Re: 2.4.10pre VM changes: Potential race condition on swap code

[Marcelo Tosatti]

On Thu, 13 Sep 2001, Marcelo Tosatti wrote:

> 
> 
> On Thu, 13 Sep 2001, Marcelo Tosatti wrote:
> 
> > 
> > 
> > On Thu, 13 Sep 2001, Marcelo Tosatti wrote:
> > 
> > > 
> > > 
> > > On Thu, 13 Sep 2001, Hugh Dickins wrote:
> > > 
> > > > On Wed, 12 Sep 2001, Marcelo Tosatti wrote:
> > > > > On Tue, 11 Sep 2001, Hugh Dickins wrote:
> > > > > > It may be made more likely by my swapoff changes (not bumping swap
> > > > > > count in valid_swaphandles), but it's not been introduced by those
> > > > > > changes.  Though usually swapin_readahead/valid_swaphandles covers
> > > > > > (includes) the particular swap entry which do_swap_page actually
> > > > > > wants to bring in, under pressure that's not always so, and then
> > > > > > the race you outline can occur with the "bare" read_swap_cache_async
> > > > > > for which there was no bumping.  Furthermore, you can play your
> > > > > > scenario with valid_swaphandles through to add_to_swap_cache on CPU0
> > > > > > interposed between the get_swap_page and add_to_swap_cache on CPU1
> > > > > > (if interrupt on CPU1 diverts it).
> > > > > 
> > > > > I don't think so. A "bare" read_swap_cache_async() call only happens on
> > > > > swap entries which already have additional references. That is, its
> > > > > guaranteed that a "bare" read_swap_cache_async() call only happens for
> > > > > swap map entries which already have a reference, so we're guaranteed that
> > > > > it cannot be reused.
> > > > 
> > > > Almost agreed, but there may be a long interval between when that reference
> > > > was observed in the page table, and when read_swap_cache_async upon it is
> > > > actually performed (waiting for BKL, waiting to allocate pages for prior
> > > > swapin_readahead).  In that interval the reference can be removed:
> > > > certainly by swapoff, certainly by vm_swap_full removal, anything else?
> > > 
> > > Not sure about swapoff(). 
> > > 
> > > vm_swap_full() is only going to remove the reference _after_ we did the
> > > swapin, so I don't see how the race can happen with it.
> >
> > 
> > Ooh I see:
> > 
> > CPU0			CPU1			CPU2
> > do_swap_page()		try_to_swap_out()	swapin_readahead()
> > 						swapin_readahead() finds valid swap
> > 						map entry and considers it "readable"
> > 		
> > swap_free(entry);				
> > 			get_swap_page() <----- (1)
> > 
> > if (exclusive_swap_p..) {
> > if (vm_swap_full()) {
> > delete_from_swap_cache_nolock(page);
> > pte = pte_mkdirty(pte);
> > }
> > }
> > UnlockPage(page);
			get_swap_page() <------ (2)

Obviously the get_swap_page() cannot "succeed" in (1), but only in (2).

The race is still valid: I just wanted to make sure you get the idea.

> > 
> > 						__find_get_page() fails on swapin_readahead()
> > 						swap_duplicate() succeeds.
> > 						add_to_swap_cache()
> > 			add_to_swap_cache()
> > 
> > BOOM.
> > 
> > Now, if we get additional references at valid_swaphandles() the above race
> > is NOT possible: we're guaranteed that any get_swap_page() will not find
> 
> Err I mean _will_ find the swap map entry used and not use it, then.
> 
> > the swap map entry used. See?