Re: [Qemu-devel] Fix refcounting in hugetlbfs quota handling

[David Gibson]

On Fri, Aug 12, 2011 at 12:15:21PM -0700, Hugh Dickins wrote:
> On Fri, 12 Aug 2011, Minchan Kim wrote:
> > On Fri, Aug 12, 2011 at 9:48 AM, Linus Torvalds
> > <address@hidden> wrote:
> > > On Wed, Aug 10, 2011 at 11:40 PM, David Gibson
> > > <address@hidden> wrote:
> > >>
> > >> This patch, therefore, stores a pointer to the inode instead of the
> > >> address_space in the page private data for hugepages.  More
> > >> importantly it correctly adjusts the reference count on the inodes
> > >> when they're added to the page private data.  This ensures that the
> > >> inode (and therefore the super block) will not be freed before we use
> > >> it from free_huge_page.
> > >
> > > Looks sane, but I *really* want some acks from people who use/know
> > > hugetlbfs. Who would that be? I'm adding random people who have
> > > acked/signed-off patches to hugetlbfs recently..
> > 
> > At least, code itself looks good to me but your random choice was failed.
> > Maybe people you want are as follows.
> > http://marc.info/?t=126928975800003&r=1&w=2
> > 
> > Ccing right persons.
> 
> I don't know much about hugetlbfs these days, but I think the patch
> is very wrong.
> 
> The real change is where alloc_huge_page() does igrab(inode) and
> free_huge_pages() does iput(inode)?
> 
> That makes me very nervous, partly because a final iput() is a complex
> operation, which we wouldn't expect to be doing when "freeing" a page.
> 
> My first worry was that free_huge_page() could actually get called at
> interrupt time (when it's in a pagevec of pages to be freed as a batch,
> then another put_page is done at interrupt time which frees that batch):
> I worried that we use spin_lock not spin_lock_irqsave on inode->i_lock.
> To be honest though, I've not followed up whether that's actually a
> possibility, the compound page path is too twisty for a quick answer;
> and even if it's a possibility, it's one that's already ignored in the
> case of hugetlb_lock.
> 
> Setting that aside, I think this thing of grabbing a reference to inode
> for each page just does not work as you wish: when we unlink an inode,
> all its pages should be freed; but because they are themselves holding
> references to the inode, it and its pages stick around forever.

Ugh, yes.  You're absolutely right.  That circular reference will mess
everything up.  Thinking it through and testing fail.

> A quick experiment with your patch versus without confirmed that:
> meminfo HugePages_Free stayed down with your patch, but went back to
> HugePages_Total without it.  Please check, perhaps I'm just mistaken.
> 
> Sorry, I've not looked into what a constructive alternative might be;
> and it's not the first time we've had this difficulty - it came up last
> year when the ->freepage function was added, that the inode may be gone
> by the time ->freepage(page) is called.

Ok, so.  In fact the quota functions we call at free time only need
the super block, not the inode per se.  If we put a superblock pointer
instead of an inode pointer in page private, and refcounted that, I
think that should remove the circular ref.  The only reason I didn't
do it before is that the superblock refcounting functions didn't seem
to be globally visible in an obvious way.

Does that sound like a reasonable approach?

> On a side note, very good description - thank you, but I wish you'd
> split the patch into two, the fix and then the inode-instead-of-mapping
> cleanup.  Though personally I'd prefer not to make that "cleanup": it's
> normal for a struct address space * to be used in struct page (if I delved
> I guess I'd find good reason why this one is in page->private instead of
> page->mapping: perhaps because it's needed after page->mapping is reset
> to NULL, perhaps because it's needed on COWed copies of hugetlbfs pages).

That is an interesting question.  But it doesn't address the basic
point.  mappings aren't refcounted themselves, and as far as I can
tell their lifetime is bound to that of their inode.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson