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Subject: Re: [PATCH v7 09/24] mm: Put readahead pages in cache earlier
To: Matthew Wilcox <willy@infradead.org>, <linux-fsdevel@vger.kernel.org>
References: <20200219210103.32400-1-willy@infradead.org>
 <20200219210103.32400-10-willy@infradead.org>
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From: John Hubbard <jhubbard@nvidia.com>
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On 2/19/20 1:00 PM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> When populating the page cache for readahead, mappings that use
> ->readpages must populate the page cache themselves as the pages are
> passed on a linked list which would normally be used for the page cache's
> LRU.  For mappings that use ->readpage or the upcoming ->readahead method,
> we can put the pages into the page cache as soon as they're allocated,
> which solves a race between readahead and direct IO.  It also lets us
> remove the gfp argument from read_pages().
> 
> Use the new readahead_page() API to implement the repeated calls to
> ->readpage(), just like most filesystems will.  This iterator also
> supports huge pages, even though none of the filesystems have been
> converted to use them yet.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/linux/pagemap.h | 20 +++++++++++++++++
>  mm/readahead.c          | 48 +++++++++++++++++++++++++----------------
>  2 files changed, 49 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 55fcea0249e6..4989d330fada 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -647,8 +647,28 @@ struct readahead_control {
>  /* private: use the readahead_* accessors instead */
>  	pgoff_t _index;
>  	unsigned int _nr_pages;
> +	unsigned int _batch_count;
>  };
>  
> +static inline struct page *readahead_page(struct readahead_control *rac)
> +{
> +	struct page *page;
> +
> +	BUG_ON(rac->_batch_count > rac->_nr_pages);
> +	rac->_nr_pages -= rac->_batch_count;
> +	rac->_index += rac->_batch_count;
> +	rac->_batch_count = 0;


Is it intentional, to set rac->_batch_count twice (here, and below)? The
only reason I can see is if a caller needs to use ->_batch_count in the
"return NULL" case, which doesn't seem to come up...


> +
> +	if (!rac->_nr_pages)
> +		return NULL;
> +
> +	page = xa_load(&rac->mapping->i_pages, rac->_index);
> +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> +	rac->_batch_count = hpage_nr_pages(page);
> +
> +	return page;
> +}
> +
>  /* The number of pages in this readahead block */
>  static inline unsigned int readahead_count(struct readahead_control *rac)
>  {
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 83df5c061d33..aaa209559ba2 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -113,15 +113,14 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
>  
>  EXPORT_SYMBOL(read_cache_pages);
>  
> -static void read_pages(struct readahead_control *rac, struct list_head *pages,
> -		gfp_t gfp)
> +static void read_pages(struct readahead_control *rac, struct list_head *pages)
>  {
>  	const struct address_space_operations *aops = rac->mapping->a_ops;
> +	struct page *page;
>  	struct blk_plug plug;
> -	unsigned page_idx;
>  
>  	if (!readahead_count(rac))
> -		return;
> +		goto out;
>  
>  	blk_start_plug(&plug);
>  
> @@ -130,23 +129,23 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
>  				readahead_count(rac));
>  		/* Clean up the remaining pages */
>  		put_pages_list(pages);
> -		goto out;
> -	}
> -
> -	for (page_idx = 0; page_idx < readahead_count(rac); page_idx++) {
> -		struct page *page = lru_to_page(pages);
> -		list_del(&page->lru);
> -		if (!add_to_page_cache_lru(page, rac->mapping, page->index,
> -				gfp))
> +		rac->_index += rac->_nr_pages;
> +		rac->_nr_pages = 0;
> +	} else {
> +		while ((page = readahead_page(rac))) {
>  			aops->readpage(rac->file, page);
> -		put_page(page);
> +			put_page(page);
> +		}
>  	}
>  
> -out:
>  	blk_finish_plug(&plug);
>  
>  	BUG_ON(!list_empty(pages));
> -	rac->_nr_pages = 0;
> +	BUG_ON(readahead_count(rac));
> +
> +out:
> +	/* If we were called due to a conflicting page, skip over it */


Tiny documentation nit: What if we were *not* called due to a conflicting page? 
(And what is a "conflicting page", in this context, btw?) The next line unconditionally 
moves the index ahead, so the "if" part of the comment really confuses me.


> +	rac->_index++;
>  }
>  
>  /*
> @@ -165,9 +164,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  	LIST_HEAD(page_pool);
>  	loff_t isize = i_size_read(inode);
>  	gfp_t gfp_mask = readahead_gfp_mask(mapping);
> +	bool use_list = mapping->a_ops->readpages;
>  	struct readahead_control rac = {
>  		.mapping = mapping,
>  		.file = filp,
> +		._index = index,
>  		._nr_pages = 0,
>  	};
>  	unsigned long i;
> @@ -184,6 +185,8 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  		if (index + i > end_index)
>  			break;
>  
> +		BUG_ON(index + i != rac._index + rac._nr_pages);
> +
>  		page = xa_load(&mapping->i_pages, index + i);
>  		if (page && !xa_is_value(page)) {
>  			/*
> @@ -191,15 +194,22 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  			 * contiguous pages before continuing with the next
>  			 * batch.
>  			 */
> -			read_pages(&rac, &page_pool, gfp_mask);
> +			read_pages(&rac, &page_pool);
>  			continue;
>  		}
>  
>  		page = __page_cache_alloc(gfp_mask);
>  		if (!page)
>  			break;
> -		page->index = index + i;
> -		list_add(&page->lru, &page_pool);
> +		if (use_list) {
> +			page->index = index + i;
> +			list_add(&page->lru, &page_pool);
> +		} else if (add_to_page_cache_lru(page, mapping, index + i,
> +					gfp_mask) < 0) {


I still think you'll want to compare against !=0, rather than < 0, here.


> +			put_page(page);
> +			read_pages(&rac, &page_pool);


Doing a read_pages() in the error case is because...actually, I'm not sure yet.
Why do we do this? Effectively it's a retry?


> +			continue;
> +		}
>  		if (i == nr_to_read - lookahead_size)
>  			SetPageReadahead(page);
>  		rac._nr_pages++;
> @@ -210,7 +220,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
>  	 * uptodate then the caller will launch readpage again, and
>  	 * will then handle the error.
>  	 */
> -	read_pages(&rac, &page_pool, gfp_mask);
> +	read_pages(&rac, &page_pool);
>  }
>  
>  /*
> 

Didn't spot any actual errors, just mainly my own questions here. :)


thanks,
-- 
John Hubbard
NVIDIA