linux-kernel - Re: [PATCH v2] mm: shmem: implement POSIX_FADV

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Date:   Thu, 2 Dec 2021 13:27:05 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Charan Teja Reddy <quic_charante@...cinc.com>
Cc:     hughd@...gle.com, akpm@...ux-foundation.org, vbabka@...e.cz,
        rientjes@...gle.com, david@...hat.com, mhocko@...e.com,
        surenb@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Charan Teja Reddy <charante@...eaurora.org>
Subject: Re: [PATCH v2] mm: shmem: implement POSIX_FADV_[WILL|DONT]NEED for
 shmem

On Thu, Dec 02, 2021 at 04:20:53PM +0530, Charan Teja Reddy wrote:
> +static int shmem_fadvise_willneed(struct address_space *mapping,
> +				 pgoff_t start, pgoff_t long end)
> +{
> +	XA_STATE(xas, &mapping->i_pages, start);
> +	struct page *page;
> +
> +	rcu_read_lock();
> +	page = xas_find(&xas, end);
> +	rcu_read_unlock();
> +
> +	while (page) {
> +		if (xa_is_value(page)) {
> +			page = shmem_read_mapping_page(mapping, xas.xa_index);
> +			if (!IS_ERR(page))
> +				put_page(page);
> +		}
> +
> +		if (need_resched()) {
> +			xas_pause(&xas);
> +			cond_resched();
> +		}
> +
> +		rcu_read_lock();
> +		page = xas_next_entry(&xas, end);
> +		rcu_read_unlock();
> +	}
> +
> +	return 0;
> +}

What part of the XArray documentation led you to believe that this is a
safe thing to do?  Because it needs to be rewritten immediately!