linux-kernel - Re: [RFC PATCH V2 2/4] mm: Add get_user_pages_cma

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Message-Id: <cbcae00a-e42c-0279-1ccb-9192e486abf1@linux.ibm.com>
Date:   Thu, 6 Sep 2018 18:53:55 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     akpm@...ux-foundation.org, Alexey Kardashevskiy <aik@...abs.ru>,
        mpe@...erman.id.au, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH V2 2/4] mm: Add get_user_pages_cma_migrate

On 09/06/2018 06:15 PM, Michal Hocko wrote:
> On Thu 06-09-18 11:13:40, Aneesh Kumar K.V wrote:
>> This helper does a get_user_pages_fast and if it find pages in the CMA area
>> it will try to migrate them before taking page reference. This makes sure that
>> we don't keep non-movable pages (due to page reference count) in the CMA area.
>> Not able to move pages out of CMA area result in CMA allocation failures.
> 
> Again, there is no user so it is hard to guess the intention completely.
> There is no documentation to describe the expected context and
> assumptions about locking etc.
> 

patch 4 is the user for the new helper. I will add the documentation 
update.

> As noted in the previous email. You should better describe why you are
> bypassing hugetlb pools. I assume that the reason is to guarantee a
> forward progress because those might be sitting in the CMA pools
> already, right?
> 

The reason for that is explained in the code

+		struct hstate *h = page_hstate(page);
+		/*
+		 * We don't want to dequeue from the pool because pool pages will
+		 * mostly be from the CMA region.
+		 */
+		return alloc_migrate_huge_page(h, gfp_mask, nid, NULL);

-aneesh