[bug report] lib/test_hmm: add zone device private THP test infrastructure

[Dan Carpenter <dan.carpenter@linaro.org>]

Hello Balbir Singh,

Commit 6098c878b175 ("lib/test_hmm: add zone device private THP test
infrastructure") from Sep 8, 2025 (linux-next), leads to the
following Smatch static checker warning:

	lib/test_hmm.c:721 dmirror_migrate_alloc_and_copy()
	error: we previously assumed 'spage' could be null (see line 718)

lib/test_hmm.c
    675 static void dmirror_migrate_alloc_and_copy(struct migrate_vma *args,
    676                                            struct dmirror *dmirror)
    677 {
    678         const unsigned long *src = args->src;
    679         unsigned long *dst = args->dst;
    680         unsigned long addr;
    681 
    682         for (addr = args->start; addr < args->end; ) {
    683                 struct page *spage;
    684                 struct page *dpage;
    685                 struct page *rpage;
    686                 bool is_large = *src & MIGRATE_PFN_COMPOUND;
    687                 int write = (*src & MIGRATE_PFN_WRITE) ? MIGRATE_PFN_WRITE : 0;
    688                 unsigned long nr = 1;
    689 
    690                 if (!(*src & MIGRATE_PFN_MIGRATE))
    691                         goto next;
    692 
    693                 /*
    694                  * Note that spage might be NULL which is OK since it is an
    695                  * unallocated pte_none() or read-only zero page.
    696                  */
    697                 spage = migrate_pfn_to_page(*src);
    698                 if (WARN(spage && is_zone_device_page(spage),
    699                      "page already in device spage pfn: 0x%lx\n",
    700                      page_to_pfn(spage)))
    701                         goto next;
    702 
    703                 if (dmirror->flags & HMM_DMIRROR_FLAG_FAIL_ALLOC) {
    704                         dmirror->flags &= ~HMM_DMIRROR_FLAG_FAIL_ALLOC;
    705                         dpage = NULL;
    706                 } else
    707                         dpage = dmirror_devmem_alloc_page(dmirror, is_large);
    708 
    709                 if (!dpage) {
    710                         struct folio *folio;
    711                         unsigned long i;
    712                         unsigned long spfn = *src >> MIGRATE_PFN_SHIFT;
    713                         struct page *src_page;
    714 
    715                         if (!is_large)
    716                                 goto next;
    717 
    718                         if (!spage && is_large) {
                                    ^^^^^^    ^^^^^^^^
If spage is NULL but is_large is false

    719                                 nr = HPAGE_PMD_NR;
    720                         } else {
--> 721                                 folio = page_folio(spage);
                                                           ^^^^^
the this will crash.

    722                                 nr = folio_nr_pages(folio);
    723                         }
    724 
    725                         for (i = 0; i < nr && addr < args->end; i++) {

regards,
dan carpenter