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Date:   Thu, 18 Feb 2021 12:15:03 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Subject: Re: page->index limitation on 32bit system?
Message-ID: <20210218121503.GQ2858050@casper.infradead.org>
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On Thu, Feb 18, 2021 at 04:54:46PM +0800, Qu Wenruo wrote:
> Recently we got a strange bug report that, one 32bit systems like armv6
> or non-64bit x86, certain large btrfs can't be mounted.
> 
> It turns out that, since page->index is just unsigned long, and on 32bit
> systemts, that can just be 32bit.
> 
> And when filesystems is utilizing any page offset over 4T, page->index
> get truncated, causing various problems.

4TB?  I think you mean 16TB (4kB * 4GB)

Yes, this is a known limitation.  Some vendors have gone to the trouble
of introducing a new page_index_t.  I'm not convinced this is a problem
worth solving.  There are very few 32-bit systems with this much storage
on a single partition (everything should work fine if you take a 20TB
drive and partition it into two 10TB partitions).

As usual, the best solution is for people to stop buying 32-bit systems.