A better dump_page()

[Matthew Wilcox]

On Tue, Jan 03, 2023 at 11:42:11AM +0100, Vlastimil Babka wrote:
> Separately we should also make the __dump_page() more resilient.

Right.  It's not ideal when one of our best debugging tools obfuscates
the problem we're trying to debug.  I've seen probems like this before,
and the problem is that somebody calls dump_page() on a page that they
don't own a refcount on.  That lets the page mutate under us in some
fairly awkward ways (as you've seen here, it seems to be part of several
different compound allocations at various points during the dump
process).

One possibility I thought about was taking our own refcount on the
page at the start of dump_page().  That would kill off the possibility
of ever passing in a const struct page, and it would confuse people.
Also, what if somebody passes in a pointer to something that's not a
struct page?  Then we've (tried to) modify memory that's not a refcount.

I think the best we can do is to snapshot the struct page and the folio
it appears to belong to at the start of dump_page().  It'll take a
little care (for example, folio_pfn() must be passed the original
folio, and not the snapshot), but I think it's doable.