makedumpfile filters out wrong pages

[Masaki Tachibana]

Hi Petr,

Thank you for important information.
I will wait for the patch.

Thanks
Tachibana


> -----Original Message-----
> From: Petr Tesarik [mailto:ptesarik at suse.cz]
> Sent: Thursday, April 12, 2018 6:02 PM
> To: Tachibana Masaki(橘 正樹) <mas-tachibana at vf.jp.nec.com>; Nakayama Takuya(中山 卓也) <tak-nakayama at tg.jp.nec.com>;
> kexec-ml <kexec at lists.infradead.org>
> Subject: makedumpfile filters out wrong pages
> 
> 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