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

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Date:   Wed, 12 Jan 2022 19:05:47 +0530
From:   Charan Teja Kalla <quic_charante@...cinc.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Mark Hemment <markhemm@...glemail.com>, <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.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

Thanks Matthew for the review.

On 1/12/2022 6:49 PM, Matthew Wilcox wrote:
> On Wed, Jan 12, 2022 at 01:51:55PM +0530, Charan Teja Kalla wrote:
>>>>> +       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();
>>> 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.
> 
> Yes.  This should obviously be an xa_for_each() loop.
> 

ACK.

>>> 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?
> 
> No.  You aren't handling retry entries at all; you clearly don't
> expect to see them.  Just let the XArray code handle them itself.

ACK.

>