On Wed 05-12-18 13:28:51, David Hildenbrand wrote: > The usage of PG_reserved and how PG_reserved pages are to be treated is > burried deep down in different parts of the kernel. Let's shine some light > onto these details by documenting (most?) current users and expected > behavior. > > I don't see a reason why we have to document "Some of them might not even > exist". If there is a user, we should document it. E.g. for balloon > drivers we now use PG_offline to indicate that a page might currently > not be backed by memory in the hypervisor. And that is independent from > PG_reserved. > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> > Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> > Cc: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx> > Cc: Miles Chen <miles.chen@xxxxxxxxxxxx> > Cc: yi.z.zhang@xxxxxxxxxxxxxxx > Cc: Dan Williams <dan.j.williams@xxxxxxxxx> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> This looks like an improvement. The essential part is that PG_reserved page belongs to its user and no generic code should touch it. The rest is a description of current users which I haven't checked due to to lack of time but yeah, I like the updated wording because I have seen multiple people confused from the swapped out part which is not true for many many years. I have tried to dig out when it was actually the case but failed. So I cannot give my Ack because I didn't really do a real review but I like this FWIW. > --- > include/linux/page-flags.h | 18 ++++++++++++++++-- > 1 file changed, 16 insertions(+), 2 deletions(-) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index 68b8495e2fbc..112526f5ba61 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -17,8 +17,22 @@ > /* > * Various page->flags bits: > * > - * PG_reserved is set for special pages, which can never be swapped out. Some > - * of them might not even exist... > + * PG_reserved is set for special pages. The "struct page" of such a page > + * should in general not be touched (e.g. set dirty) except by their owner. > + * Pages marked as PG_reserved include: > + * - Kernel image (including vDSO) and similar (e.g. BIOS, initrd) > + * - Pages allocated early during boot (bootmem, memblock) > + * - Zero pages > + * - Pages that have been associated with a zone but are not available for > + * the page allocator (e.g. excluded via online_page_callback()) > + * - Pages to exclude from the hibernation image (e.g. loaded kexec images) > + * - MMIO pages (communicate with a device, special caching strategy needed) > + * - MCA pages on ia64 (pages with memory errors) > + * - Device memory (e.g. PMEM, DAX, HMM) > + * Some architectures don't allow to ioremap pages that are not marked > + * PG_reserved (as they might be in use by somebody else who does not respect > + * the caching strategy). Consequently, PG_reserved for a page mapped into > + * user space can indicate the zero page, the vDSO, MMIO pages or device memory. > * > * The PG_private bitflag is set on pagecache pages if they contain filesystem > * specific data (which is normally at page->private). It can be used by > -- > 2.17.2 > -- Michal Hocko SUSE Labs
