Messages in this thread | | | Date | Thu, 23 Jul 2020 12:29:26 +0100 | From | Catalin Marinas <> | Subject | Re: 答复: 答复: [PATC H] arm64: mm: fre e unused memmap for sparse memory model that define VMEMMAP |
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On Wed, Jul 22, 2020 at 01:40:34PM +0000, liwei (CM) wrote: > Catalin Marinas wrote: > > On Wed, Jul 22, 2020 at 08:41:17AM +0000, liwei (CM) wrote: > > > Mike Rapoport wrote: > > > > On Tue, Jul 21, 2020 at 03:32:03PM +0800, Wei Li wrote: > > > > > For the memory hole, sparse memory model that define > > > > > SPARSEMEM_VMEMMAP do not free the reserved memory for the page > > > > > map, this patch do it. > > > > > > > > Are there numbers showing how much memory is actually freed? > > > > > > > > The freeing of empty memmap would become rather complex with these > > > > changes, do the memory savings justify it? > > > > > > In the sparse memory model, the size of a section is 1 GB > > > (SECTION_SIZE_BITS 30) by default. > > > > Can we reduce SECTION_SIZE_BITS instead? Say 26? > > Yes, you are right, reduce SECTION_SIZE_BITS to 26 can save almost the > same memory as the patch. > > 1) However, it is not clear whether changing the section size has any > other impact.
Well, we should analyse this.
> 2) Just like the flat memory model and the sparse memory model that > does not define VMEMMAP, both of them have their own ways to free > unused memmap. I think we've given a similar way for sparse memory > define VMEMMAP.
I think we did it for flatmem initially (on arm32) and added support for sparsemem later on, so free_unused_memmap() had to cope with sparse sections. On arm64 we introduced vmemmap support and didn't bother with the freeing at all because of the added complexity of the vmemmap page tables.
I wonder whether we should just disallow flatmem and non-vmemmap sparsemem on arm64. Is there any value in keeping them around?
> 3) This explicit free unused memmap method does reduce unnecessary > memory waste for users who do not notice the section size > modification.
But if we changed SECTION_SIZE_BITS in the mainline kernel, then we wouldn't need additional code to free the unused memmap.
-- Catalin
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