Re: [PATCH] zsmalloc: zs_page_migrate: not check inuse if migrate_mode is not MIGRATE_ASYNC

[Minchan Kim <minchan@kernel.org>]

Hello Hui,

On Fri, Jul 14, 2017 at 03:51:07PM +0800, Hui Zhu wrote:
> Got some -EBUSY from zs_page_migrate that will make migration
> slow (retry) or fail (zs_page_putback will schedule_work free_work,
> but it cannot ensure the success).

I think EAGAIN(migration retrial) is better than EBUSY(bailout) because
expectation is that zsmalloc will release the empty zs_page soon so
at next retrial, it will be succeeded.
About schedule_work, as you said, we don't make sure when it happens but
I believe it will happen in a migration iteration most of case.
How often do you see that case?

> 
> And I didn't find anything that make zs_page_migrate cannot work with
> a ZS_EMPTY zspage.
> So make the patch to not check inuse if migrate_mode is not
> MIGRATE_ASYNC.

At a first glance, I think it work but the question is that it a same problem
ith schedule_work of zs_page_putback. IOW, Until the work is done, compaction
cannot succeed. Do you have any number before and after?

Thanks.

> 
> Signed-off-by: Hui Zhu <zhuhui@xiaomi.com>
> ---
>  mm/zsmalloc.c | 66 +++++++++++++++++++++++++++++++++--------------------------
>  1 file changed, 37 insertions(+), 29 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index d41edd2..c298e5c 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1982,6 +1982,7 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
>  	unsigned long old_obj, new_obj;
>  	unsigned int obj_idx;
>  	int ret = -EAGAIN;
> +	int inuse;
>  
>  	VM_BUG_ON_PAGE(!PageMovable(page), page);
>  	VM_BUG_ON_PAGE(!PageIsolated(page), page);
> @@ -1996,21 +1997,24 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
>  	offset = get_first_obj_offset(page);
>  
>  	spin_lock(&class->lock);
> -	if (!get_zspage_inuse(zspage)) {
> +	inuse = get_zspage_inuse(zspage);
> +	if (mode == MIGRATE_ASYNC && !inuse) {
>  		ret = -EBUSY;
>  		goto unlock_class;
>  	}
>  
>  	pos = offset;
>  	s_addr = kmap_atomic(page);
> -	while (pos < PAGE_SIZE) {
> -		head = obj_to_head(page, s_addr + pos);
> -		if (head & OBJ_ALLOCATED_TAG) {
> -			handle = head & ~OBJ_ALLOCATED_TAG;
> -			if (!trypin_tag(handle))
> -				goto unpin_objects;
> +	if (inuse) {
> +		while (pos < PAGE_SIZE) {
> +			head = obj_to_head(page, s_addr + pos);
> +			if (head & OBJ_ALLOCATED_TAG) {
> +				handle = head & ~OBJ_ALLOCATED_TAG;
> +				if (!trypin_tag(handle))
> +					goto unpin_objects;
> +			}
> +			pos += class->size;
>  		}
> -		pos += class->size;
>  	}
>  
>  	/*
> @@ -2020,20 +2024,22 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
>  	memcpy(d_addr, s_addr, PAGE_SIZE);
>  	kunmap_atomic(d_addr);
>  
> -	for (addr = s_addr + offset; addr < s_addr + pos;
> -					addr += class->size) {
> -		head = obj_to_head(page, addr);
> -		if (head & OBJ_ALLOCATED_TAG) {
> -			handle = head & ~OBJ_ALLOCATED_TAG;
> -			if (!testpin_tag(handle))
> -				BUG();
> -
> -			old_obj = handle_to_obj(handle);
> -			obj_to_location(old_obj, &dummy, &obj_idx);
> -			new_obj = (unsigned long)location_to_obj(newpage,
> -								obj_idx);
> -			new_obj |= BIT(HANDLE_PIN_BIT);
> -			record_obj(handle, new_obj);
> +	if (inuse) {
> +		for (addr = s_addr + offset; addr < s_addr + pos;
> +						addr += class->size) {
> +			head = obj_to_head(page, addr);
> +			if (head & OBJ_ALLOCATED_TAG) {
> +				handle = head & ~OBJ_ALLOCATED_TAG;
> +				if (!testpin_tag(handle))
> +					BUG();
> +
> +				old_obj = handle_to_obj(handle);
> +				obj_to_location(old_obj, &dummy, &obj_idx);
> +				new_obj = (unsigned long)
> +					location_to_obj(newpage, obj_idx);
> +				new_obj |= BIT(HANDLE_PIN_BIT);
> +				record_obj(handle, new_obj);
> +			}
>  		}
>  	}
>  
> @@ -2055,14 +2061,16 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
>  
>  	ret = MIGRATEPAGE_SUCCESS;
>  unpin_objects:
> -	for (addr = s_addr + offset; addr < s_addr + pos;
> +	if (inuse) {
> +		for (addr = s_addr + offset; addr < s_addr + pos;
>  						addr += class->size) {
> -		head = obj_to_head(page, addr);
> -		if (head & OBJ_ALLOCATED_TAG) {
> -			handle = head & ~OBJ_ALLOCATED_TAG;
> -			if (!testpin_tag(handle))
> -				BUG();
> -			unpin_tag(handle);
> +			head = obj_to_head(page, addr);
> +			if (head & OBJ_ALLOCATED_TAG) {
> +				handle = head & ~OBJ_ALLOCATED_TAG;
> +				if (!testpin_tag(handle))
> +					BUG();
> +				unpin_tag(handle);
> +			}
>  		}
>  	}
>  	kunmap_atomic(s_addr);
> -- 
> 1.9.1
> 
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