Re: [PATCH -mm -v4 03/21] mm, THP, swap: Support PMD swap mapping in swap_duplicate()

[Dave Hansen <dave.hansen@linux.intel.com>]

> +static inline bool thp_swap_supported(void)
> +{
> +	return IS_ENABLED(CONFIG_THP_SWAP);
> +}

This seems like rather useless abstraction.  Why do we need it?

...
> -static inline int swap_duplicate(swp_entry_t swp)
> +static inline int swap_duplicate(swp_entry_t *swp, bool cluster)
>  {
>  	return 0;
>  }

FWIW, I despise true/false function arguments like this.  When I see
this in code:

	swap_duplicate(&entry, false);

I have no idea what false does.  I'd much rather see:

enum do_swap_cluster {
	SWP_DO_CLUSTER,
	SWP_NO_CLUSTER
};

So you see:

	swap_duplicate(&entry, SWP_NO_CLUSTER);

vs.

	swap_duplicate(&entry, SWP_DO_CLUSTER);


> diff --git a/mm/memory.c b/mm/memory.c
> index e9cac1c4fa69..f3900282e3da 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -951,7 +951,7 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  		swp_entry_t entry = pte_to_swp_entry(pte);
>  
>  		if (likely(!non_swap_entry(entry))) {
> -			if (swap_duplicate(entry) < 0)
> +			if (swap_duplicate(&entry, false) < 0)
>  				return entry.val;
>  
>  			/* make sure dst_mm is on swapoff's mmlist. */

I'll also point out that in a multi-hundred-line patch, adding arguments
to a existing function would not be something I'd try to include in the
patch.  I'd break it out separately unless absolutely necessary.

> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index f42b1b0cdc58..48e2c54385ee 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -49,6 +49,9 @@ static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
>  				 unsigned char);
>  static void free_swap_count_continuations(struct swap_info_struct *);
>  static sector_t map_swap_entry(swp_entry_t, struct block_device**);
> +static int add_swap_count_continuation_locked(struct swap_info_struct *si,
> +					      unsigned long offset,
> +					      struct page *page);
>  
>  DEFINE_SPINLOCK(swap_lock);
>  static unsigned int nr_swapfiles;
> @@ -319,6 +322,11 @@ static inline void unlock_cluster_or_swap_info(struct swap_info_struct *si,
>  		spin_unlock(&si->lock);
>  }
>  
> +static inline bool is_cluster_offset(unsigned long offset)
> +{
> +	return !(offset % SWAPFILE_CLUSTER);
> +}
> +
>  static inline bool cluster_list_empty(struct swap_cluster_list *list)
>  {
>  	return cluster_is_null(&list->head);
> @@ -1166,16 +1174,14 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
>  	return NULL;
>  }
>  
> -static unsigned char __swap_entry_free(struct swap_info_struct *p,
> -				       swp_entry_t entry, unsigned char usage)
> +static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
> +					      struct swap_cluster_info *ci,
> +					      unsigned long offset,
> +					      unsigned char usage)
>  {
> -	struct swap_cluster_info *ci;
> -	unsigned long offset = swp_offset(entry);
>  	unsigned char count;
>  	unsigned char has_cache;
>  
> -	ci = lock_cluster_or_swap_info(p, offset);
> -
>  	count = p->swap_map[offset];
>  
>  	has_cache = count & SWAP_HAS_CACHE;
> @@ -1203,6 +1209,17 @@ static unsigned char __swap_entry_free(struct swap_info_struct *p,
>  	usage = count | has_cache;
>  	p->swap_map[offset] = usage ? : SWAP_HAS_CACHE;
>  
> +	return usage;
> +}
> +
> +static unsigned char __swap_entry_free(struct swap_info_struct *p,
> +				       swp_entry_t entry, unsigned char usage)
> +{
> +	struct swap_cluster_info *ci;
> +	unsigned long offset = swp_offset(entry);
> +
> +	ci = lock_cluster_or_swap_info(p, offset);
> +	usage = __swap_entry_free_locked(p, ci, offset, usage);
>  	unlock_cluster_or_swap_info(p, ci);
>  
>  	return usage;
> @@ -3450,32 +3467,12 @@ void si_swapinfo(struct sysinfo *val)
>  	spin_unlock(&swap_lock);
>  }
>  
> -/*
> - * Verify that a swap entry is valid and increment its swap map count.
> - *
> - * Returns error code in following case.
> - * - success -> 0
> - * - swp_entry is invalid -> EINVAL
> - * - swp_entry is migration entry -> EINVAL
> - * - swap-cache reference is requested but there is already one. -> EEXIST
> - * - swap-cache reference is requested but the entry is not used. -> ENOENT
> - * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
> - */
> -static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
> +static int __swap_duplicate_locked(struct swap_info_struct *p,
> +				   unsigned long offset, unsigned char usage)
>  {
> -	struct swap_info_struct *p;
> -	struct swap_cluster_info *ci;
> -	unsigned long offset;
>  	unsigned char count;
>  	unsigned char has_cache;
> -	int err = -EINVAL;
> -
> -	p = get_swap_device(entry);
> -	if (!p)
> -		goto out;
> -
> -	offset = swp_offset(entry);
> -	ci = lock_cluster_or_swap_info(p, offset);
> +	int err = 0;
>  
>  	count = p->swap_map[offset];
>  
> @@ -3485,12 +3482,11 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
>  	 */
>  	if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
>  		err = -ENOENT;
> -		goto unlock_out;
> +		goto out;
>  	}
>  
>  	has_cache = count & SWAP_HAS_CACHE;
>  	count &= ~SWAP_HAS_CACHE;
> -	err = 0;
>  
>  	if (usage == SWAP_HAS_CACHE) {
>  
> @@ -3517,11 +3513,39 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
>  
>  	p->swap_map[offset] = count | has_cache;
>  
> -unlock_out:
> +out:
> +	return err;
> +}

