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

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Message-ID: <61212ffd-3505-04d7-5403-932a10d676e7@quicinc.com>
Date:   Wed, 12 Jan 2022 13:51:55 +0530
From:   Charan Teja Kalla <quic_charante@...cinc.com>
To:     Mark Hemment <markhemm@...glemail.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

Hello Mark,

On 1/10/2022 3:51 PM, Charan Teja Kalla 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();
>>> +       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.
> Agree here. I should have the better test case to verify my changes.
> 
>> Also, not necessary to re-call xas_pause() before cond_resched (it is
>> a no-op).
> In the event when CONFIG_DEBUG_ATOMIC_SLEEP is enabled users may still
> need to call the xas_pause(), as we are dropping the rcu lock. NO?
> 
> static inline void cond_resched_rcu(void)
> {
> #if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU)
>         rcu_read_unlock();
>         cond_resched();
>         rcu_read_lock();
> #endif
> }
> 
>> Would be better to check need_resched() before
>> rcu_read_lock().
> Okay, I can directly use cond_resched() if used 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).
> Even the xarray documentation says that: If most entries found during a
> walk require you to call xas_pause(), the xa_for_each() iterator may be
> more appropriate.
> 
> Since every value entry found in the xarray requires me to do the
> xas_pause(), I do agree that xa_for_each() is the appropriate call here.
> Will switch to this in the next spin. Waiting for further review
> comments on this patch.

I also found the below documentation:
xa_for_each() will spin if it hits a retry entry; if you intend to see
retry entries, you should use the xas_for_each() iterator instead.

Since retry entries are expected, I should be using the xas_for_each()
with the corrections you had pointed out. Isn't it?

>