linux-kernel - Re: [PATCH] mm: page_alloc: update the COMPACT[STALL|FAIL] events properly

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Message-ID: <8a03203b-e6dd-3a86-5b83-33d9c6adcc11@codeaurora.org>
Date:   Sat, 6 Feb 2021 07:27:04 +0530
From:   Charan Teja Kalla <charante@...eaurora.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz, mhocko@...e.com,
        vinmenon@...eaurora.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: page_alloc: update the COMPACT[STALL|FAIL] events
 properly

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.

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