Messages in this thread | | | Date | Mon, 16 Dec 2013 19:36:18 +0100 | From | Oleg Nesterov <> | Subject | Re: PATCH? introduce get_compound_page (Was: process 'stuck' at exit) |
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On 12/13, Andrea Arcangeli wrote: > > On Fri, Dec 13, 2013 at 05:22:40PM +0100, Oleg Nesterov wrote: > > > > I'll try to make v2 based on -mm and your suggestions. > > Ok great!
Yes, it would be great, but I need your help again ;)
Let me quote the pseudo-code you sent me:
put_compound_tail(page) { page_head = compound_trans_head(page); if (!__compound_tail_refcounted(page_head)) { ... return ...; }
flags = compound_lock_irqsave(page_head); ...
Sure, put_compound_tail() should be the simplified version of put_compound_page() which doesn't dec page_head->_count, this is clear.
But afaics, compound_lock_irqsave() above looks unsafe without get_page_unless_zero(page_head) ? If we race with _split, page_head can be freed and compound_lock() can race with, say, free_pages_check() which plays with page->flags ?
So it seems that put_compound_tail() should also do get/put(head) like put_compound_page() does, and this probably means we should factor out the common code somehow.
Or I missed something?
OTOH, I can't really understand
if (likely(page != page_head && get_page_unless_zero(page_head)))
in __get_page_tail() and put_compound_page().
First of all, is it really possible that page == compound_trans_head(page)? We already verified that PG_tail was set. Of course this bit can be cleared and ->first_page can be a dangling pointer but it can never be changed to point to this page? (in fact, afaics it can be changed at all as long as we have a reference, but this doesn't matter).
And compound_lock_irqsave() looks racy even after get_page_unless_zero().
For example, suppose that page_head was already freed and then re-allocated as (say) alloc_pages(__GFP_COMP, 1). get_page_unless_zero() can succeed right after prep_new_page() does set_page_refcounted(). Now, can't compound_lock() race with the non-atomic prep_compound_page()->__SetPageHead() ?
Finally. basepage_index(page) after put_page(page) in get_futex_key() looks confusing imho. I think this is correct, we already checked PageAnon() so it can't be a thp page. But probably this needs a comment and __basepage_index() should do BUG_ON(!PageHuge()).
Oleg.
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