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xen-devel
Re: [Xen-devel] [RFC] linux: add p[mug]d_clear_full() accessors
Jan Beulich wrote:
One question though - in our 2.6.23 merge (where pv-ops-Xen's
PG_pinned appeared as an alias of PG_owner_priv_1, and where
PG_arch_1 got assigned a meaning for native x86, so PG_pinned
for the traditional patches needed to change anyway) I intentionally
didn't follow pv-ops for our patches, since PG_pinned is among the
flags bad_page() checks for (in the XenSource tree, and I think this
should really also be done in upstream Linux), and hence re-using
the bit here would change behavior for other parts of the kernel.
I don't think so. So long as its clear by the time you free the page,
it doesn't matter how it gets used in the meantime (after all, you
should never free a pinned pagetable page).
But isn't that one of the purposes of those page flags checks - checking
whether e.g. a page may validly get freed (free_pages_check())? All
of bad_page(), free_pages_check(), and prep_new_page() currently
add PG_pinned into the set of flags cleared/checked, and I continue to
think that that is the right thing to do.
Well, yes, it would be helpful for free_pages_check to test whether
PG_owner_priv_1 is set on a page you're attempting to free, since that
would definitely be a bug. But its also a bug which would turn up
fairly quickly anyway, since Xen would complain about any subsequent
users of that page. Regardless, its not actually a bug that has turned
up, since the lifespan of pagetable pages is pretty well controlled, and
there's only a couple of places where they get allocated and freed.
So, not a big issue either way, I think.
J
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