On Tue 15-05-18 13:51:24, Pavel Tatashin wrote: > It is unsafe to do virtual to physical translations before mm_init() is > called if struct page is needed in order to determine the memory section > number (see SECTION_IN_PAGE_FLAGS). This is because only in mm_init() we > initialize struct pages for all the allocated memory when deferred struct > pages are used. > > My recent fix exposed this problem, because it greatly reduced number of > pages that are initialized before mm_init(), but the problem existed even > before my fix, as Fengguang Wu found. > > Below is a more detailed explanation of the problem. > > We initialize struct pages in four places: > > 1. Early in boot a small set of struct pages is initialized to fill > the first section, and lower zones. > 2. During mm_init() we initialize "struct pages" for all the memory > that is allocated, i.e reserved in memblock. > 3. Using on-demand logic when pages are allocated after mm_init call (when > memblock is finished) > 4. After smp_init() when the rest free deferred pages are initialized. > > The problem occurs if we try to do va to phys translation of a memory > between steps 1 and 2. Because we have not yet initialized struct pages for > all the reserved pages, it is inherently unsafe to do va to phys if the > translation itself requires access of "struct page" as in case of this > combination: CONFIG_SPARSE && !CONFIG_SPARSE_VMEMMAP > > The following path exposes the problem: > > start_kernel() > trap_init() > setup_cpu_entry_areas() > setup_cpu_entry_area(cpu) > get_cpu_gdt_paddr(cpu) > per_cpu_ptr_to_phys(addr) > pcpu_addr_to_page(addr) > virt_to_page(addr) > pfn_to_page(__pa(addr) >> PAGE_SHIFT) > > We disable this path by not allowing NEED_PER_CPU_KM with deferred struct > pages feature. > > The problems are discussed in these threads: > http://lkml.kernel.org/r/20180418135300.inazvpxjxowogyge@xxxxxxxxxxxxxxxxxxxxxx > http://lkml.kernel.org/r/20180419013128.iurzouiqxvcnpbvz@xxxxxxxxxxxxxxxxxxxxxx > http://lkml.kernel.org/r/20180426202619.2768-1-pasha.tatashin@xxxxxxxxxx > > Fixes: 3a80a7fa7989 ("mm: meminit: initialise a subset of struct pages if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set") > Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Thanks a lot! > --- > mm/Kconfig | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/mm/Kconfig b/mm/Kconfig > index d5004d82a1d6..e14c01513bfd 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -636,6 +636,7 @@ config DEFERRED_STRUCT_PAGE_INIT > default n > depends on NO_BOOTMEM > depends on !FLATMEM > + depends on !NEED_PER_CPU_KM > help > Ordinarily all struct pages are initialised during early boot in a > single thread. On very large machines this can take a considerable > -- > 2.17.0 -- Michal Hocko SUSE Labs
