Messages in this thread Patch in this message | | | Date | Tue, 17 Aug 1999 01:26:56 +0200 (CEST) | From | Andrea Arcangeli <> | Subject | Re: [bigmem-patch] 4GB with Linux on IA32 |
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This other incremental patch will make the bigmem code safe w.r.t. raw-io:
--- 2.3.13-bigmem-L/mm/memory.c Fri Aug 13 00:31:59 1999 +++ 2.3.13-bigmem/mm/memory.c Tue Aug 17 00:59:37 1999 @@ -436,6 +436,10 @@ map = mem_map + MAP_NR(page); if (PageReserved(map)) return 0; +#ifdef CONFIG_BIGMEM + if (PageBIGMEM(map)) + return 0; +#endif return map; } But now IMO there's to choose between one of the below options:
1) should we change all device drivers to allow us to do I/O over bigmem pages? NOTE: all DMA engine are just fine since virt_to_bus just works right as Gerhard pointed out to me. The only problem is for drivers that reads and writes to the b_data in software. 2) should we change ll_rw_block to force an high limit of bh queued in the same request and then remap the b_data in the ll_rw_block layer with a NR_REQUEST*MAX_BH_PER_REQUEST array of virtual-pages in the fixmap area? (many tlb_flush_all... or at least many SMP-invlpg with a smarter cross-CPU-invlpg message) virt_to_bus must be able to resolve the bus address starting from the fixmap virtual address. 3) using the remap trick that I am just using in the swapout/swapin code, I could just do raw-io on anonymous memory but I get stuck with the shm memory where I can't simply realloc a page without browsing all processes VM. Should I take a list of all pte that are mapping each smp page and doing the remap trick also on shm memory? 4) should I avoid raw-io in the shm memory and use the remap trick with the anonymous memory? 5) should I avoid bigmem in shm memory and simply use the remap trick with the anonymous memory?
I guess big databases uses the shm memory as cache. And I guess they use raw-io to fill the shm memory with proper data. Am I right about this? If so I can't choose (4). And since I would like to use the bigmem as shm memory I would like to avoid also (5).
(3) looks dirty and add a performance hit in the shm_nopage hander.
(2) looks dirty and slow due the SMP tlb flushes.
(1) looks clean and efficient (100% efficient in the DMA case!) but it breaks all drivers out there... :(((
Theorically the cleanest solution would be (1) but I don't know if this will be a good choice on the long run (theorically on 2038 we won't need CONFIG_BIGMEM anymore...).
Right now I temporary applyed solution (0): the patch at the top of this email so if you want to use raw-io on anonymous or shm memory you'll have to recompile with CONFIG_BIGMEM not set.
Comments? (very welcome :)
Andrea
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