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

[Alexandru Elisei <alexandru.elisei@arm.com>]

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
> > 
> >