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

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Message-Id: <20170629134810.3a5b09dbdea001cca72080ce@linux-foundation.org>
Date:   Thu, 29 Jun 2017 13:48:10 -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>,
        linux-mm@...ck.org (open list:MEMORY MANAGEMENT),
        linux-kernel@...r.kernel.org (open list)
Subject: Re: [PATCH] cma: fix calculation of aligned offset

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.