Re: [PATCH V2] mm: Introduce GFP_PGTABLE

On 01/16/2019 06:00 PM, Matthew Wilcox wrote:

On Wed, Jan 16, 2019 at 07:57:03AM +0100, Michal Hocko wrote:

quoted

On Wed 16-01-19 11:51:32, Anshuman Khandual wrote:

quoted

All architectures have been defining their own PGALLOC_GFP as (GFP_KERNEL |
__GFP_ZERO) and using it for allocating page table pages. This causes some
code duplication which can be easily avoided. GFP_KERNEL allocated and
cleared out pages (__GFP_ZERO) are required for page tables on any given
architecture. This creates a new generic GFP flag flag which can be used
for any page table page allocation. Does not cause any functional change.

GFP_PGTABLE is being added into include/asm-generic/pgtable.h which is the
generic page tabe header just to prevent it's potential misuse as a general
allocation flag if included in include/linux/gfp.h.

I haven't reviewed the patch yet but I am wondering whether this is
really worth it without going all the way down to unify the common code
and remove much more code duplication. Or is this not possible for some
reason?

Exactly what I suggested doing in response to v1.

Also, the approach taken here is crazy.  x86 has a feature that no other
architecture has bothered to implement yet -- accounting page tables
to the process.  Yet instead of spreading that goodness to all other
architectures, Anshuman has gone to more effort to avoid doing that.

The basic objective for this patch is to create a common minimum allocation
flag that can be used by architectures but that still allows archs to add
on additional constraints if they see fit. This patch does not intend to
change functionality for any arch.

Yes. There is opportunity for further clean up and consolidation like the
one you mentioned about accounting. Uses pages tables should have
__GFP_ACCOUNT and kernel ones should not. IIUC unfortunately not all arch
implement this right now. It is something which should not be arch specific.
Accounting semantics should be common for all archs. A default function
switching between GFP_PGTABLE for kernel and GFP_PGTABLE | __GFP_ACCOUNT
looking into mm_struct can help here.

Then there is __GFP_RETRY_MAYFAIL. Some archs use it for multi order page
allocation but some use for a single page as well.

If there is an agreement on __GFP_ACCOUNT and __GFP_RETRY_MAYFAIL we can
clean this up further.