linux-kernel - Re: [PATCH 2/6] mm/page_io: use a folio in sio_read

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Message-ID: <ZLVEyG7SZMzgY7ov@casper.infradead.org>
Date:   Mon, 17 Jul 2023 14:40:24 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Peng Zhang <zhangpeng362@...wei.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        sidhartha.kumar@...cle.com, akpm@...ux-foundation.org,
        wangkefeng.wang@...wei.com, sunnanyong@...wei.com,
        Kent Overstreet <kent.overstreet@...il.com>
Subject: Re: [PATCH 2/6] mm/page_io: use a folio in sio_read_complete()

On Mon, Jul 17, 2023 at 09:25:58PM +0800, Peng Zhang wrote:
> +++ b/mm/page_io.c
> @@ -406,19 +406,19 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
>  
>  	if (ret == sio->len) {
>  		for (p = 0; p < sio->pages; p++) {
> -			struct page *page = sio->bvec[p].bv_page;
> +			struct folio *folio = page_folio(sio->bvec[p].bv_page);
>  
> -			SetPageUptodate(page);
> -			unlock_page(page);
> +			folio_mark_uptodate(folio);
> +			folio_unlock(folio);
>  		}

I'm kind of shocked this works today.  Usually bvecs coalesce adjacent
pages into a single entry, so you need to use a real iterator like
bio_for_each_folio_all() to extract individual pages from a bvec.
Maybe the sio bvec is constructed inefficiently.

I think Kent had some bvec folio iterators in progress?

>  		count_vm_events(PSWPIN, sio->pages);
>  	} else {
>  		for (p = 0; p < sio->pages; p++) {
> -			struct page *page = sio->bvec[p].bv_page;
> +			struct folio *folio = page_folio(sio->bvec[p].bv_page);
>  
> -			SetPageError(page);
> -			ClearPageUptodate(page);
> -			unlock_page(page);
> +			folio_set_error(folio);
> +			folio_clear_uptodate(folio);
> +			folio_unlock(folio);

Similar questions to the last patch -- who checks the error flag on this
page/folio, and isn't the folio already !uptodate?

>  		}
>  		pr_alert_ratelimited("Read-error on swap-device\n");
>  	}
> -- 
> 2.25.1
>