Re: [PATCH RFC v2 21/27] mm: arm64: Handle tag storage pages mapped before mprotect(PROT_MTE)

Alexandru Elisei
2023-11-30T17:43:47Z

Hi Peter,

On Mon, Nov 27, 2023 at 09:39:17PM -0800, Peter Collingbourne wrote:
> Hi Alexandru,
>
> On Sun, Nov 19, 2023 at 8:59 AM Alexandru Elisei
> <alexandru.elisei@arm.com> wrote:
> >
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > ---
> > arch/arm64/include/asm/mte_tag_storage.h | 1 +
> > arch/arm64/kernel/mte_tag_storage.c | 15 +++++++
> > arch/arm64/mm/fault.c | 55 ++++++++++++++++++++++++
> > include/linux/migrate.h | 8 +++-
> > include/linux/migrate_mode.h | 1 +
> > mm/internal.h | 6 ---
> > 6 files changed, 78 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/mte_tag_storage.h b/arch/arm64/include/asm/mte_tag_storage.h
> > index b97406d369ce..6a8b19a6a758 100644
> > --- a/arch/arm64/include/asm/mte_tag_storage.h
> > +++ b/arch/arm64/include/asm/mte_tag_storage.h
> > @@ -33,6 +33,7 @@ int reserve_tag_storage(struct page *page, int order, gfp_t gfp);
> > void free_tag_storage(struct page *page, int order);
> >
> > bool page_tag_storage_reserved(struct page *page);
> > +bool page_is_tag_storage(struct page *page);
> >
> > vm_fault_t handle_page_missing_tag_storage(struct vm_fault *vmf);
> > vm_fault_t handle_huge_page_missing_tag_storage(struct vm_fault *vmf);
> > diff --git a/arch/arm64/kernel/mte_tag_storage.c b/arch/arm64/kernel/mte_tag_storage.c
> > index a1cc239f7211..5096ce859136 100644
> > --- a/arch/arm64/kernel/mte_tag_storage.c
> > +++ b/arch/arm64/kernel/mte_tag_storage.c
> > @@ -500,6 +500,21 @@ bool page_tag_storage_reserved(struct page *page)
> > return test_bit(PG_tag_storage_reserved, &page->flags);
> > }
> >
> > +bool page_is_tag_storage(struct page *page)
> > +{
> > + unsigned long pfn = page_to_pfn(page);
> > + struct range *tag_range;
> > + int i;
> > +
> > + for (i = 0; i < num_tag_regions; i++) {
> > + tag_range = &tag_regions[i].tag_range;
> > + if (tag_range->start <= pfn && pfn <= tag_range->end)
> > + return true;
> > + }
> > +
> > + return false;
> > +}
> > +
> > int reserve_tag_storage(struct page *page, int order, gfp_t gfp)
> > {
> > unsigned long start_block, end_block;
> > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > index 6730a0812a24..964c5ae161a3 100644
> > --- a/arch/arm64/mm/fault.c
> > +++ b/arch/arm64/mm/fault.c
> > @@ -12,6 +12,7 @@
> > #include <linux/extable.h>
> > #include <linux/kfence.h>
> > #include <linux/signal.h>
> > +#include <linux/migrate.h>
> > #include <linux/mm.h>
> > #include <linux/hardirq.h>
> > #include <linux/init.h>
> > @@ -956,6 +957,50 @@ void tag_clear_highpage(struct page *page)
> > }
> >
> > #ifdef CONFIG_ARM64_MTE_TAG_STORAGE
> > +
> > +#define MR_TAGGED_TAG_STORAGE MR_ARCH_1
> > +
> > +extern bool isolate_lru_page(struct page *page);
> > +extern void putback_movable_pages(struct list_head *l);
>
> Could we move these declarations to a non-mm-internal header and
> #include it instead of manually declaring them here?

Yes, that's better than this hackish way of doing it.

>
> > +
> > +/* Returns with the page reference dropped. */
> > +static void migrate_tag_storage_page(struct page *page)
> > +{
> > + struct migration_target_control mtc = {
> > + .nid = NUMA_NO_NODE,
> > + .gfp_mask = GFP_HIGHUSER_MOVABLE | __GFP_TAGGED,
> > + };
> > + unsigned long i, nr_pages = compound_nr(page);
> > + LIST_HEAD(pagelist);
> > + int ret, tries;
> > +
> > + lru_cache_disable();
> > +
> > + for (i = 0; i < nr_pages; i++) {
> > + if (!isolate_lru_page(page + i)) {
> > + ret = -EAGAIN;
> > + goto out;
> > + }
> > + /* Isolate just grabbed another reference, drop ours. */
> > + put_page(page + i);
> > + list_add_tail(&(page + i)->lru, &pagelist);
> > + }
> > +
> > + tries = 5;
> > + while (tries--) {
> > + ret = migrate_pages(&pagelist, alloc_migration_target, NULL, (unsigned long)&mtc,
> > + MIGRATE_SYNC, MR_TAGGED_TAG_STORAGE, NULL);
> > + if (ret == 0 || ret != -EBUSY)
>
> This could be simplified to:
>
> if (ret != -EBUSY)

Indeed! I can do the same thing in reserve_tag_storage(), in the loop where I
call alloc_contig_range().

Thanks,
Alex