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Date:   Tue, 29 Dec 2020 22:23:11 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Toralf Förster <toralf.foerster@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org
Subject: Re: [PATCH -mm] mm: readahead: apply a default readahead size

On Tue, Dec 29, 2020 at 01:26:34PM -0800, Randy Dunlap wrote:
> UBSAN reports an invalid shift size:
> 
> mr-fox kernel: UBSAN: shift-out-of-bounds in ./include/linux/log2.h:57:13
> mr-fox kernel: shift exponent 64 is too large for 64-bit type 'long unsigned int'
> 
> Original report:
> https://lore.kernel.org/lkml/c6e5eb81-680f-dd5c-8a81-62041a5ce50c@gmx.de/
> 
> Follow-up report:
> https://lore.kernel.org/lkml/0c283ea9-b446-0e40-6dc8-e9585ae171b4@gmx.de/T/#m9b604660925f9e8a544f7453130c31d083c1e5bb
> 
> 
> Willy suggested that get_init_ra_size() was being called with a size of 0,
> which would cause this (instead of some Huge value), so add a check in
> that function for size == 0, and if 0, default it to 32 (pages).

No, this is wrong.  'size' in this case is the size of the read.
And it's zero.  Is this fixed by commit
3644e2d2dda78e21edd8f5415b6d7ab03f5f54f3

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