linux-kernel - Re: [PATCH v7 19/20] fs/dax: Properly refcount fs dax pages

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Message-ID: <6a6773ae-a21a-437c-be6f-d8e53267e534@redhat.com>
Date: Mon, 10 Feb 2025 20:01:28 +0100
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org,
 dan.j.williams@...el.com, linux-mm@...ck.org
Cc: Alison Schofield <alison.schofield@...el.com>, lina@...hilina.net,
 zhang.lyra@...il.com, gerald.schaefer@...ux.ibm.com,
 vishal.l.verma@...el.com, dave.jiang@...el.com, logang@...tatee.com,
 bhelgaas@...gle.com, jack@...e.cz, jgg@...pe.ca, catalin.marinas@....com,
 will@...nel.org, mpe@...erman.id.au, npiggin@...il.com,
 dave.hansen@...ux.intel.com, ira.weiny@...el.com, willy@...radead.org,
 djwong@...nel.org, tytso@....edu, linmiaohe@...wei.com, peterx@...hat.com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
 nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
 linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
 david@...morbit.com, chenhuacai@...nel.org, kernel@...0n.name,
 loongarch@...ts.linux.dev
Subject: Re: [PATCH v7 19/20] fs/dax: Properly refcount fs dax pages


>   
> -static inline unsigned long dax_page_share_put(struct page *page)
> +static inline unsigned long dax_folio_put(struct folio *folio)
>   {
> -	WARN_ON_ONCE(!page->share);
> -	return --page->share;
> +	unsigned long ref;
> +
> +	if (!dax_folio_is_shared(folio))
> +		ref = 0;
> +	else
> +		ref = --folio->share;
> +
> +	WARN_ON_ONCE(ref < 0);
> +	if (!ref) {
> +		folio->mapping = NULL;
> +		if (folio_order(folio)) {


I'd have made this easier to read by doing

if (ref)
	return ref;
folio->mapping = NULL;

order = folio_order(folio)
if (!order)
	return 0;

pgmap = page_pgmap(&folio->page);
for (i = 0; i < (1UL << order); i++) {
	// loop body see below
}
return 0;


In the context of similar split users and related discussions in the 
future with memdescs -- see 89a41a0263293856678189981e5407375261c4ff -- 
I would further do within the loop (avoiding messing with page-> 
completely):

...
	struct page *page = folio_page(folio, i);
	/* Careful: see __split_huge_page_tail() */
	struct folio *new_folio = (struct folio *)page;

	ClearPageHead(page);
	clear_compound_head(page);

	new_folio->mapping = NULL;	
	/*
	 * Reset pgmap which was over-written by
	 * prep_compound_page().
	 */
	new_folio->pgmap = pgmap;
	new_folio->share = 0;
	WARN_ON_ONCE(folio_ref_count(new_folio));
...


I scanned over the other stuff in here, but I'm not an expert on the 
pfn_t etc thingies.

-- 
Cheers,

David / dhildenb