linux-kernel - Re: [PATCH 2/3] mm,thp,shm: limit gfp mask to no more than specified

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Message-ID: <0bf4037559563bd51f6ac68d659e9f886387fa7e.camel@surriel.com>
Date:   Mon, 30 Nov 2020 09:40:01 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     hughd@...gle.com, xuyu@...ux.alibaba.com,
        akpm@...ux-foundation.org, mgorman@...e.de, aarcange@...hat.com,
        willy@...radead.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, linux-mm@...ck.org, vbabka@...e.cz
Subject: Re: [PATCH 2/3] mm,thp,shm: limit gfp mask to no more than specified

On Mon, 2020-11-30 at 11:00 +0100, Michal Hocko wrote:
> On Fri 27-11-20 14:03:39, Rik van Riel wrote:
> > On Fri, 2020-11-27 at 08:52 +0100, Michal Hocko wrote:
> > > On Thu 26-11-20 13:04:14, Rik van Riel wrote:
> > > > I would be more than happy to implement things differently,
> > > > but I am not sure what alternative you are suggesting.
> > > 
> > > Simply do not alter gfp flags? Or warn in some cases of a serious
> > > mismatch.
> > > E.g. GFP_ZONEMASK mismatch because there are already GFP_KERNEL
> > > users
> > > of
> > > shmem.
> > 
> > Not altering the gfp flags is not really an option,
> > because that would leads to attempting to allocate THPs
> > with GFP_HIGHUSER, which is what is used to allocate
> > regular tmpfs pages.
> 
> Right but that is a completely different reason to alter the mask and
> it
> would be really great to know whether this is a theoretical concern
> or
> those users simply do not ever use THPs. Btw. should they be using
> THPs
> even if they opt themselves into GFP_KERNEL restriction?

I suppose disabling THPs completely if the gfp_mask
passed to shmem_getpage_gfp() is not GFP_HIGHUSER
is another option.

That seems like it might come with its own pitfalls,
though, both functionality wise and maintenance wise.

Does anyone have
strong feelings between "limit gfp_mask"
and "disable THP for !GFP_HIGHUSER"?

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