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, 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()

if (exclusive_swap_p..) {
if (vm_swap_full()) {
delete_from_swap_cache_nolock(page);
pte = pte_mkdirty(pte);
}
}
UnlockPage(page);

						__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
the swap map entry used. See?