On 10/26/21 11:50 AM, Pasha Tatashin wrote:
On Tue, Oct 26, 2021 at 2:45 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
On 10/26/21 10:38 AM, Pasha Tatashin wrote:
prep_compound_gigantic_page() calls set_page_count(0, p), but it is not
needed because page_ref_freeze(p, 1) already sets refcount to 0.
Using, set_page_count() is dangerous, because it unconditionally resets
refcount from the current value to unrestrained value, and therefore
should be minimized.
Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
Thanks!
My bad for not removing the set_page_count when adding the page_ref_freeze.
FYI, there have been additional changes to this routine in Andrew's
tree. Not really sure if we want/need the VM_BUG_ON_PAGE as that would
only check if there was a 'bug' in page_ref_freeze.
I would like to keep it. Part of the idea of this series is to reduce
reliance on comments such as:
/* No worries, refcount is A therefore we can do B */
And instead enforce that via VM_BUG_ON(). It should be able to
prevent existing and future _refcount related bugs from manifesting as
memory corruptions.
Ok, but that seems a bit redundant to me.
There is actually a VM_BUG_ON_PAGE(page_count(p), p) in the 'non-demote'
case in Andrew's tree. This is in the path where we do not call
page_ref_freeze to zero page ref. That seems sufficient to me.
Since you did point out the unnecessary set_page_count, I will submit a
code cleanup patch to remove it. I think that is independent of your
efforts here, and adding VM_BUG_ON can be discussed in the context of
this series.
--
Mike Kravetz
