Messages in this thread | | | Subject | Re: [PATCH V2] mm: Introduce GFP_PGTABLE | From | Christophe Leroy <> | Date | Wed, 16 Jan 2019 14:47:16 +0100 |
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Le 16/01/2019 à 14:18, Matthew Wilcox a écrit : > On Wed, Jan 16, 2019 at 06:42:22PM +0530, Anshuman Khandual wrote: >> On 01/16/2019 06:00 PM, Matthew Wilcox wrote: >>> On Wed, Jan 16, 2019 at 07:57:03AM +0100, Michal Hocko wrote: >>>> On Wed 16-01-19 11:51:32, Anshuman Khandual wrote: >>>>> 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. > > I disagree with your objective. Making more code common is a great idea, > but this patch is too unambitious. We should be heading towards one or > two page table allocation functions instead of having every architecture do > its own thing. > > So start there. Move the x86 function into common code and convert one > other architecture to use it too.
Are we talking about pte_alloc_one_kernel() and pte_alloc_one() ?
I'm not sure x86 function is the best common one, as it seems to allocate a multiple of PAGE_SIZE only.
Some arches like powerpc use pagetables which are smaller than a page, for instance powerpc 8xx uses 4k pagetables even with 16k pages, which means a single page can be used by 4 pagetables.
Therefore, I would suggest to start with powerpc functions.
Christophe
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