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    SubjectRe: [PATCH 5/5] proc: export more page flags in /proc/kpageflags
    On Tue, 28 Apr 2009 18:31:09 -0500
    Matt Mackall <mpm@selenic.com> wrote:

    > On Tue, 2009-04-28 at 16:02 -0700, Andrew Morton wrote:
    > > 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?
    >
    > As I mentioned just a bit ago, it's really not an unreasonable use case
    > to want to do this on every page in the system back to back. So per page
    > overhead matters. And the odds of stalling on a locked page when
    > visiting 1M pages while under load are probably not negligible.

    The chances of stalling on a locked page are pretty good, and the
    duration of the stall might be long indeed. Perhaps a trylock is a
    decent compromise - it depends on the value of this metric, and I've
    forgotten what we're talking about ;)

    umm, seems that this flag is needed to enable PG_error, PG_dirty,
    PG_uptodate and PG_writeback reporting. So simply removing this code
    would put a huge hole in the patchset, no?

    > Our lock primitives are pretty low overhead in the fast path, but every
    > cycle counts. The new tests and branches this code already adds are a
    > bit worrisome, but on balance probably worth it.

    That should be easy to quantify (hint).


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