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Date:   Mon, 3 Oct 2022 11:59:18 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Brian Geffon <bgeffon@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan@kernel.org>,
        Nitin Gupta <ngupta@vflare.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        linux-kernel@vger.kernel.org,
        Suleiman Souhlal <suleiman@google.com>,
        Rom Lemarchand <romlem@google.com>, linux-mm@kvack.org
Subject: Re: [PATCH] zram: Always expose rw_page
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 <20220930195215.2360317-1-bgeffon@google.com>
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On (22/09/30 15:52), Brian Geffon wrote:
> Currently zram will adjust its fops to a version which does not
> contain rw_page when a backing device has been assigned. This is
> done to prevent upper layers from assuming a synchronous operation
> when a page may have been written back. This forces every operation
> through bio which has overhead associated with bio_alloc/frees.
> 
> The code can be simplified to always expose a rw_page method and
> only in the rare event that a page is written back we instead will
> return -EOPNOTSUPP forcing the upper layer to fallback to bio.

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

> @@ -1267,6 +1253,9 @@ static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
>  		struct bio_vec bvec;
>  
>  		zram_slot_unlock(zram, index);
> +		/* If we don't have a bio we came via rw_page, we must fallback to bio */
> +		if (!bio)
> +			return -EOPNOTSUPP;

The comment is above 80 cols.