Re: [PATCH RFC v2 16/27] arm64: mte: Manage tag storage on page allocation

Alexandru Elisei
2023-11-29T13:34:17Z

Hi,

On Wed, Nov 29, 2023 at 06:10:40PM +0900, Hyesoo Yu wrote:
> On Sun, Nov 19, 2023 at 04:57:10PM +0000, Alexandru Elisei wrote:
> > [..]
> > +static int order_to_num_blocks(int order)
> > +{
> > + return max((1 << order) / 32, 1);
> > +}
> > [..]
> > +int reserve_tag_storage(struct page *page, int order, gfp_t gfp)
> > +{
> > + unsigned long start_block, end_block;
> > + struct tag_region *region;
> > + unsigned long block;
> > + unsigned long flags;
> > + unsigned int tries;
> > + int ret = 0;
> > +
> > + VM_WARN_ON_ONCE(!preemptible());
> > +
> > + if (page_tag_storage_reserved(page))
> > + return 0;
> > +
> > + /*
> > + * __alloc_contig_migrate_range() ignores gfp when allocating the
> > + * destination page for migration. Regardless, massage gfp flags and
> > + * remove __GFP_TAGGED to avoid recursion in case gfp stops being
> > + * ignored.
> > + */
> > + gfp &= ~__GFP_TAGGED;
> > + if (!(gfp & __GFP_NORETRY))
> > + gfp |= __GFP_RETRY_MAYFAIL;
> > +
> > + ret = tag_storage_find_block(page, &start_block, &region);
> > + if (WARN_ONCE(ret, "Missing tag storage block for pfn 0x%lx", page_to_pfn(page)))
> > + return 0;
> > + end_block = start_block + order_to_num_blocks(order) * region->block_size;
> > +
>
> Hello.
>
> If the page size is 4K, block size is 2 (block size bytes 8K), and order is 6,
> then we need 2 pages for the tag. However according to the equation, order_to_num_blocks
> is 2 and block_size is also 2, so end block will be incremented by 4.
>
> However we actually only need 8K of tag, right for 256K ?
> Could you explain order_to_num_blocks * region->block_size more detail ?

I think you are correct, thank you for pointing it out. The formula should
probably be something like:

static int order_to_num_blocks(int order, u32 block_size)
{
int num_tag_pages = max((1 << order) / 32, 1);

return DIV_ROUND_UP(num_tag_pages, block_size);
}

and that will make end_block = start_block + 2 in your scenario.

Does that look correct to you?

