Re: [PATCH v3] x86/e820: put !E820_TYPE_RAM regions into memblock.reserved

[Oscar Salvador]

On Fri, Jun 15, 2018 at 10:00:00AM -0400, Pavel Tatashin wrote:
> On 18-06-15 10:41:42, Michal Hocko wrote:
> > On Fri 15-06-18 07:29:48, Naoya Horiguchi wrote:
> > [...]
> > > From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > > Date: Thu, 14 Jun 2018 16:04:36 +0900
> > > Subject: [PATCH] x86/e820: put !E820_TYPE_RAM regions into memblock.reserved
> > > 
> > > There is a kernel panic that is triggered when reading /proc/kpageflags
> > > on the kernel booted with kernel parameter 'memmap=nn[KMG]!ss[KMG]':
> > > 
> > >   BUG: unable to handle kernel paging request at fffffffffffffffe
> > >   PGD 9b20e067 P4D 9b20e067 PUD 9b210067 PMD 0
> > >   Oops: 0000 [#1] SMP PTI
> > >   CPU: 2 PID: 1728 Comm: page-types Not tainted 4.17.0-rc6-mm1-v4.17-rc6-180605-0816-00236-g2dfb086ef02c+ #160
> > >   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.fc28 04/01/2014
> > >   RIP: 0010:stable_page_flags+0x27/0x3c0
> > >   Code: 00 00 00 0f 1f 44 00 00 48 85 ff 0f 84 a0 03 00 00 41 54 55 49 89 fc 53 48 8b 57 08 48 8b 2f 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 01 0f 84 10 03 00 00 31 db 49 8b 54 24 08 4c 89 e7
> > >   RSP: 0018:ffffbbd44111fde0 EFLAGS: 00010202
> > >   RAX: fffffffffffffffe RBX: 00007fffffffeff9 RCX: 0000000000000000
> > >   RDX: 0000000000000001 RSI: 0000000000000202 RDI: ffffed1182fff5c0
> > >   RBP: ffffffffffffffff R08: 0000000000000001 R09: 0000000000000001
> > >   R10: ffffbbd44111fed8 R11: 0000000000000000 R12: ffffed1182fff5c0
> > >   R13: 00000000000bffd7 R14: 0000000002fff5c0 R15: ffffbbd44111ff10
> > >   FS:  00007efc4335a500(0000) GS:ffff93a5bfc00000(0000) knlGS:0000000000000000
> > >   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >   CR2: fffffffffffffffe CR3: 00000000b2a58000 CR4: 00000000001406e0
> > >   Call Trace:
> > >    kpageflags_read+0xc7/0x120
> > >    proc_reg_read+0x3c/0x60
> > >    __vfs_read+0x36/0x170
> > >    vfs_read+0x89/0x130
> > >    ksys_pread64+0x71/0x90
> > >    do_syscall_64+0x5b/0x160
> > >    entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > >   RIP: 0033:0x7efc42e75e23
> > >   Code: 09 00 ba 9f 01 00 00 e8 ab 81 f4 ff 66 2e 0f 1f 84 00 00 00 00 00 90 83 3d 29 0a 2d 00 00 75 13 49 89 ca b8 11 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 34 c3 48 83 ec 08 e8 db d3 01 00 48 89 04 24
> > > 
> > > According to kernel bisection, this problem became visible due to commit
> > > f7f99100d8d9 which changes how struct pages are initialized.
> > > 
> > > Memblock layout affects the pfn ranges covered by node/zone. Consider
> > > that we have a VM with 2 NUMA nodes and each node has 4GB memory, and
> > > the default (no memmap= given) memblock layout is like below:
> > > 
> > >   MEMBLOCK configuration:
> > >    memory size = 0x00000001fff75c00 reserved size = 0x000000000300c000
> > >    memory.cnt  = 0x4
> > >    memory[0x0]     [0x0000000000001000-0x000000000009efff], 0x000000000009e000 bytes on node 0 flags: 0x0
> > >    memory[0x1]     [0x0000000000100000-0x00000000bffd6fff], 0x00000000bfed7000 bytes on node 0 flags: 0x0
> > >    memory[0x2]     [0x0000000100000000-0x000000013fffffff], 0x0000000040000000 bytes on node 0 flags: 0x0
> > >    memory[0x3]     [0x0000000140000000-0x000000023fffffff], 0x0000000100000000 bytes on node 1 flags: 0x0
> > >    ...
> > > 
> > > If you give memmap=1G!4G (so it just covers memory[0x2]),
> > > the range [0x100000000-0x13fffffff] is gone:
> > > 
> > >   MEMBLOCK configuration:
> > >    memory size = 0x00000001bff75c00 reserved size = 0x000000000300c000
> > >    memory.cnt  = 0x3
> > >    memory[0x0]     [0x0000000000001000-0x000000000009efff], 0x000000000009e000 bytes on node 0 flags: 0x0
> > >    memory[0x1]     [0x0000000000100000-0x00000000bffd6fff], 0x00000000bfed7000 bytes on node 0 flags: 0x0
> > >    memory[0x2]     [0x0000000140000000-0x000000023fffffff], 0x0000000100000000 bytes on node 1 flags: 0x0
> > >    ...
> > > 
> > > This causes shrinking node 0's pfn range because it is calculated by
> > > the address range of memblock.memory. So some of struct pages in the
> > > gap range are left uninitialized.
> > > 
> > > We have a function zero_resv_unavail() which does zeroing the struct
> > > pages within the reserved unavailable range (i.e. memblock.memory &&
> > > !memblock.reserved). This patch utilizes it to cover all unavailable
> > > ranges by putting them into memblock.reserved.
> > > 
> > > Fixes: f7f99100d8d9 ("mm: stop zeroing memory during allocation in vmemmap")
> > > Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > > Tested-by: Oscar Salvador <osalvador@xxxxxxx>
> > 
> > OK, this makes sense to me. It is definitely much better than the
> > original attempt.
> > 
> > Unless I am missing something this should be correct
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> 
> First of all thank you Naoya for finding and root causing this issue.
> 
> So, with this fix we reserve any hole and !E820_TYPE_RAM or
> !E820_TYPE_RESERVED_KERN in e820.  I think, this will work because we
> do pfn_valid() check in zero_resv_unavail(), so the ranges that do not have
> backing struct pages will be skipped. But, I am worried on the performance
> implications of when the holes of invalid memory are rather large. We would
> have to loop through it in zero_resv_unavail() one pfn at a time.
> 
> Therefore, we might also need to optimize zero_resv_unavail() a little like
> this:
> 
> 6407			if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages)))
> 6408				continue;
> 
> Add before "continue":
> 	pfn = ALIGN_DOWN(pfn, pageblock_nr_pages) + pageblock_nr_pageas - 1.
> At least, this way, we would skip a section of invalid memory at a time.
> 
> For the patch above:
> Reviewed-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> 
> But, I think the 2nd patch with the optimization above should go along this
> this fix.

