linux-kernel - Re: [PATCH v2 06/28] gup: Fix some contiguous memmap assumptions

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Message-ID: <YdvuhjweiC1SjVF4@infradead.org>
Date:   Mon, 10 Jan 2022 00:29:58 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        William Kucharski <william.kucharski@...cle.com>,
        linux-kernel@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH v2 06/28] gup: Fix some contiguous memmap assumptions

On Mon, Jan 10, 2022 at 04:23:44AM +0000, Matthew Wilcox (Oracle) wrote:
> Several functions in gup.c assume that a compound page has virtually
> contiguous page structs.  This isn't true for SPARSEMEM configs unless
> SPARSEMEM_VMEMMAP is also set.  Fix them by using nth_page() instead of
> plain pointer arithmetic.

So is this an actualy bug that need a Fixes tag, or do all architectures
that support THP and sparsemem use SPARSEMEM_VMEMMAP?

> +	page = nth_page(head, (addr & (sz-1)) >> PAGE_SHIFT);

Would be nice to fix the indeation for sz - 1 while you're at it.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@....de>