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Date:   Thu, 30 Nov 2023 17:43:01 +0000
From:   Alexandru Elisei <alexandru.elisei@arm.com>
To:     Peter Collingbourne <pcc@google.com>
Cc:     catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev,
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Subject: Re: [PATCH RFC v2 21/27] mm: arm64: Handle tag storage pages mapped
 before mprotect(PROT_MTE)
Message-ID: <ZWjJpaSahUUM3GKs@raptor>
References: <20231119165721.9849-1-alexandru.elisei@arm.com>
 <20231119165721.9849-22-alexandru.elisei@arm.com>
 <CAMn1gO5WYC5Xx7LfBxN_j-xqkVT+tjXP5PqDfrvhgqPOa0ZCsA@mail.gmail.com>
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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