linux-kernel - Re: [PATCH v2] mm: zswap: handle incorrect attempts to load of large folios

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Message-ID: <2821278f-bc94-c147-d0fe-8cc52dbdccb1@redhat.com>
Date: Sat, 8 Jun 2024 07:08:35 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Yosry Ahmed <yosryahmed@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>,
 Chengming Zhou <chengming.zhou@...ux.dev>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Barry Song <21cnbao@...il.com>,
 Chris Li <chrisl@...nel.org>, Ryan Roberts <ryan.roberts@....com>,
 David Hildenbrand <david@...hat.com>, Matthew Wilcox <willy@...radead.org>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: zswap: handle incorrect attempts to load of large
 folios

On 6/8/24 05:36, Yosry Ahmed wrote:
> diff --git a/mm/zswap.c b/mm/zswap.c
> index b9b35ef86d9be..ebb878d3e7865 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1557,6 +1557,26 @@ bool zswap_load(struct folio *folio)
>  
>  	VM_WARN_ON_ONCE(!folio_test_locked(folio));
>  
> +	/*
> +	 * Large folios should not be swapped in while zswap is being used, as
> +	 * they are not properly handled. Zswap does not properly load large
> +	 * folios, and a large folio may only be partially in zswap.
> +	 *
> +	 * If any of the subpages are in zswap, reading from disk would result
> +	 * in data corruption, so return true without marking the folio uptodate
> +	 * so that an IO error is emitted (e.g. do_swap_page() will sigfault).
> +	 *
> +	 * Otherwise, return false and read the folio from disk.
> +	 */
> +	if (folio_test_large(folio)) {
> +		if (xa_find(tree, &offset,
> +			    offset + folio_nr_pages(folio) - 1, XA_PRESENT)) {
> +			WARN_ON_ONCE(1);
> +			return true;
> +		}

How does that work? Should it be xa_find_after() to not always find
current entry?

And does it still mean those subsequent entries map to same folio?


--Mika