Subject: Re: [PATCH v3 RESEND] mm: shmem: implement POSIX_FADV_[WILL|DONT]NEED
 for shmem
To:     Matthew Wilcox <willy@infradead.org>
CC:     Mark Hemment <markhemm@googlemail.com>, <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, <vbabka@suse.cz>,
        <rientjes@google.com>, <mhocko@suse.com>, <surenb@google.com>,
        <shakeelb@google.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>,
        Charan Teja Reddy <charante@codeaurora.org>
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 <61212ffd-3505-04d7-5403-932a10d676e7@quicinc.com>
 <Yd7VZId4IlKd4VpC@casper.infradead.org>
From:   Charan Teja Kalla <quic_charante@quicinc.com>
Message-ID: <eb056201-a6e2-a58e-dd77-666f124d1896@quicinc.com>
Date:   Wed, 12 Jan 2022 19:05:47 +0530
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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.

>