linux-kernel - Re: [PATCH v3 RESEND] mm: shmem: implement POSIX_FADV

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Date:   Fri, 7 Jan 2022 12:10:49 +0000
From:   Mark Hemment <markhemm@...glemail.com>
To:     Charan Teja Reddy <quic_charante@...cinc.com>
Cc:     hughd@...gle.com, Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>, vbabka@...e.cz,
        rientjes@...gle.com, mhocko@...e.com, surenb@...gle.com,
        shakeelb@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Charan Teja Reddy <charante@...eaurora.org>
Subject: Re: [PATCH v3 RESEND] mm: shmem: implement POSIX_FADV_[WILL|DONT]NEED
 for shmem

On Thu, 6 Jan 2022 at 17:06, Charan Teja Reddy
<quic_charante@...cinc.com> wrote:
>
> From: Charan Teja Reddy <charante@...eaurora.org>
>
> Currently fadvise(2) is supported only for the files that doesn't
> associated with noop_backing_dev_info thus for the files, like shmem,
> fadvise results into NOP. But then there is file_operations->fadvise()
> that lets the file systems to implement their own fadvise
> implementation. Use this support to implement some of the POSIX_FADV_XXX
> functionality for shmem files.
>
> [snip]

> +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();
> +       xas_for_each(&xas, page, end) {
> +               if (!xa_is_value(page))
> +                       continue;
> +               xas_pause(&xas);
> +               rcu_read_unlock();
> +
> +               page = shmem_read_mapping_page(mapping, xas.xa_index);
> +               if (!IS_ERR(page))
> +                       put_page(page);
> +
> +               rcu_read_lock();
> +               if (need_resched()) {
> +                       xas_pause(&xas);
> +                       cond_resched_rcu();
> +               }
> +       }
> +       rcu_read_unlock();
> +
> +       return 0;

I have a doubt on referencing xa_index after calling xas_pause().
xas_pause() walks xa_index forward, so will not be the value expected
for the current page.
Also, not necessary to re-call xas_pause() before cond_resched (it is
a no-op).  Would be better to check need_resched() before
rcu_read_lock().

As this loop may call xas_pause() for most iterations, should consider
using xa_for_each() instead (I *think* - still getting up to speed
with XArray).

Mark