linux-kernel - Re: [PATCH v2 2/6] shmem: return freed pages in shmem_free

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Message-ID: <ZQm2hIj7W1kJ8C7n@casper.infradead.org>
Date:   Tue, 19 Sep 2023 15:56:04 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Daniel Gomez <da.gomez@...sung.com>
Cc:     "minchan@...nel.org" <minchan@...nel.org>,
        "senozhatsky@...omium.org" <senozhatsky@...omium.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "gost.dev@...sung.com" <gost.dev@...sung.com>,
        Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v2 2/6] shmem: return freed pages in shmem_free_swap

On Tue, Sep 19, 2023 at 01:55:47PM +0000, Daniel Gomez wrote:
> +++ b/mm/shmem.c
> @@ -846,16 +846,18 @@ static void shmem_delete_from_page_cache(struct folio *folio, void *radswap)
>  /*
>   * Remove swap entry from page cache, free the swap and its page cache.
>   */
> -static int shmem_free_swap(struct address_space *mapping,
> +static long shmem_free_swap(struct address_space *mapping,
>  			   pgoff_t index, void *radswap)
>  {
>  	void *old;
>  
>  	old = xa_cmpxchg_irq(&mapping->i_pages, index, radswap, NULL, 0);
>  	if (old != radswap)
> -		return -ENOENT;
> +		return 0;
> +
>  	free_swap_and_cache(radix_to_swp_entry(radswap));
> -	return 0;
> +
> +	return folio_nr_pages((struct folio *)radswap);
>  }

Oh my goodness.  I have led you astray; my apologies.

shmem_free_swap() is called when the 'folio' is NOT actually a folio.
It's an 'exceptional' / 'value' entry.  We can't do this.

Do we encode the size of the swap entry in the swp_entry_t or do
we have to get that information from the XArray (which no longer
knows it after we've stored a NULL there)?