linux-ext4 - Re: [PATCH 04/23] page-writeback: Convert write_cache_pages() to use filemap_get_folios

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Message-ID: <20221018210152.GH2703033@dread.disaster.area>
Date:   Wed, 19 Oct 2022 08:01:52 +1100
From:   Dave Chinner <david@...morbit.com>
To:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-afs@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        ceph-devel@...r.kernel.org, linux-cifs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        cluster-devel@...hat.com, linux-nilfs@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 04/23] page-writeback: Convert write_cache_pages() to use
 filemap_get_folios_tag()

On Thu, Sep 01, 2022 at 03:01:19PM -0700, Vishal Moola (Oracle) wrote:
> Converted function to use folios throughout. This is in preparation for
> the removal of find_get_pages_range_tag().
> 
> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@...il.com>
> ---
>  mm/page-writeback.c | 44 +++++++++++++++++++++++---------------------
>  1 file changed, 23 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 032a7bf8d259..087165357a5a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2285,15 +2285,15 @@ int write_cache_pages(struct address_space *mapping,
>  	int ret = 0;
>  	int done = 0;
>  	int error;
> -	struct pagevec pvec;
> -	int nr_pages;
> +	struct folio_batch fbatch;
> +	int nr_folios;
>  	pgoff_t index;
>  	pgoff_t end;		/* Inclusive */
>  	pgoff_t done_index;
>  	int range_whole = 0;
>  	xa_mark_t tag;
>  
> -	pagevec_init(&pvec);
> +	folio_batch_init(&fbatch);
>  	if (wbc->range_cyclic) {
>  		index = mapping->writeback_index; /* prev offset */
>  		end = -1;
> @@ -2313,17 +2313,18 @@ int write_cache_pages(struct address_space *mapping,
>  	while (!done && (index <= end)) {
>  		int i;
>  
> -		nr_pages = pagevec_lookup_range_tag(&pvec, mapping, &index, end,
> -				tag);
> -		if (nr_pages == 0)
> +		nr_folios = filemap_get_folios_tag(mapping, &index, end,
> +				tag, &fbatch);

This can find and return dirty multi-page folios if the filesystem
enables them in the mapping at instantiation time, right?

> +
> +		if (nr_folios == 0)
>  			break;
>  
> -		for (i = 0; i < nr_pages; i++) {
> -			struct page *page = pvec.pages[i];
> +		for (i = 0; i < nr_folios; i++) {
> +			struct folio *folio = fbatch.folios[i];
>  
> -			done_index = page->index;
> +			done_index = folio->index;
>  
> -			lock_page(page);
> +			folio_lock(folio);
>  
>  			/*
>  			 * Page truncated or invalidated. We can freely skip it
> @@ -2333,30 +2334,30 @@ int write_cache_pages(struct address_space *mapping,
>  			 * even if there is now a new, dirty page at the same
>  			 * pagecache address.
>  			 */
> -			if (unlikely(page->mapping != mapping)) {
> +			if (unlikely(folio->mapping != mapping)) {
>  continue_unlock:
> -				unlock_page(page);
> +				folio_unlock(folio);
>  				continue;
>  			}
>  
> -			if (!PageDirty(page)) {
> +			if (!folio_test_dirty(folio)) {
>  				/* someone wrote it for us */
>  				goto continue_unlock;
>  			}
>  
> -			if (PageWriteback(page)) {
> +			if (folio_test_writeback(folio)) {
>  				if (wbc->sync_mode != WB_SYNC_NONE)
> -					wait_on_page_writeback(page);
> +					folio_wait_writeback(folio);
>  				else
>  					goto continue_unlock;
>  			}
>  
> -			BUG_ON(PageWriteback(page));
> -			if (!clear_page_dirty_for_io(page))
> +			BUG_ON(folio_test_writeback(folio));
> +			if (!folio_clear_dirty_for_io(folio))
>  				goto continue_unlock;
>  
>  			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
> -			error = (*writepage)(page, wbc, data);
> +			error = writepage(&folio->page, wbc, data);

Yet, IIUC, this treats all folios as if they are single page folios.
i.e. it passes the head page of a multi-page folio to a callback
that will treat it as a single PAGE_SIZE page, because that's all
the writepage callbacks are currently expected to be passed...

So won't this break writeback of dirty multipage folios?

-Dave.
-- 
Dave Chinner
david@...morbit.com