> > page but that could change in the future if THP ever backs files. This
> > would then break again except it would be harder to fix because THP pages
> > can be collapsed underneath you after the futex key has been generated.
> > 
> > As this problem is hugetlbfs-specific should the fix be firmly in hugetlbfs
> > land? Something like the following untested and only partial diff? Is the
> > use of PageCompound in the futex path like this going to be problematic?
> 
> Why should it ?
>  

The comment for it states that it is "generally not used in hot code
paths" but it's a light-weight check that the cache lines should already
be fetched for. I doubt that the overhead of this check versus
page_head == page is noticable.

> > @@ -365,7 +366,7 @@ again:
> >  	} else {
> >  		key->both.offset |= FUT_OFF_INODE; /* inode-based key */
> >  		key->shared.inode = page_head->mapping->host;
> > -		key->shared.pgoff = page_head->index;
> > +		key->shared.pgoff = basepage_index(page_head);
> 
>   That want's to be  basepage_index(page), right ?
> 

BAH, yes.

-- 
Mel Gorman
SUSE Labs
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