linux-kernel - Re: [PATCH] cma: fix calculation of aligned offset

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Date:   Thu, 29 Jun 2017 17:49:40 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Doug Berger <opendmb@...il.com>
Cc:     Gregory Fong <gregory.0xf0@...il.com>,
        Angus Clark <angus@...usclark.org>,
        Laura Abbott <labbott@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Lucas Stach <l.stach@...gutronix.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Shiraz Hashim <shashim@...eaurora.org>,
        Jaewon Kim <jaewon31.kim@...sung.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cma: fix calculation of aligned offset

On Thu, 29 Jun 2017 17:43:18 -0700 Doug Berger <opendmb@...il.com> wrote:

> On 06/29/2017 01:48 PM, Andrew Morton wrote:
> > On Wed, 28 Jun 2017 10:07:41 -0700 Doug Berger <opendmb@...il.com> wrote:
> > 
> >> The align_offset parameter is used by bitmap_find_next_zero_area_off()
> >> to represent the offset of map's base from the previous alignment
> >> boundary; the function ensures that the returned index, plus the
> >> align_offset, honors the specified align_mask.
> >>
> >> The logic introduced by commit b5be83e308f7 ("mm: cma: align to
> >> physical address, not CMA region position") has the cma driver
> >> calculate the offset to the *next* alignment boundary.  In most cases,
> >> the base alignment is greater than that specified when making
> >> allocations, resulting in a zero offset whether we align up or down.
> >> In the example given with the commit, the base alignment (8MB) was
> >> half the requested alignment (16MB) so the math also happened to work
> >> since the offset is 8MB in both directions.  However, when requesting
> >> allocations with an alignment greater than twice that of the base,
> >> the returned index would not be correctly aligned.
> >>
> >> Also, the align_order arguments of cma_bitmap_aligned_mask() and
> >> cma_bitmap_aligned_offset() should not be negative so the argument
> >> type was made unsigned.
> > 
> > The changelog doesn't describe the user-visible effects of the bug.  It
> > should do so please, so that others can decide which kernel(s) need the fix.
> > 
> > Since the bug has been there for three years, I'll assume that -stable
> > backporting is not needed.
> > 
> I'm afraid I'm confused by what you are asking me to do since it appears
> that you have already signed-off on this patch.
> 
> The direct user-visible effect of the bug is that if the user requests a
> CMA allocation that is aligned with a granule that is more than twice
> the base alignment of the CMA region she will receive an allocation that
> does not have that alignment.
> 
> As I indicated to Gregory, the follow-on consequences of the address not
> satisfying the required alignment depend on why the alignment was
> requested.  In our case it was a system crash, but it could also
> manifest as data corruption on a network interface for example.
> 
> In general I would expect it to be unusual for anyone to request an
> allocation alignment that is larger than the CMA base alignment which is
> probably why the bug has been hiding for three years.
> 

OK, it sounds like it isn't very critical so I'll remove the cc:stable
and the patch will appear in 4.12 and no earlier kernels.