makedumpfile filters out wrong pages

[Petr Tesarik]

Hello,

I've just hit a nasty bug in makedumpfile when page cache is filtered
out (dump level bitmap includes 2 or 4). In short, makedumpfile checks
PG_swapcache, but since commit 6326fec1122cde256bd2a8c63f2606e08e44ce1d
this bit is an alias for PG_owner_priv_1. This bit is also used by
filesystem code (PG_checked) and Xen (PG_pinned and PG_foreign).

FTR I haven't hit an issue with the filsystem usage in practice, but
all my Xen dumps are unusable, because PG_pinned is by design set for
all page table pages.

Anyway, it appears that PG_swapcache is valid only if PG_swapbacked is
set. I have already submitted a patch to the Linux kernel to export the
value of PG_swapbacked in VMCOREINFO:

http://ozlabs.org/~akpm/mmotm/broken-out/kexec-export-pg_swapbacked-to-vmcoreinfo.patch

I plan to make the corresponding makedumpfile change soon. However, it
seems to me that PG_swapbacked alone should determine whether a page
should be kept in the dump file, or filtered out. Why do we check
PG_swapcache instead? PG_swapbacked first appeared in kernel 2.6.28,
i.e. much later than makedumpfile. Is it simply that nobody has
reviewed the logic since then?

Petr T