On Wed, Jun 5, 2024 at 5:21 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > Not all pages may apply to pgtable check. One example is ZONE_DEVICE > pages: they map PFNs directly, and they don't allocate page_ext at all even > if there's struct page around. One may reference devm_memremap_pages(). > > When both ZONE_DEVICE and page-table-check enabled, then try to map some > dax memories, one can trigger kernel bug constantly now when the kernel was > trying to inject some pfn maps on the dax device: > > kernel BUG at mm/page_table_check.c:55! > > While it's pretty legal to use set_pxx_at() for ZONE_DEVICE pages for page > fault resolutions, skip all the checks if page_ext doesn't even exist in > pgtable checker, which applies to ZONE_DEVICE but maybe more. Thank you for reporting this bug. A few comments below: > > Cc: Dan Williams <dan.j.williams@xxxxxxxxx> > Cc: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> > --- > mm/page_table_check.c | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) > > diff --git a/mm/page_table_check.c b/mm/page_table_check.c > index 4169576bed72..509c6ef8de40 100644 > --- a/mm/page_table_check.c > +++ b/mm/page_table_check.c > @@ -73,6 +73,9 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt) > page = pfn_to_page(pfn); > page_ext = page_ext_get(page); > > + if (!page_ext) > + return; I would replace the above with the following, here and in other places: if (!page_ext) { WARN_ONCE(!is_zone_device_page(page), "page_ext is missing for a non-device page\n"); return; } > + > BUG_ON(PageSlab(page)); > anon = PageAnon(page); > > @@ -110,6 +113,9 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt, > page = pfn_to_page(pfn); > page_ext = page_ext_get(page); > > + if (!page_ext) > + return; > + > BUG_ON(PageSlab(page)); > anon = PageAnon(page); > > @@ -140,7 +146,10 @@ void __page_table_check_zero(struct page *page, unsigned int order) > BUG_ON(PageSlab(page)); > > page_ext = page_ext_get(page); > - BUG_ON(!page_ext); > + > + if (!page_ext) > + return; > + > for (i = 0; i < (1ul << order); i++) { > struct page_table_check *ptc = get_page_table_check(page_ext); > > -- > 2.45.0 >