... and that all looks like refactoring, not actively implementing PMD
swap support.  That's unfortunate.

> +/*
> + * Verify that a swap entry is valid and increment its swap map count.
> + *
> + * Returns error code in following case.
> + * - success -> 0
> + * - swp_entry is invalid -> EINVAL
> + * - swp_entry is migration entry -> EINVAL
> + * - swap-cache reference is requested but there is already one. -> EEXIST
> + * - swap-cache reference is requested but the entry is not used. -> ENOENT
> + * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
> + */
> +static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
> +{
> +	struct swap_info_struct *p;
> +	struct swap_cluster_info *ci;
> +	unsigned long offset;
> +	int err = -EINVAL;
> +
> +	p = get_swap_device(entry);
> +	if (!p)
> +		goto out;

Is this an error, or just for running into something like a migration
entry?  Comments please.

> +	offset = swp_offset(entry);
> +	ci = lock_cluster_or_swap_info(p, offset);
> +	err = __swap_duplicate_locked(p, offset, usage);
>  	unlock_cluster_or_swap_info(p, ci);
> +
> +	put_swap_device(p);
>  out:
> -	if (p)
> -		put_swap_device(p);
>  	return err;
>  }

Not a comment on this patch, but lock_cluster_or_swap_info() is woefully
uncommented.

> @@ -3534,6 +3558,81 @@ void swap_shmem_alloc(swp_entry_t entry)
>  	__swap_duplicate(entry, SWAP_MAP_SHMEM);
>  }
>  
> +#ifdef CONFIG_THP_SWAP
> +static int __swap_duplicate_cluster(swp_entry_t *entry, unsigned char usage)
> +{
> +	struct swap_info_struct *si;
> +	struct swap_cluster_info *ci;
> +	unsigned long offset;
> +	unsigned char *map;
> +	int i, err = 0;

Instead of an #ifdef, is there a reason we can't just do:

	if (!IS_ENABLED(THP_SWAP))
		return 0;

?

> +	si = get_swap_device(*entry);
> +	if (!si) {
> +		err = -EINVAL;
> +		goto out;
> +	}
> +	offset = swp_offset(*entry);
> +	ci = lock_cluster(si, offset);

Could you explain a bit why we do lock_cluster() and not
lock_cluster_or_swap_info() here?

> +	if (cluster_is_free(ci)) {
> +		err = -ENOENT;
> +		goto unlock;
> +	}

Needs comments on how this could happen.  We just took the lock, so I
assume this is some kind of race, but can you elaborate?

> +	if (!cluster_is_huge(ci)) {
> +		err = -ENOTDIR;
> +		goto unlock;
> +	}

Yikes!  This function is the core of the new functionality and its
comment count is exactly 0.  There was quite a long patch description,
which will be surely lost to the ages, but nothing in the code that
folks _will_ be looking at for decades to come.

Can we fix that?

> +	VM_BUG_ON(!is_cluster_offset(offset));
> +	VM_BUG_ON(cluster_count(ci) < SWAPFILE_CLUSTER);

So, by this point, we know we are looking at (or supposed to be looking
at) a cluster on the device?

