linux-kernel - Re: [PATCH] MM: discard __GFP

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Message-id: <163727727803.13692.15470049610672496362@noble.neil.brown.name>
Date:   Fri, 19 Nov 2021 10:14:38 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Matthew Wilcox" <willy@...radead.org>
Cc:     "Andrew Morton" <akpm@...ux-foundation.org>,
        "Michal Hocko" <mhocko@...e.com>,
        "Thierry Reding" <thierry.reding@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MM: discard __GFP_ATOMIC

On Thu, 18 Nov 2021, Matthew Wilcox wrote:
> On Wed, Nov 17, 2021 at 03:39:30PM +1100, NeilBrown wrote:
> > +++ b/drivers/iommu/tegra-smmu.c
> > @@ -676,12 +676,12 @@ static struct page *as_get_pde_page(struct tegra_smmu_as *as,
> >  	 * allocate page in a sleeping context if GFP flags permit. Hence
> >  	 * spinlock needs to be unlocked and re-locked after allocation.
> >  	 */
> > -	if (!(gfp & __GFP_ATOMIC))
> > +	if (gfp & __GFP_DIRECT_RECLAIM)
> >  		spin_unlock_irqrestore(&as->lock, *flags);
> >  
> >  	page = alloc_page(gfp | __GFP_DMA | __GFP_ZERO);
> >  
> > -	if (!(gfp & __GFP_ATOMIC))
> > +	if (gfp & __GFP_DIRECT_RECLAIM)
> >  		spin_lock_irqsave(&as->lock, *flags);
> >  
> >  	/*
> 
> Surely this should be gfpflags_allow_blocking() instead of poking about
> in the innards of gfp flags?

Possibly.  Didn't know about gfpflags_allow_blocking().  From a quick
grep in the kernel, a whole lot of other people don't know about it
either, though clearly some do.

Maybe we should reaname "__GFP_DIRECT_RECLAIM" to
"__GFP_ALLOW_BLOCKING", because that is what most users seems to care
about.

If not, then we probably want a gfpflags_without_block() function that
removes that flag, as lots of code wants to do that - and using the flag
for one, and an inline for the other is not consistent.

My leaning would be to __GFP_ALLOW_BLOCKING

NeilBrown


> 
> This patch seems like a good simplification to me.
> 
> Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>

Thanks,
NeilBrown