Tight check of pfn_valid on sparsemem
in [Kernel]
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From: KAMEZAWA Hiroyuki on 13 Jul 2010 02:50 On Tue, 13 Jul 2010 15:04:00 +0900 Minchan Kim <minchan.kim(a)gmail.com> wrote: > >> > 2. This can't be help for a case where a section has multiple small holes. > >> > >> I agree. But this(not punched hole but not filled section problem) > >> isn't such case. But it would be better to handle it altogether. :) > >> > >> > > >> > Then, my proposal for HOLES_IN_MEMMAP sparsemem is below. > >> > == > >> > Some architectures unmap memmap[] for memory holes even with SPARSEMEM. > >> > To handle that, pfn_valid() should check there are really memmap or not. > >> > For that purpose, __get_user() can be used. > >> > >> Look at free_unused_memmap. We don't unmap pte of hole memmap. > >> Is __get_use effective, still? > >> > > __get_user() works with TLB and page table, the vaddr is really mapped or not. > > If you got SEGV, __get_user() returns -EFAULT. It works per page granule. > > I mean following as. > For example, there is a struct page in on 0x20000000. > > int pfn_valid_mapped(unsigned long pfn) > { > struct page *page = pfn_to_page(pfn); /* hole page is 0x2000000 */ > char *lastbyte = (char *)(page+1)-1; /* lastbyte is 0x2000001f */ > char byte; > > /* We pass this test since free_unused_memmap doesn't unmap pte */ > if(__get_user(byte, page) != 0) > return 0; why ? When the page size is 4096 byte. 0x1ffff000 - 0x1ffffffff 0x20000000 - 0x200000fff are on the same page. And memory is mapped per page. What we access by above __get_user() is a byte at [0x20000000, 0x20000001) and it's unmapped if 0x20000000 is unmapped. Thanks, -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Russell King - ARM Linux on 13 Jul 2010 03:30 On Tue, Jul 13, 2010 at 03:04:00PM +0900, Minchan Kim wrote: > > __get_user() works with TLB and page table, the vaddr is really mapped or not. > > If you got SEGV, __get_user() returns -EFAULT. It works per page granule. Not in kernel space. It works on 1MB sections there. Testing whether a page is mapped by __get_user is a hugely expensive way to test whether a PFN is valid. It'd be cheaper to use our flatmem implementation of pfn_valid() instead. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: KAMEZAWA Hiroyuki on 13 Jul 2010 03:40 On Tue, 13 Jul 2010 08:20:09 +0100 Russell King - ARM Linux <linux(a)arm.linux.org.uk> wrote: > On Tue, Jul 13, 2010 at 03:04:00PM +0900, Minchan Kim wrote: > > > __get_user() works with TLB and page table, the vaddr is really mapped or not. > > > If you got SEGV, __get_user() returns -EFAULT. It works per page granule. > > Not in kernel space. It works on 1MB sections there. > > Testing whether a page is mapped by __get_user is a hugely expensive > way to test whether a PFN is valid. Note: pfn_valid() is for checking "there is memmap". > It'd be cheaper to use our flatmem implementation of pfn_valid() instead. > Hmm. IIUC, pfn_valid() succeeds in almost all case if there is a section. But yes, I'm not familar with ARM. I love another idea as I've already shown as preparing _a_ page filled with 0x00004000 and map it into the all holes. PG_reserved will help almost all case even if it's ugly. Anyway, sparsemem is designed to be aligned to SECTION_SIZE of memmap. Please avoid adding new Spaghetti code without proper configs. Thanks, -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: KAMEZAWA Hiroyuki on 13 Jul 2010 04:10 On Tue, 13 Jul 2010 16:58:08 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> wrote: > On Tue, 13 Jul 2010 16:34:17 +0900 > KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> wrote: > > > Anyway, sparsemem is designed to be aligned to SECTION_SIZE of memmap. > > Please avoid adding new Spaghetti code without proper configs. > > Thanks, > > Ok, I realized I misunderstand all. Arm doesn't unmap memmap but reuse the page > for memmap without modifing ptes. My routine only works when ARM uses sparsemem_vmemmap. > But yes, it isn't. > > Hmm...How about using pfn_valid() for FLATMEM or avoid using SPARSEMEM ? > If you want conrols lower than SPARSEMEM, FLATMEM works better because ARM unmaps memmap. allocation of memmap in lower granule than SPARSEMEM. How about stop using SPARSEMEM ? What's the benefit ? It just eats up memory for mem_section[]. Sorry, -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Minchan Kim on 13 Jul 2010 04:10
On Tue, Jul 13, 2010 at 3:40 PM, KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> wrote: > On Tue, 13 Jul 2010 15:04:00 +0900 > Minchan Kim <minchan.kim(a)gmail.com> wrote: > >> >> > �2. This can't be help for a case where a section has multiple small holes. >> >> >> >> I agree. But this(not punched hole but not filled section problem) >> >> isn't such case. But it would be better to handle it altogether. :) >> >> >> >> > >> >> > Then, my proposal for HOLES_IN_MEMMAP sparsemem is below. >> >> > == >> >> > Some architectures unmap memmap[] for memory holes even with SPARSEMEM. >> >> > To handle that, pfn_valid() should check there are really memmap or not. >> >> > For that purpose, __get_user() can be used. >> >> >> >> Look at free_unused_memmap. We don't unmap pte of hole memmap. >> >> Is __get_use effective, still? >> >> >> > __get_user() works with TLB and page table, the vaddr is really mapped or not. >> > If you got SEGV, __get_user() returns -EFAULT. It works per page granule. >> >> I mean following as. >> For example, there is a struct page in on 0x20000000. >> >> int pfn_valid_mapped(unsigned long pfn) >> { >> � � � �struct page *page = pfn_to_page(pfn); /* hole page is 0x2000000 */ >> � � � �char *lastbyte = (char *)(page+1)-1; �/* lastbyte is 0x2000001f */ >> � � � �char byte; >> >> � � � �/* We pass this test since free_unused_memmap doesn't unmap pte */ >> � � � �if(__get_user(byte, page) != 0) >> � � � � � � � �return 0; > > why ? When the page size is 4096 byte. > > � � �0x1ffff000 - 0x1ffffffff > � � �0x20000000 - 0x200000fff are on the same page. And memory is mapped per page. sizeof(struct page) is 32 byte. So lastbyte is address of struct page + 32 byte - 1. > What we access by above __get_user() is a byte at [0x20000000, 0x20000001) Right. > and it's unmapped if 0x20000000 is unmapped. free_unused_memmap doesn't unmap pte although it returns the page to free list of buddy. > > Thanks, > -Kame > > -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |