linux-kernel - Re: [PATCH v6 6/6] zsmalloc: Implement writeback mechanism for zsmalloc

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Message-ID: <Y3xuKXmNjs87q3AW@google.com>
Date:   Tue, 22 Nov 2022 15:37:29 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, minchan@...nel.org,
        ngupta@...are.org, senozhatsky@...omium.org, sjenning@...hat.com,
        ddstreet@...e.org, vitaly.wool@...sulko.com
Subject: Re: [PATCH v6 6/6] zsmalloc: Implement writeback mechanism for
 zsmalloc

On (22/11/18 16:15), Nhat Pham wrote:
[..]
> +static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries)
> +{
> +	int i, obj_idx, ret = 0;
> +	unsigned long handle;
> +	struct zspage *zspage;
> +	struct page *page;
> +	enum fullness_group fullness;
> +
> +	/* Lock LRU and fullness list */
> +	spin_lock(&pool->lock);
> +	if (list_empty(&pool->lru)) {
> +		spin_unlock(&pool->lock);
> +		return -EINVAL;
> +	}
> +
> +	for (i = 0; i < retries; i++) {
> +		struct size_class *class;
> +
> +		zspage = list_last_entry(&pool->lru, struct zspage, lru);
> +		list_del(&zspage->lru);
> +
> +		/* zs_free may free objects, but not the zspage and handles */
> +		zspage->under_reclaim = true;
> +
> +		class = zspage_class(pool, zspage);
> +		fullness = get_fullness_group(class, zspage);
> +
> +		/* Lock out object allocations and object compaction */
> +		remove_zspage(class, zspage, fullness);
> +
> +		spin_unlock(&pool->lock);
> +
> +		/* Lock backing pages into place */
> +		lock_zspage(zspage);
> +
> +		obj_idx = 0;
> +		page = zspage->first_page;
> +		while (1) {
> +			handle = find_alloced_obj(class, page, &obj_idx);
> +			if (!handle) {
> +				page = get_next_page(page);
> +				if (!page)
> +					break;
> +				obj_idx = 0;
> +				continue;
> +			}
> +
> +			/*
> +			 * This will write the object and call zs_free.
> +			 *
> +			 * zs_free will free the object, but the
> +			 * under_reclaim flag prevents it from freeing
> +			 * the zspage altogether. This is necessary so
> +			 * that we can continue working with the
> +			 * zspage potentially after the last object
> +			 * has been freed.
> +			 */
> +			ret = pool->zpool_ops->evict(pool->zpool, handle);
> +			if (ret)
> +				goto next;
> +
> +			obj_idx++;
> +		}
> +
> +next:
> +		/* For freeing the zspage, or putting it back in the pool and LRU list. */
> +		spin_lock(&pool->lock);
> +		zspage->under_reclaim = false;
> +
> +		if (!get_zspage_inuse(zspage)) {
> +			/*
> +			 * Fullness went stale as zs_free() won't touch it
> +			 * while the page is removed from the pool. Fix it
> +			 * up for the check in __free_zspage().
> +			 */
> +			zspage->fullness = ZS_EMPTY;
> +
> +			__free_zspage(pool, class, zspage);
> +			spin_unlock(&pool->lock);
> +			return 0;
> +		}
> +
> +		putback_zspage(class, zspage);
> +		list_add(&zspage->lru, &pool->lru);
> +		unlock_zspage(zspage);

We probably better to cond_resched() somewhere here. Or in zs_zpool_shrink()
loop.

> +	}
> +
> +	spin_unlock(&pool->lock);
> +	return -EAGAIN;
> +}