On Fri, May 28, 2021 at 10:54:03AM -0700, Yang Shi wrote:
> So I prepared this patch to show a possible approach to get some
> feedback. The same thing could be decoded by the reader of page dump
> as well by using the same formula used by this patch. However it sounds
> more convenient to have kernel do the math.
You haven't taken enough things into consideration ...
> + bool is_slab = PageSlab(head);
We should probably have a separate dump_slab_page(). Almost nothing
in __dump_page() is really useful for slab pages (eg, mapping, index,
mapcount, compound_mapcount, compound_pincount, aops), and the flags
(such as are used) have different meanings.
> + nr = compound_nr(head);
> + if (is_slab)
> + total_mapcount = 0;
> + else if (PageHuge(head))
> + total_mapcount = comp_mapcnt;
> + else {
> + if (mapping) {
> + if (!PageAnon(head))
> + nr = nr * (comp_mapcnt + 1) - comp_mapcnt;
> + } else
> + nr = 0;
> + total_mapcount = refcount - pincount - nr;
I see what you're trying to do here, but there are so many other things
which take a refcount on a page. The LRU, the page cache, private fs
data, random temporary "gets" (eg, buffered reads, buffered writes,
get_user_pages(), readahead, truncate, migration). I think this is
likely to be so inaccurate as to be confusing.
I had to think hard about it though. I like what you're trying to do,
I just don't think it works ;-(
