Re: [PATCH] ARM: print cma-reserved pages from show_mem

[Gregory Fong]

Hello,

On Tue, Feb 10, 2015 at 3:32 AM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> On Mon, Feb 09, 2015 at 11:55:54AM -0800, Gregory Fong wrote:
>> On Fri, Feb 6, 2015 at 1:41 PM, Laura Abbott <lauraa@xxxxxxxxxxxxxx> wrote:
>> > So it looks like the lib/show_mem.c does something different
>> > #ifdef CONFIG_CMA
>> >         printk("%lu pages reserved\n", (reserved - totalcma_pages));
>> >         printk("%lu pages cma reserved\n", totalcma_pages);
>> > #else
>> >         printk("%lu pages reserved\n", reserved);
>> > #endif
>> >
>> >
>> > No need to change the name, instead I'd say fix up arm to match what
>> > the generic showmem is doing.
>>
>> The trouble is that lib/show_mem.c and ARM's show_mem use the
>> 'reserved' variable to hold different info, which was not a problem I
>> was aiming to tackle here, and am not sure I understand what's going
>> on well enough to do so.  But let's give it a shot:
>>
>> In lib/show_mem.c, reserved is calculated by iterating over all online
>> nodes, then increasing reserved by (zone->present_pages -
>> zone->managed_pages).  This count includes CMA pages and so when
>> reserved pages is printed it should be 'reserved' - totalcma_pages, as
>> it currently is.
>
> So, some digging is needed into why the generic version is different.
> You have to remember that many of the algorithms for this kind of thing
> were based on the x86 implementation, so differences like this are
> probably down to ARM being annoyingly overlooked or ignored when generic
> changes happen.
>

Revisiting this finally, it looks like this was changed by Mel about a
year and a half ago in commit c78e93630d15b5f5774213aad9bdc9f52473a89b
"mm: do not walk all of system memory during show_mem"[1], which
removes the pfn walk and gets this info from struct zone instead,
saving a lot of time.  Is there any reason to not to change the ARM
show_mem to do this as well?  With that, I'm not sure I understand why
there would need to be an ARM-specific implementation at all anymore,
but maybe I'm missing something.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=c78e93630d15b5f5774213aad9bdc9f52473a89b

Best regards,
Gregory
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