linux-kernel - Re: [PATCH v9 19/20] fs/dax: Properly refcount fs dax pages

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Message-ID: <67e46be7e0a25_152c29414@dwillia2-mobl3.amr.corp.intel.com.notmuch>
Date: Wed, 26 Mar 2025 17:04:39 -0400
From: Dan Williams <dan.j.williams@...el.com>
To: David Hildenbrand <david@...hat.com>, Alistair Popple
	<apopple@...dia.com>, <akpm@...ux-foundation.org>,
	<dan.j.williams@...el.com>, <linux-mm@...ck.org>
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	<zhang.lyra@...il.com>, <gerald.schaefer@...ux.ibm.com>,
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Subject: Re: [PATCH v9 19/20] fs/dax: Properly refcount fs dax pages

David Hildenbrand wrote:
> 
> > -static inline unsigned long dax_folio_share_put(struct folio *folio)
> > +static inline unsigned long dax_folio_put(struct folio *folio)
> >   {
> > -	return --folio->page.share;
> > +	unsigned long ref;
> > +	int order, i;
> > +
> > +	if (!dax_folio_is_shared(folio))
> > +		ref = 0;
> > +	else
> > +		ref = --folio->share;
> > +
> 
> It would still be good to learn how this non-atomic update here is safe 
> (@Dan?), but that's independent of this series.

Apologies, I missed this question earlier.

All these manipulations are happening under xas_lock_irq() for @entry
where each @entry is 1:1 correlated with a folio. So concurrent attempts
to associate/disassociate a reflinked block in a file should synchronize
there.

> Staring at it, I would have thought we have to us an atomic_t here.
> 
> Acked-by: David Hildenbrand <david@...hat.com>

Thanks for taking a look!