linux-kernel - Re: Correlation CMA size and FORCE_MAX

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Date:   Mon, 19 Sep 2022 11:57:30 +0200
From:   Michael Nazzareno Trimarchi <michael@...rulasolutions.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Mike Rapoport <rppt@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Correlation CMA size and FORCE_MAX_ZONEORDER

Hi

On Mon, Sep 19, 2022 at 11:31 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 19.09.22 11:17, Michael Nazzareno Trimarchi wrote:
> > Hi David
> >
> > On Mon, Sep 19, 2022 at 10:38 AM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 15.09.22 23:36, Michael Nazzareno Trimarchi wrote:
> >>> Hi all
> >>
> >> Hi,
> >>
> >>>
> >>> Working on a small device with 128MB of memory and using imx_v6_v7
> >>> defconfig I found that CMA_SIZE_MBYTES, CMA_SIZE_PERCENTAGE
> >>> are not respected. The calculation done does not allow the requested
> >>> size. I think that this should be somehow documented and described but
> >>> I did not
> >>> find the documentation. Does it work this way?
> >>>
> >>> With CMA_SIZE of 8MB I need to have FORCE_MAX_ZONEORDER=12 if I have
> >>> the default FORCE_MAX_ZONEORDER=14 the min size is 32Mb
> >>
> >> The underlying constraint is that CMA regions require a certain minimum
> >> alignment+size. They cannot be arbitrarily in size.
> >>
> >> CMA_MIN_ALIGNMENT_BYTES expresses that, and corresponds in upstream
> >> kernels to the size of a single pageblock.
> >>
> >> In previous kernels, it used to be the size of the largest buddy
> >> allocation granularity (derived from MAX_ORDER, derived from
> >> FORCE_MAX_ZONEORDER).
> >>
> >> On upstream kernels, the FORCE_MAX_ZONEORDER constraint should no longer
> >> apply. On most archs, the minimum alignment+size should be 2 MiB
> >> (x86-64, aarch64 with 4k base pages) -- the size of a single pageblock.
> >>
> >> So far the theory. Are you still running into this limitation on
> >> upstream kernels?
> >>
> >
> > I can run 6-rc2 on my board. I test again but according to it, if I
> > put 4M as CMA in cma=4M in boot
> > parameters, the result is 32Mb of CMA. Apart of that seems that
> > process lime tiny membench can not even start
> > to mblock memory
> >
>
> The CMA alignemnt change went into v5.19. If "cma=4M" still gives you >
> 4M, can you post /proc/meminfo and the early console output?
>

cat /proc/cmdline
cma=4M mtdparts=gpmi-nand:4m(nandboot),1m(env),24m(kernel),1m(nanddtb),-(rootfs)
root=ubi0:root rw ubi.mtd=ro
otfs rootfstype=ubifs rootwait=1
# cat /proc/meminfo
MemTotal:         109560 kB
MemFree:           56084 kB
MemAvailable:      56820 kB
Buffers:               0 kB
Cached:            39680 kB
SwapCached:            0 kB
Active:               44 kB
Inactive:            644 kB
Active(anon):         44 kB
Inactive(anon):      644 kB
Active(file):          0 kB
Inactive(file):        0 kB
Unevictable:       39596 kB
Mlocked:               0 kB
HighTotal:             0 kB
HighFree:              0 kB
LowTotal:         109560 kB
LowFree:           56084 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:           628 kB
Mapped:             1480 kB
Shmem:                84 kB
KReclaimable:       4268 kB
Slab:               8456 kB
SReclaimable:       4268 kB
SUnreclaim:         4188 kB
KernelStack:         392 kB
PageTables:           88 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:       54780 kB
Committed_AS:       1876 kB
VmallocTotal:     901120 kB
VmallocUsed:        2776 kB
VmallocChunk:          0 kB
Percpu:               72 kB
CmaTotal:          32768 kB
CmaFree:           32484 kB
# uname -a
Linux buildroot 6.0.0-rc5 #20 SMP Mon Sep 19 11:51:26 CEST 2022 armv7l GNU/Linux
#

Then here https://pastebin.com/6MUB2VBM dmesg

CONFIG_ARM_MODULE_PLTS=y
CONFIG_FORCE_MAX_ZONEORDER=14
CONFIG_ALIGNMENT_TRAP=y
...
CONFIG_CMA
CONFIG_CMA_AREAS=7
...

CONFIG_CMA_SIZE_MBYTES=8
CONFIG_CMA_SIZE_SEL_MBYTES=y

Michael

>
> --
> Thanks,
>
> David / dhildenb
>
--
Michael Nazzareno Trimarchi
Co-Founder & Chief Executive Officer
M. +39 347 913 2170
michael@...rulasolutions.com
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