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

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Date:   Wed, 28 Jun 2017 23:23:52 -0700
From:   Gregory Fong <gregory.0xf0@...il.com>
To:     Doug Berger <opendmb@...il.com>
Cc:     Angus Clark <angus@...usclark.org>,
        Andrew Morton <akpm@...ux-foundation.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>,
        Danesh Petigara <dpetigara@...adcom.com>
Subject: Re: [PATCH] cma: fix calculation of aligned offset

On Wed, Jun 28, 2017 at 10:07 AM, 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.

Wow, I had that completely backward, nice catch.

> 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.

It may be worth explaining what impact incorrect alignment has for an
end user, then considering for inclusion in stable.

>
> 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.
>
> Fixes: b5be83e308f7 ("mm: cma: align to physical address, not CMA region position")
> Signed-off-by: Angus Clark <angus@...usclark.org>
> Signed-off-by: Doug Berger <opendmb@...il.com>

Acked-by: Gregory Fong <gregory.0xf0@...il.com>