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    SubjectRe: [PATCH v2] mm: include CMA pages in lowmem_reserve at boot
    On Fri, 14 Aug 2020 09:49:26 -0700 Doug Berger <opendmb@gmail.com> wrote:

    > The lowmem_reserve arrays provide a means of applying pressure
    > against allocations from lower zones that were targeted at
    > higher zones. Its values are a function of the number of pages
    > managed by higher zones and are assigned by a call to the
    > setup_per_zone_lowmem_reserve() function.
    >
    > The function is initially called at boot time by the function
    > init_per_zone_wmark_min() and may be called later by accesses
    > of the /proc/sys/vm/lowmem_reserve_ratio sysctl file.
    >
    > The function init_per_zone_wmark_min() was moved up from a
    > module_init to a core_initcall to resolve a sequencing issue
    > with khugepaged. Unfortunately this created a sequencing issue
    > with CMA page accounting.
    >
    > The CMA pages are added to the managed page count of a zone
    > when cma_init_reserved_areas() is called at boot also as a
    > core_initcall. This makes it uncertain whether the CMA pages
    > will be added to the managed page counts of their zones before
    > or after the call to init_per_zone_wmark_min() as it becomes
    > dependent on link order. With the current link order the pages
    > are added to the managed count after the lowmem_reserve arrays
    > are initialized at boot.
    >
    > This means the lowmem_reserve values at boot may be lower than
    > the values used later if /proc/sys/vm/lowmem_reserve_ratio is
    > accessed even if the ratio values are unchanged.
    >
    > In many cases the difference is not significant, but for example
    > an ARM platform with 1GB of memory and the following memory layout
    > [ 0.000000] cma: Reserved 256 MiB at 0x0000000030000000
    > [ 0.000000] Zone ranges:
    > [ 0.000000] DMA [mem 0x0000000000000000-0x000000002fffffff]
    > [ 0.000000] Normal empty
    > [ 0.000000] HighMem [mem 0x0000000030000000-0x000000003fffffff]
    >
    > would result in 0 lowmem_reserve for the DMA zone. This would allow
    > userspace to deplete the DMA zone easily.

    Sounds fairly serious for thos machines. Was a cc:stable considered?

    > Funnily enough
    > $ cat /proc/sys/vm/lowmem_reserve_ratio
    > would fix up the situation because it forces
    > setup_per_zone_lowmem_reserve as a side effect.
    >
    > This commit breaks the link order dependency by invoking
    > init_per_zone_wmark_min() as a postcore_initcall so that the
    > CMA pages have the chance to be properly accounted in their
    > zone(s) and allowing the lowmem_reserve arrays to receive
    > consistent values.
    >

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