linux-kernel - Re: [PATCH 3/3] s390: supplement for ptdesc conversion

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Date: Tue, 26 Mar 2024 12:48:09 -0700
From: Vishal Moola <vishal.moola@...il.com>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
	rppt@...nel.org, willy@...radead.org, muchun.song@...ux.dev,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Janosch Frank <frankja@...ux.ibm.com>,
	Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org,
	linux-s390@...r.kernel.org
Subject: Re: [PATCH 3/3] s390: supplement for ptdesc conversion

On Mon, Mar 04, 2024 at 07:07:20PM +0800, Qi Zheng wrote:
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -206,9 +206,11 @@ static void gmap_free(struct gmap *gmap)
>  
>  	/* Free additional data for a shadow gmap */
>  	if (gmap_is_shadow(gmap)) {
> +		struct ptdesc *ptdesc;
> +
>  		/* Free all page tables. */
> -		list_for_each_entry_safe(page, next, &gmap->pt_list, lru)
> -			page_table_free_pgste(page);
> +		list_for_each_entry_safe(ptdesc, next, &gmap->pt_list, pt_list)
> +			page_table_free_pgste(ptdesc);

An important note: ptdesc allocation/freeing is different than the
standard alloc_pages()/free_pages() routines architectures are used to.
Are we sure we don't have memory leaks here?

We always allocate and free ptdescs as compound pages; for page table
struct pages, most archictectures do not. s390 has CRST_ALLOC_ORDER
pagetables, meaning if we free anything using the ptdesc api, we better
be sure it was allocated using the ptdesc api as well.

Like you, I don't have a s390 to test on, so hopefully some s390 expert
can chime in to let us know if we need a fix for this.