On Wed, Sep 10, 2025 at 05:52:04PM +0200, Pratyush Yadav wrote:
> On Wed, Sep 10 2025, Matthew Wilcox wrote:
>
> > On Wed, Sep 10, 2025 at 05:34:40PM +0200, Pratyush Yadav wrote:
> >> +#define KHO_PAGE_MAGIC 0x4b484f50U /* ASCII for 'KHOP' */
> >> +
> >> +/*
> >> + * KHO uses page->private, which is an unsigned long, to store page
> >> metadata.
> >> + * Use it to store both the magic and the order.
> >> + */
> >> +union kho_page_info {
> >> + unsigned long page_private;
> >> + struct {
> >> + unsigned int order;
> >> + unsigned int magic;
> >> + };
> >
> > KHO is only supported on 64-bit?
>
> Yes. Currently only x86_64 and ARM64. It is mainly for hypervisor live
> update so there isn't much reason to support it on 32-bit platforms.
Presumably this will eventually change to use some special coding on the memdesc
pointer?
> >> @@ -210,16 +226,16 @@ static void kho_restore_page(struct page *page,
> >> unsigned int order)
> >> struct folio *kho_restore_folio(phys_addr_t phys)
> >> {
> >> struct page *page = pfn_to_online_page(PHYS_PFN(phys));
> >> - unsigned long order;
> >> + union kho_page_info info;
> >>
> >> if (!page)
> >> return NULL;
> >>
> >> - order = page->private;
> >> - if (order > MAX_PAGE_ORDER)
> >> + info.page_private = page->private;
> >> + if (info.magic != KHO_PAGE_MAGIC || info.order > MAX_PAGE_ORDER)
All the impossible checks shoudl be WARN_ON()
> >> return NULL;
> >>
> >> - kho_restore_page(page, order);
> >> + kho_restore_page(page, info.order);
> >> return page_folio(page);
> >
> > This all looks very confused. Before your patch as well as after it.
> > I don't see anything in the current KHO code that requires the
> > phys_addr_t to be order-aligned.
>
> Right, good point. I can send that as a follow up patch. But I think
> this patch stands on its own without that fix too.
Maybe it is worth adding some KHO_DEBUG kconfig to protect some of
these extra checks?
phys should be pfn_valid, phys should be aligned, the page should be
in the right state, order should be valid, etc. All worth checking.
Jason
