linux-kernel - Re: [PATCH v8 1/4] mm/cma: Add PF flag to force non cma alloc

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Date:   Thu, 28 Feb 2019 15:34:05 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        akpm@...ux-foundation.org, Alexey Kardashevskiy <aik@...abs.ru>,
        David Gibson <david@...son.dropbear.id.au>,
        Andrea Arcangeli <aarcange@...hat.com>, mpe@...erman.id.au,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v8 1/4] mm/cma: Add PF flag to force non cma alloc

On Thu 28-02-19 13:20:03, Vlastimil Babka wrote:
> On 2/27/19 3:47 PM, Aneesh Kumar K.V wrote:
> > This patch adds PF_MEMALLOC_NOCMA which make sure any allocation in that context
> > is marked non-movable and hence cannot be satisfied by CMA region.
> > 
> > This is useful with get_user_pages_longterm where we want to take a page pin by
> > migrating pages from CMA region. Marking the section PF_MEMALLOC_NOCMA ensures
> > that we avoid unnecessary page migration later.
> > 
> > Suggested-by: Andrea Arcangeli <aarcange@...hat.com>
> > Reviewed-by: Andrea Arcangeli <aarcange@...hat.com>
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> 
> +CC scheduler guys
> 
> Do we really take the last available PF flag just so that "we avoid
> unnecessary page migration later"?
> If yes, that's a third PF_MEMALLOC flag, should we get separate variable
> for gfp context at this point?

Yes, that sounds like a reasonable thing to do. Just note that xfs still
uses current_{set,restore}* api to handle PF_MEMALLOC_NOFS so that would
have to be moved over to the memalloc_nofs_{save,restore} API.

-- 
Michal Hocko