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Date:   Tue, 29 Dec 2020 22:23:11 +0000
From:   Matthew Wilcox <>
To:     Randy Dunlap <>
        Toralf Förster <>,
        Andrew Morton <>,
        Jens Axboe <>,
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:
> Follow-up report:
> 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

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