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Message-ID: <ZNFstXmztIriaKOX@casper.infradead.org>
Date: Mon, 7 Aug 2023 23:14:13 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Russell King <linux@...linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH] arm: dma-mapping: fix potential endless loop in
__dma_page_dev_to_cpu()
On Mon, Aug 07, 2023 at 05:26:57PM +0200, Marek Szyprowski wrote:
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index 70cb7e63a9a5..02250106e5ed 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -718,7 +718,7 @@ static void __dma_page_dev_to_cpu(struct page *page, unsigned long off,
> folio = folio_next(folio);
> }
>
> - while (left >= (ssize_t)folio_size(folio)) {
> + while (left && left >= (ssize_t)folio_size(folio)) {
> set_bit(PG_dcache_clean, &folio->flags);
> left -= folio_size(folio);
> folio = folio_next(folio);
I've been thinking about this and I think this is the right fix for the
wrong reason. I don't understand how it can produce the failure you
saw, but we shouldn't be calling folio_next() if left is zero, let alone
calling folio_size() on it. So I'd rather see this fix:
while (left >= (ssize_t)folio_size(folio)) {
set_bit(PG_dcache_clean, &folio->flags);
left -= folio_size(folio);
+ if (!left)
+ break;
folio = folio_next(folio);
}
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