Re: [Xen-devel] [PATCH v3] xen/arm: Do not allocate pte entries for MAP_SMALL_PAGES

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.03.15 at 07:59, <vijay.kilari@xxxxxxxxx> wrote:
> From: Vijaya Kumar K <Vijaya.Kumar@xxxxxxxxxxxxxxxxxx>
> 
> On x86, for the pages mapped with PAGE_HYPERVISOR attribute
> non-leaf page tables are allocated with valid pte entries.
> and with MAP_SMALL_PAGES attribute only non-leaf page tables are
> allocated with invalid (valid bit set to 0) pte entries.
> However on arm this is not the case. On arm for the pages
> mapped with PAGE_HYPERVISOR and MAP_SMALL_PAGES both
> non-leaf and leaf level page table are allocated with valid bit
> in pte entries.
> 
> This behaviour in arm makes common vmap code fail to
> allocate memory beyond 128MB as described below.
> 
> In vm_init, map_pages_to_xen() is called for mapping
> vm_bitmap. Initially one page of vm_bitmap is allocated
> and mapped using PAGE_HYPERVISOR attribute.
> For the rest of vm_bitmap pages, MAP_SMALL_PAGES attribute
> is used to map.
> 
> In ARM for both PAGE_HYPERVISOR and MAP_SMALL_PAGES, valid bit
> is set to 1 in pte entry for these mapping.
> 
> In vm_alloc(), map_pages_to_xen() is failing for >128MB because
> for this next vm_bitmap page the mapping is already set in vm_init()
> with valid bit set in pte entry. So map_pages_to_xen() in
> ARM returns error.
> 
> With this patch, MAP_SMALL_PAGES attribute will only allocate
> non-leaf page tables only and arch specific populate_pt_range()
> api is introduced to populate non-leaf page table entries
> for the requested pages.
> 
> Here we use bit[16] in the attribute flag to know if leaf page
> tables should be allocated or not.
> 
> This bit is set only for MAP_SMALL_PAGES attribute.
> 
> Signed-off-by: Vijaya Kumar K<Vijaya.Kumar@xxxxxxxxxxxxxxxxxx>

Non-ARM bits
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>


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