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Subject: Re: [RFC PATCH 2/2] mm: readahead: handle LARGE input to
 get_init_ra_size()
To:     Randy Dunlap <rdunlap@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
        linux-mm@kvack.org
References: <20201220211051.1416-1-rdunlap@infradead.org>
 <20201222173533.c9e28416835d7487b0e28cda@linux-foundation.org>
 <6a595671-20a8-e63f-f3ea-f4749a574efa@infradead.org>
From:   =?UTF-8?Q?Toralf_F=c3=b6rster?= <toralf.foerster@gmx.de>
Message-ID: <d2edfb69-93b4-d938-faf0-5f7c0f1158b9@gmx.de>
Date:   Tue, 29 Dec 2020 19:01:55 +0100
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On 12/23/20 2:50 AM, Randy Dunlap wrote:
>> What motivates this change?  Is there any reason to think this can
>> happen?
> Spotted in the wild:
I run 2 hardened Gentoo systems, a server and a desktop.

I patched the server with this:

mr-fox ~ # cat ubsan.patch
=2D-- linux-5.10.1.orig/mm/readahead.c
+++ linux-5.10.1/mm/readahead.c
@@ -310,7 +310,11 @@ void force_page_cache_ra(struct readahea
   */
  static unsigned long get_init_ra_size(unsigned long size, unsigned
long max)
  {
-       unsigned long newsize =3D roundup_pow_of_two(size);
+       unsigned long newsize;
+
+       if (!size)
+               size =3D 32;
+       newsize =3D roundup_pow_of_two(size);

         if (newsize <=3D max / 32)
                 newsize =3D newsize * 4;


and the issue did no longer occurred at the server (5.10.2).

I did not patched the desktop system and the issue occurred still 3
times since 21th of december (5.10.2/3)

=2D-
Toralf