> +	map = si->swap_map + offset;
> +	if (usage == SWAP_HAS_CACHE) {
> +		if (map[0] & SWAP_HAS_CACHE) {
> +			err = -EEXIST;
> +			goto unlock;
> +		}
> +		for (i = 0; i < SWAPFILE_CLUSTER; i++) {
> +			VM_BUG_ON(map[i] & SWAP_HAS_CACHE);
> +			map[i] |= SWAP_HAS_CACHE;
> +		}

So, it's OK to race with the first entry, but after that it's a bug
because the tail pages should agree with the head page's state?

> +	} else {
> +		for (i = 0; i < SWAPFILE_CLUSTER; i++) {
> +retry:
> +			err = __swap_duplicate_locked(si, offset + i, usage);
> +			if (err == -ENOMEM) {
> +				struct page *page;
> +
> +				page = alloc_page(GFP_ATOMIC | __GFP_HIGHMEM);

I noticed that the non-clustering analog of this function takes a GFP
mask.  Why not this one?

> +				err = add_swap_count_continuation_locked(
> +					si, offset + i, page);
> +				if (err) {
> +					*entry = swp_entry(si->type, offset+i);
> +					goto undup;
> +				}
> +				goto retry;
> +			} else if (err)
> +				goto undup;
> +		}
> +		cluster_set_count(ci, cluster_count(ci) + usage);
> +	}
> +unlock:
> +	unlock_cluster(ci);
> +	put_swap_device(si);
> +out:
> +	return err;
> +undup:
> +	for (i--; i >= 0; i--)
> +		__swap_entry_free_locked(
> +			si, ci, offset + i, usage);
> +	goto unlock;
> +}

So, we've basically created a fork of the __swap_duplicate() code for
huge pages, along with a presumably new set of bugs and a second code
path to update.  Was this unavoidable?  Can we unify this any more with
the small pages path?

>  /*
>   * Increase reference count of swap entry by 1.
>   * Returns 0 for success, or -ENOMEM if a swap_count_continuation is required
> @@ -3541,12 +3640,15 @@ void swap_shmem_alloc(swp_entry_t entry)
>   * if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which
>   * might occur if a page table entry has got corrupted.
>   */
> -int swap_duplicate(swp_entry_t entry)
> +int swap_duplicate(swp_entry_t *entry, bool cluster)
>  {
>  	int err = 0;
>  
> -	while (!err && __swap_duplicate(entry, 1) == -ENOMEM)
> -		err = add_swap_count_continuation(entry, GFP_ATOMIC);
> +	if (thp_swap_supported() && cluster)
> +		return __swap_duplicate_cluster(entry, 1);
> +
> +	while (!err && __swap_duplicate(*entry, 1) == -ENOMEM)
> +		err = add_swap_count_continuation(*entry, GFP_ATOMIC);
>  	return err;
>  }

Reading this, I wonder whether this has been refactored as much as
possible.  Both add_swap_count_continuation() and
__swap_duplciate_cluster() start off with the same get_swap_device() dance.

> @@ -3558,9 +3660,12 @@ int swap_duplicate(swp_entry_t entry)
>   * -EBUSY means there is a swap cache.
>   * Note: return code is different from swap_duplicate().
>   */
> -int swapcache_prepare(swp_entry_t entry)
> +int swapcache_prepare(swp_entry_t entry, bool cluster)
>  {
> -	return __swap_duplicate(entry, SWAP_HAS_CACHE);
> +	if (thp_swap_supported() && cluster)
> +		return __swap_duplicate_cluster(&entry, SWAP_HAS_CACHE);
> +	else
> +		return __swap_duplicate(entry, SWAP_HAS_CACHE);
>  }
>  
>  struct swap_info_struct *swp_swap_info(swp_entry_t entry)
> @@ -3590,51 +3695,13 @@ pgoff_t __page_file_index(struct page *page)
>  }
>  EXPORT_SYMBOL_GPL(__page_file_index);
>  
> -/*
> - * add_swap_count_continuation - called when a swap count is duplicated
> - * beyond SWAP_MAP_MAX, it allocates a new page and links that to the entry's
> - * page of the original vmalloc'ed swap_map, to hold the continuation count
> - * (for that entry and for its neighbouring PAGE_SIZE swap entries).  Called
> - * again when count is duplicated beyond SWAP_MAP_MAX * SWAP_CONT_MAX, etc.

This closes out with a lot of refactoring noise.  Any chance that can be
isolated into another patch?