Hi Pavel,

I think this makes a lot of sense.
Since Naoya is out until Wednesday, maybe I give it a shot next week and see if I can gather some numbers.

> 
> Thank you,
> Pasha
> 
> > 
> > > ---
> > >  arch/x86/kernel/e820.c | 15 ++++++++++++---
> > >  1 file changed, 12 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> > > index d1f25c831447..c88c23c658c1 100644
> > > --- a/arch/x86/kernel/e820.c
> > > +++ b/arch/x86/kernel/e820.c
> > > @@ -1248,6 +1248,7 @@ void __init e820__memblock_setup(void)
> > >  {
> > >  	int i;
> > >  	u64 end;
> > > +	u64 addr = 0;
> > >  
> > >  	/*
> > >  	 * The bootstrap memblock region count maximum is 128 entries
> > > @@ -1264,13 +1265,21 @@ void __init e820__memblock_setup(void)
> > >  		struct e820_entry *entry = &e820_table->entries[i];
> > >  
> > >  		end = entry->addr + entry->size;
> > > +		if (addr < entry->addr)
> > > +			memblock_reserve(addr, entry->addr - addr);
> > > +		addr = end;
> > >  		if (end != (resource_size_t)end)
> > >  			continue;
> > >  
> > > +		/*
> > > +		 * all !E820_TYPE_RAM ranges (including gap ranges) are put
> > > +		 * into memblock.reserved to make sure that struct pages in
> > > +		 * such regions are not left uninitialized after bootup.
> > > +		 */
> > >  		if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
> > > -			continue;
> > > -
> > > -		memblock_add(entry->addr, entry->size);
> > > +			memblock_reserve(entry->addr, entry->size);
> > > +		else
> > > +			memblock_add(entry->addr, entry->size);
> > >  	}
> > >  
> > >  	/* Throw away partial pages: */
> > > -- 
> > > 2.7.4
> > 
> > -- 
> > Michal Hocko
> > SUSE Labs
> > 
> 

Best Regards
Oscar Salvador