On Tue, 2016-02-09 at 10:10 +0100, Ingo Molnar wrote: > * Toshi Kani <toshi.kani@xxxxxxx> wrote: > > > Since 4.1, ioremap() supports large page (pud/pmd) mappings in x86_64 > > and PAE. vmalloc_fault() however assumes that the vmalloc range is > > limited to pte mappings. > > > > pgd_ctor() sets the kernel's pgd entries to user's during fork(), which > > makes user processes share the same page tables for the kernel > > ranges. When a call to ioremap() is made at run-time that leads to > > allocate a new 2nd level table (pud in 64-bit and pmd in PAE), user > > process needs to re-sync with the updated kernel pgd entry with > > vmalloc_fault(). > > > > Following changes are made to vmalloc_fault(). > > So what were the effects of this shortcoming? Were large page ioremap()s > unusable? Was this harmless because no driver used this facility? > > If so then the changelog needs to spell this out clearly ... Large page support of ioremap() has been used for persistent memory mappings for a while. In order to hit this problem, i.e. causing a vmalloc fault, a large mount of ioremap allocations at run-time is required. The following example repeats allocation of 16GB range. # cat /proc/vmallocinfo | grep memremap 0xffffc90040000000-0xffffc90440001000 17179873280 memremap+0xb4/0x110 phys=480000000 ioremap 0xffffc90480000000-0xffffc90880001000 17179873280 memremap+0xb4/0x110 phys=480000000 ioremap 0xffffc908c0000000-0xffffc90cc0001000 17179873280 memremap+0xb4/0x110 phys=c80000000 ioremap 0xffffc90d00000000-0xffffc91100001000 17179873280 memremap+0xb4/0x110 phys=c80000000 ioremap 0xffffc91140000000-0xffffc91540001000 17179873280 memremap+0xb4/0x110 phys=480000000 ioremap : 0xffffc97300000000-0xffffc97700001000 17179873280 memremap+0xb4/0x110 phys=c80000000 ioremap 0xffffc97740000000-0xffffc97b40001000 17179873280 memremap+0xb4/0x110 phys=480000000 ioremap 0xffffc97b80000000-0xffffc97f80001000 17179873280 memremap+0xb4/0x110 phys=c80000000 ioremap 0xffffc97fc0000000-0xffffc983c0001000 17179873280 memremap+0xb4/0x110 phys=480000000 ioremap The last ioremap call above crossed a 512GB boundary (0x8000000000), which allocated a new pud table and updated the kernel pgd entry to point it. Because user process's page table does not have this pgd entry update, a read/write syscall request to the range will hit a vmalloc fault. Since vmalloc_fault() does not handle a large page properly, this causes an Oops as follows. BUG: unable to handle kernel paging request at ffff880840000ff8 IP: [<ffffffff810664ae>] vmalloc_fault+0x1be/0x300 PGD c7f03a067 PUD 0 Oops: 0000 [#1] SM : Call Trace: [<ffffffff81067335>] __do_page_fault+0x285/0x3e0 [<ffffffff810674bf>] do_page_fault+0x2f/0x80 [<ffffffff810d6d85>] ? put_prev_entity+0x35/0x7a0 [<ffffffff817a6888>] page_fault+0x28/0x30 [<ffffffff813bb976>] ? memcpy_erms+0x6/0x10 [<ffffffff817a0845>] ? schedule+0x35/0x80 [<ffffffffa006350a>] ? pmem_rw_bytes+0x6a/0x190 [nd_pmem] [<ffffffff817a3713>] ? schedule_timeout+0x183/0x240 [<ffffffffa028d2b3>] btt_log_read+0x63/0x140 [nd_btt] : [<ffffffff811201d0>] ? __symbol_put+0x60/0x60 [<ffffffff8122dc60>] ? kernel_read+0x50/0x80 [<ffffffff81124489>] SyS_finit_module+0xb9/0xf0 [<ffffffff817a4632>] entry_SYSCALL_64_fastpath+0x1a/0xa4 Note that this issue is limited to 64-bit. 32-bit only uses index 3 of the pgd entry to cover the entire vmalloc range, which is always valid. I will add this information to the change log. Thanks, -Toshi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>