On Fri, 6 Jul 2018 17:01:11 +0800 Jia He <[email protected]> wrote:
> From: Jia He <[email protected]> > > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns > where possible") optimized the loop in memmap_init_zone(). But it causes > possible panic bug. So Daniel Vacek reverted it later. > > But as suggested by Daniel Vacek, it is fine to using memblock to skip > gaps and finding next valid frame with CONFIG_HAVE_ARCH_PFN_VALID. > Daniel said: > "On arm and arm64, memblock is used by default. But generic version of > pfn_valid() is based on mem sections and memblock_next_valid_pfn() does > not always return the next valid one but skips more resulting in some > valid frames to be skipped (as if they were invalid). And that's why > kernel was eventually crashing on some !arm machines." > > About the performance consideration: > As said by James in b92df1de5, > "I have tested this patch on a virtual model of a Samurai CPU > with a sparse memory map. The kernel boot time drops from 109 to > 62 seconds." > > Thus it would be better if we remain memblock_next_valid_pfn on arm/arm64. > We're making a bit of a mess here. mmzone.h: ... #ifndef CONFIG_HAVE_ARCH_PFN_VALID ... #define next_valid_pfn(pfn) (pfn + 1) #endif ... #ifdef CONFIG_HAVE_MEMBLOCK_PFN_VALID #define next_valid_pfn(pfn) memblock_next_valid_pfn(pfn) ... #else ... #ifndef next_valid_pfn #define next_valid_pfn(pfn) (pfn + 1) #endif I guess it works OK, since CONFIG_HAVE_MEMBLOCK_PFN_VALID depends on CONFIG_HAVE_ARCH_PFN_VALID. But it could all do with some cleanup and modernization. - Perhaps memblock_next_valid_pfn() should just be called pfn_valid(). So the header file's responsibility is to provide pfn_valid() and next_valid_pfn(). - CONFIG_HAVE_ARCH_PFN_VALID should go away. The current way of doing such thnigs is for the arch (or some Kconfig combination) to define pfn_valid() and next_valid_pfn() in some fashion and to then ensure that one of them is #defined to something, to indicate that both of these have been set up. Or something like that. Secondly, in memmap_init_zone() > - if (!early_pfn_valid(pfn)) > + if (!early_pfn_valid(pfn)) { > + pfn = next_valid_pfn(pfn) - 1; > continue; > + } > + This is weird-looking. next_valid_pfn(pfn) is usually (pfn+1) so it's a no-op. Sometimes we're calling memblock_next_valid_pfn() and then backing up one, presumably because the `for' loop ends in `pfn++'. Or something. Can this please be fully commented or cleaned up?
