Re: 答复: 答复: [PATCH] arm64: mm: free unused memmap for sparse memory model that define VMEMMAP
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2020-07-23 11:29:33
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On Wed, Jul 22, 2020 at 01:40:34PM +0000, liwei (CM) wrote:
Catalin Marinas wrote:quoted
On Wed, Jul 22, 2020 at 08:41:17AM +0000, liwei (CM) wrote:quoted
Mike Rapoport wrote:quoted
On Tue, Jul 21, 2020 at 03:32:03PM +0800, Wei Li wrote:quoted
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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel