How to understand lowmem_reserve in a zone?

[Peter Teoh]

On Sat, Aug 18, 2012 at 9:06 PM, Parmenides <mobile.parmenides at gmail.com>wrote:

> Hi,
>
>     I have a question about the reseved page frames in a zone. The
> physical memory is splitted into some node, which is  further divied
> into some zone. For each zone, the kernel try to reserve some page
> frames to statisfy requests on low memory condition. There is a
> lowmem_reserve[] in a zone descriptor, its defined like:
>
>      struct zone {
>                     ....
>                     unsigned long lowmem_reserve[MAX_NR_ZONES];
>                     ...
>      };
>
> It is obvious that lowmem_reserve[] contains MAX_NR_ZONES elements.
>

for mem, MAX_NR_ZONES is 4, which therefore mean 4 zone exists, and for
each zone there is a lowmem_reserve value, make sense?

The function:    setup_per_zone_lowmem_reserve(void) will setup the value,
which is user-definable via a sysctl called "lowmem_reserve_ratio".

cat /proc/sys/vm/lowmem_reserve_ratio
256 256 32

and read the source code for explanation:

mm/page_alloc.c:

/*
 * results with 256, 32 in the lowmem_reserve sysctl:
 *      1G machine -> (16M dma, 800M-16M normal, 1G-800M high)
 *      1G machine -> (16M dma, 784M normal, 224M high)
 *      NORMAL allocation will leave 784M/256 of ram reserved in the
ZONE_DMA
 *      HIGHMEM allocation will leave 224M/32 of ram reserved in ZONE_NORMAL
 *      HIGHMEM allocation will (224M+784M)/256 of ram reserved in ZONE_DMA
 *
 * TBD: should special case ZONE_DMA32 machines here - in those we normally
 * don't need any ZONE_NORMAL reservation
 */
int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES-1] = {



> But, I think just a interger is enough to record the number of  a
> zone's reserved page frames. Why do we have to use a
> array?


for each zone, and the zone name is defined in mm/page_alloc.c as well.


> Furthermore, lowmem_reserve[] merely stand for the amount. How
> does the kernel mark a page frame to be reserved?
>
> API is SetPageReserved(page), and queried with PageReserved(page) etc.

This API will use just one bit to store the reservation information for one
page.

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-- 
Regards,
Peter Teoh
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