Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Sender: charante=codeaurora.org@mg.codeaurora.org
DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 9DFC5C433C6
Subject: Re: [PATCH] mm: page_alloc: update the COMPACT[STALL|FAIL] events
 properly
To:     David Rientjes <rientjes@google.com>
Cc:     akpm@linux-foundation.org, vbabka@suse.cz, mhocko@suse.com,
        vinmenon@codeaurora.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <1612187338-19100-1-git-send-email-charante@codeaurora.org>
 <d9d1dd7b-1fe7-67b6-6ba4-fb1a6faa8fae@google.com>
 <160ba3b5-2cd4-5ff0-1348-fb477cefd33d@codeaurora.org>
 <1213f4c6-7557-268d-253e-23f8fea55b19@google.com>
From:   Charan Teja Kalla <charante@codeaurora.org>
Message-ID: <8a03203b-e6dd-3a86-5b83-33d9c6adcc11@codeaurora.org>
Date:   Sat, 6 Feb 2021 07:27:04 +0530
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101
 Thunderbird/78.7.0
MIME-Version: 1.0
In-Reply-To: <1213f4c6-7557-268d-253e-23f8fea55b19@google.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 2/6/2021 3:58 AM, David Rientjes wrote:
>> In the code, when COMPACT_SKIPPED is being returned, the page will
>> always be NULL. So, I'm not sure how much useful it is for the page ==
>> NULL check here. Or I failed to understand your point here?
>>
> Your code is short-circuiting the rest of  __alloc_pages_direct_compact() 
> where the return value is dictated by whether page is NULL or non-NULL.  
> We can't leak a captured page if we are testing for it being NULL or 
> non-NULL, which is what the rest of __alloc_pages_direct_compact() does 
> *before* your change.  So the idea was to add a check the page is actually 
> NULL here since you are now relying on the return value of 
> compact_zone_order() to be COMPACT_SKIPPED to infer page == NULL.
> 
> I agree that's currently true in the code, I was trying to catch any 
> errors where current->capture_control.page was non-NULL but 
> try_to_compact_pages() returns COMPACT_SKIPPED.  There's some complexity 
> here.

Thanks for the detailed explanation. This looks fine to me. I will send
V2 with this information in the commit log.

-- 
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
Forum, a Linux Foundation Collaborative Project