Thanks,
Alex

>
> Thanks,
> Regards.
>
> > + mutex_lock(&tag_blocks_lock);
> > +
> > + /* Check again, this time with the lock held. */
> > + if (page_tag_storage_reserved(page))
> > + goto out_unlock;
> > +
> > + /* Make sure existing entries are not freed from out under out feet. */
> > + xa_lock_irqsave(&tag_blocks_reserved, flags);
> > + for (block = start_block; block < end_block; block += region->block_size) {
> > + if (tag_storage_block_is_reserved(block))
> > + block_ref_add(block, region, order);
> > + }
> > + xa_unlock_irqrestore(&tag_blocks_reserved, flags);
> > +
> > + for (block = start_block; block < end_block; block += region->block_size) {
> > + /* Refcount incremented above. */
> > + if (tag_storage_block_is_reserved(block))
> > + continue;
> > +
> > + tries = 3;
> > + while (tries--) {
> > + ret = alloc_contig_range(block, block + region->block_size, MIGRATE_CMA, gfp);
> > + if (ret == 0 || ret != -EBUSY)
> > + break;
> > + }
> > +
> > + if (ret)
> > + goto out_error;
> > +
> > + ret = tag_storage_reserve_block(block, region, order);
> > + if (ret) {
> > + free_contig_range(block, region->block_size);
> > + goto out_error;
> > + }
> > +
> > + count_vm_events(CMA_ALLOC_SUCCESS, region->block_size);
> > + }
> > +
> > + page_set_tag_storage_reserved(page, order);
> > +out_unlock:
> > + mutex_unlock(&tag_blocks_lock);
> > +
> > + return 0;
> > +
> > +out_error:
> > + xa_lock_irqsave(&tag_blocks_reserved, flags);
> > + for (block = start_block; block < end_block; block += region->block_size) {
> > + if (tag_storage_block_is_reserved(block) &&
> > + block_ref_sub_return(block, region, order) == 1) {
> > + __xa_erase(&tag_blocks_reserved, block);
> > + free_contig_range(block, region->block_size);
> > + }
> > + }
> > + xa_unlock_irqrestore(&tag_blocks_reserved, flags);
> > +
> > + mutex_unlock(&tag_blocks_lock);
> > +
> > + count_vm_events(CMA_ALLOC_FAIL, region->block_size);
> > +
> > + return ret;
> > +}
> > +
> > +void free_tag_storage(struct page *page, int order)
> > +{
> > + unsigned long block, start_block, end_block;
> > + struct tag_region *region;
> > + unsigned long flags;
> > + int ret;
> > +
> > + ret = tag_storage_find_block(page, &start_block, &region);
> > + if (WARN_ONCE(ret, "Missing tag storage block for pfn 0x%lx", page_to_pfn(page)))
> > + return;
> > +
> > + end_block = start_block + order_to_num_blocks(order) * region->block_size;
> > +
> > + xa_lock_irqsave(&tag_blocks_reserved, flags);
> > + for (block = start_block; block < end_block; block += region->block_size) {
> > + if (WARN_ONCE(!tag_storage_block_is_reserved(block),
> > + "Block 0x%lx is not reserved for pfn 0x%lx", block, page_to_pfn(page)))
> > + continue;
> > +
> > + if (block_ref_sub_return(block, region, order) == 1) {
> > + __xa_erase(&tag_blocks_reserved, block);
> > + free_contig_range(block, region->block_size);
> > + }
> > + }
> > + xa_unlock_irqrestore(&tag_blocks_reserved, flags);
> > +}
> > diff --git a/fs/proc/page.c b/fs/proc/page.c
> > index 195b077c0fac..e7eb584a9234 100644
> > --- a/fs/proc/page.c
> > +++ b/fs/proc/page.c
> > @@ -221,6 +221,7 @@ u64 stable_page_flags(struct page *page)
> > #ifdef CONFIG_ARCH_USES_PG_ARCH_X
> > u |= kpf_copy_bit(k, KPF_ARCH_2, PG_arch_2);
> > u |= kpf_copy_bit(k, KPF_ARCH_3, PG_arch_3);
> > + u |= kpf_copy_bit(k, KPF_ARCH_4, PG_arch_4);
> > #endif
> >
> > return u;
> > diff --git a/include/linux/kernel-page-flags.h b/include/linux/kernel-page-flags.h
> > index 859f4b0c1b2b..4a0d719ffdd4 100644
> > --- a/include/linux/kernel-page-flags.h
> > +++ b/include/linux/kernel-page-flags.h
> > @@ -19,5 +19,6 @@
> > #define KPF_SOFTDIRTY 40
> > #define KPF_ARCH_2 41
> > #define KPF_ARCH_3 42
> > +#define KPF_ARCH_4 43
> >
> > #endif /* LINUX_KERNEL_PAGE_FLAGS_H */
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index a88e64acebfe..7915165a51bd 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -135,6 +135,7 @@ enum pageflags {
> > #ifdef CONFIG_ARCH_USES_PG_ARCH_X
> > PG_arch_2,
> > PG_arch_3,
> > + PG_arch_4,
> > #endif
> > __NR_PAGEFLAGS,
> >
> > diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> > index 6ca0d5ed46c0..ba962fd10a2c 100644
> > --- a/include/trace/events/mmflags.h
> > +++ b/include/trace/events/mmflags.h
> > @@ -125,7 +125,8 @@ IF_HAVE_PG_HWPOISON(hwpoison) \
> > IF_HAVE_PG_IDLE(idle) \
> > IF_HAVE_PG_IDLE(young) \
> > IF_HAVE_PG_ARCH_X(arch_2) \
> > -IF_HAVE_PG_ARCH_X(arch_3)
> > +IF_HAVE_PG_ARCH_X(arch_3) \
> > +IF_HAVE_PG_ARCH_X(arch_4)
> >
> > #define show_page_flags(flags) \
> > (flags) ? __print_flags(flags, "|", \
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index f31f02472396..9beead961a65 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2474,6 +2474,7 @@ static void __split_huge_page_tail(struct folio *folio, int tail,
> > #ifdef CONFIG_ARCH_USES_PG_ARCH_X
> > (1L << PG_arch_2) |
> > (1L << PG_arch_3) |
> > + (1L << PG_arch_4) |
> > #endif
> > (1L << PG_dirty) |
> > LRU_GEN_MASK | LRU_REFS_MASK));
> > --
> > 2.42.1
> >
> >