linux-kernel - Re: [PATCH 08/11] mm: convert vma_interval

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Message-ID: <20191003204108.GF32665@bombadil.infradead.org>
Date:   Thu, 3 Oct 2019 13:41:08 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     akpm@...ux-foundation.org, walken@...gle.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        dri-devel@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 08/11] mm: convert vma_interval_tree to half closed
 intervals

On Thu, Oct 03, 2019 at 01:18:55PM -0700, Davidlohr Bueso wrote:
> +++ b/mm/nommu.c
> @@ -1793,7 +1793,7 @@ int nommu_shrink_inode_mappings(struct inode *inode, size_t size,
>  	size_t r_size, r_top;
>  
>  	low = newsize >> PAGE_SHIFT;
> -	high = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
> +	high = (size + PAGE_SIZE) >> PAGE_SHIFT;

Uhh ... are you sure about this?  size is in bytes here, and we're rounding
up to the next page size.  So if size is [1-4096], then we add on 4095 and get
the answer 1.  With your patch, if size is [0-4095], we get the answer 1.
I think you meant:

	high = ((size + PAGE_SIZE - 1) >> PAGE_SHIFT) + 1;