linux-kernel - Re: [PATCH] mm,thp,shmem: limit shmem THP alloc gfp

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Message-ID: <20201023064736.GQ23790@dhcp22.suse.cz>
Date:   Fri, 23 Oct 2020 08:47:36 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Rik van Riel <riel@...riel.com>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Mel Gorman <mgorman@...e.de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm,thp,shmem: limit shmem THP alloc gfp_mask

On Thu 22-10-20 12:06:01, Rik van Riel wrote:
> On Thu, 2020-10-22 at 17:50 +0200, Michal Hocko wrote:
> > On Thu 22-10-20 09:25:21, Rik van Riel wrote:
> > > On Thu, 2020-10-22 at 10:15 +0200, Michal Hocko wrote:
> > > > On Wed 21-10-20 23:48:46, Rik van Riel wrote:
> > > > > 
> > > > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > > > index 537c137698f8..d1290eb508e5 100644
> > > > > --- a/mm/shmem.c
> > > > > +++ b/mm/shmem.c
> > > > > @@ -1545,8 +1545,11 @@ static struct page
> > > > > *shmem_alloc_hugepage(gfp_t gfp,
> > > > >  		return NULL;
> > > > >  
> > > > >  	shmem_pseudo_vma_init(&pvma, info, hindex);
> > > > > -	page = alloc_pages_vma(gfp | __GFP_COMP | __GFP_NORETRY
> > > > > |
> > > > > __GFP_NOWARN,
> > > > > -			HPAGE_PMD_ORDER, &pvma, 0,
> > > > > numa_node_id(),
> > > > > true);
> > > > > +	/* Limit the gfp mask according to THP configuration.
> > > > > */
> > > > > +	gfp |= __GFP_COMP | __GFP_NORETRY | __GFP_NOWARN;
> > > > 
> > > > What is the reason for these when alloc_hugepage_direct_gfpmask
> > > > provides
> > > > the full mask?
> > > 
> > > The mapping_gfp_mask for the shmem file might have additional
> > > restrictions above and beyond the gfp mask returned by
> > > alloc_hugepage_direct_gfpmask, and I am not sure we should just
> > > ignore the mapping_gfp_mask.
> > 
> > No, we shouldn't. But I do not see why you should be adding the above
> > set of flags on top.
> 
> Because THP allocations are higher order and optimistic,
> and we want them to:
> 1) be annotated as compound allocations
> 2) fail (and fall back to 4kB allocations) when they cannot
>    be easily satisfied, and
> 3) not create a spew of allocation failure backtraces on
>    the (serial) console when these THP allocations fail

This all is already returned from alloc_hugepage_direct_gfpmask.
-- 
Michal Hocko
SUSE Labs