On Wed, Jun 24, 2020 at 11:33:07AM -0700, Andrew Morton wrote: > On Wed, 24 Jun 2020 18:52:25 +0100 Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > From: Steven Price <steven.price@xxxxxxx> > > For arm64 MTE support it is necessary to be able to mark pages that > > contain user space visible tags that will need to be saved/restored e.g. > > when swapped out. > > > > To support this add a new arch specific flag (PG_ARCH_2) that arch code > > can opt into using ARCH_USES_PG_ARCH_2. > > > > ... > > > > --- a/fs/proc/page.c > > +++ b/fs/proc/page.c > > @@ -217,6 +217,9 @@ u64 stable_page_flags(struct page *page) > > u |= kpf_copy_bit(k, KPF_PRIVATE_2, PG_private_2); > > u |= kpf_copy_bit(k, KPF_OWNER_PRIVATE, PG_owner_priv_1); > > u |= kpf_copy_bit(k, KPF_ARCH, PG_arch_1); > > +#ifdef CONFIG_ARCH_USES_PG_ARCH_2 > > + u |= kpf_copy_bit(k, KPF_ARCH_2, PG_arch_2); > > +#endif > > Do we need CONFIG_ARCH_USES_PG_ARCH_2? What would be the downside to > giving every architecture a PG_arch_2, but only arm64 uses it (at > present)? It turns out we have another issue with this flag. PG_arch_2 in the arm64 MTE patches is used to mark a page as having valid tags. During set_pte_at(), if the mapping type is tagged, we set PG_arch_2 (also setting it in other cases like copy_page). In combination with THP and swap (and some stress-testing to force swap-out), the kernel ends up clearing PG_arch_2 in __split_huge_page_tail(), causing a subsequent set_pte_at() to zero valid tags stored by user. The quick fix is to add an arch_huge_page_flags_split_preserve macro (need to think of a shorter name) which adds 1L << PG_arch_2 to the preserve list in the above mentioned function. However, I wonder whether it's safe to add both PG_arch_1 and PG_arch_2 to this list. At least on arm and arm64, PG_arch_1 is used to mark a page as D-cache clean (and don't need to do this again after splitting a pmd): diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 78c84bee7e29..22b3236a6dd8 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2364,6 +2364,10 @@ static void __split_huge_page_tail(struct page *head, int tail, (1L << PG_workingset) | (1L << PG_locked) | (1L << PG_unevictable) | + (1L << PG_arch_1) | +#ifdef CONFIG_64BIT + (1L << PG_arch_2) | +#endif (1L << PG_dirty))); /* ->mapping in first tail page is compound_mapcount */ Thanks. -- Catalin
