Re: [PATCH] reapswap for 2.4.5-ac10

[Hugh Dickins <hugh@veritas.com>]

On Tue, 5 Jun 2001, Marcelo Tosatti wrote:
> On Tue, 5 Jun 2001, Stephen C. Tweedie wrote:
> > On Tue, Jun 05, 2001 at 04:48:46PM -0300, Marcelo Tosatti wrote:
> > > I'm resending the reapswap patch for inclusion into -ac series. 
> > 
> > Isn't it broken in this state?  Checking page_count, page->buffers and
> > PageSwapCache without the appropriate locks is dangerous.
> 
> We hold the pagemap_lru_lock, so there will be no one doing lookups on
> this swap page (get_swapcache_page() locks pagemap_lru_lock).
> 
> Am I overlooking something here? 

mm/shmem.c:shmem_getpage_locked() and mm/swapfile.c:try_to_unuse()
call delete_from_swap_cache_nolock(), both holding page lock,
neither holding pagemap_lru_lock.

Unless you hold the page lock, PageSwapCache(page) and page->index
are volatile, but to find swap_count(page) you have to rely on both
of them.  TryLockPage()?

Hugh

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