linux-kernel - Re: [PATCH 1/1] mm/page_alloc: add scheduling point to free_unref_page

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Date:   Wed, 9 Mar 2022 17:05:17 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     wangjianxing <wangjianxing@...ngson.cn>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/page_alloc: add scheduling point to
 free_unref_page_list

On Tue, 8 Mar 2022 16:19:33 +0000 Matthew Wilcox <willy@...radead.org> wrote:

> On Tue, Mar 01, 2022 at 08:38:25PM -0500, wangjianxing wrote:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3589febc6..1b96421c8 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3479,6 +3479,9 @@ void free_unref_page_list(struct list_head *list)
> >  		 */
> >  		if (++batch_count == SWAP_CLUSTER_MAX) {
> >  			local_unlock_irqrestore(&pagesets.lock, flags);
> > +
> > +			cond_resched();
> 
> This isn't safe.  This path can be called from interrupt context
> (otherwise we'd be using local_unlock_irq() instead of irqrestore()).

What a shame it is that we don't document our interfaces :(

I can't immediately find such callers, but I could imagine
put_pages_list() (which didn't document its interface this way either)
being called from IRQ.

And drivers/iommu/dma-iommu.c:fq_ring_free() calls put_pages_list()
from inside spin_lock().