lkml.org 
[lkml]   [2009]   [Apr]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 5/5] proc: export more page flags in /proc/kpageflags
On Tue, 28 Apr 2009 17:46:34 -0500
Matt Mackall <mpm@selenic.com> wrote:

> > > +/* a helper function _not_ intended for more general uses */
> > > +static inline int page_cap_writeback_dirty(struct page *page)
> > > +{
> > > + struct address_space *mapping;
> > > +
> > > + if (!PageSlab(page))
> > > + mapping = page_mapping(page);
> > > + else
> > > + mapping = NULL;
> > > +
> > > + return mapping && mapping_cap_writeback_dirty(mapping);
> > > +}
> >
> > If the page isn't locked then page->mapping can be concurrently removed
> > and freed. This actually happened to me in real-life testing several
> > years ago.
>
> We certainly don't want to be taking locks per page to build the flags
> data here. As we don't have any pretense of being atomic, it's ok if we
> can find a way to do the test that's inaccurate when a race occurs, so
> long as it doesn't dereference null.
>
> But if there's not an obvious way to do that, we should probably just
> drop this flag bit for this iteration.

trylock_page() could be used here, perhaps.

Then again, why _not_ just do lock_page()? After all, few pages are
ever locked. There will be latency if the caller stumbles across a
page which is under read I/O, but so be it?



\
 
 \ /
  Last update: 2009-04-29 01:09    [from the cache]
©2003-2011 Jasper Spaans