Re: why inode creation with GFP_HIGHUSER?

[girish <girishvg@gmail.com>]

> > i'd like to know why page(s) for inodes are allocated with
> > GFP_HIGHUSER & not with GFP_USER mask? is there any particular need
> > that the address_space be set with GFP_HIGHUSER flag?
>
> GFP_HIGHUSER allows the use of HIGH memory but it does not require it. If
> the system has no HIGHMEM then we will just use regular memory.

i understand the policy of zone fallback. but in my case there is, in
fact bigger chunk, meory marked as high memory. please see below for
further explaination -

> > i intend to allocate highmem pages strictly to user processes. my idea
> > is to completely avoid kernel mapping for these pages. so, as a dirty
> > hack - i changed mapping_set_gfp_mask function not to honor
> > __GFP_HIGHMEM zone selector if __GFP_IO | __GFP_FS are set. in short i
> > replace  GFP_HIGHUSER with GFP_USER mask. with this change the kernel
> > comes to life. but i am still confused about the effect of this change
> > on system, that i am yet to see?
>
> I am not sure what you intend to do. The kernel already avoids mapping
> highmem pages into the kernel as much as possible.

that's the whole confusion. kernel is supposed to *avoid* allocating
from ZONE_HIGHMEM if there is some memory left in ZONE_DMA and/or
ZONE_NORMAL. but as i mentioned the zonelist selection that happens
based  on GFP_* mask (in this case GFP_HIGHUSER), makes __alloc_pages
to allocate from a list which has both HIGHMEM and DMA/NORMAL zones
listed in it. the zonelist looping/fallback is as implemented in
get_page_from_freelist (). to this function, the zonelist that is
passed contains both and in the order - HIGHMEM and DMA/NORMAL zones.
shouldn't it be NORMAl/DMA first and then HIGHMEM in the zonelist?

(ref: http://lxr.free-electrons.com/source/mm/page_alloc.c#883)

thanks.

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