On Mon, Jul 16, 2018 at 02:29:18PM +0200, Michal Hocko wrote: > On Mon 16-07-18 08:09:19, Pavel Tatashin wrote: > > > > > > On 07/16/2018 08:06 AM, Michal Hocko wrote: > > > On Sat 14-07-18 09:39:29, Pavel Tatashin wrote: > > > [...] > > >> From 95259841ef79cc17c734a994affa3714479753e3 Mon Sep 17 00:00:00 2001 > > >> From: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> > > >> Date: Sat, 14 Jul 2018 09:15:07 -0400 > > >> Subject: [PATCH] mm: zero unavailable pages before memmap init > > >> > > >> We must zero struct pages for memory that is not backed by physical memory, > > >> or kernel does not have access to. > > >> > > >> Recently, there was a change which zeroed all memmap for all holes in e820. > > >> Unfortunately, it introduced a bug that is discussed here: > > >> > > >> https://www.spinics.net/lists/linux-mm/msg156764.html > > >> > > >> Linus, also saw this bug on his machine, and confirmed that pulling > > >> commit 124049decbb1 ("x86/e820: put !E820_TYPE_RAM regions into memblock.reserved") > > >> fixes the issue. > > >> > > >> The problem is that we incorrectly zero some struct pages after they were > > >> setup. > > > > > > I am sorry but I simply do not see it. zero_resv_unavail should be > > > touching only reserved memory ranges and those are not initialized > > > anywhere. So who has reused them and put them to normal available > > > memory to be initialized by free_area_init_node[s]? > > > > > > The patch itself should be safe because reserved and available memory > > > ranges should be disjoint so the ordering shouldn't matter. The fact > > > that it matters is the crux thing to understand and document. So the > > > change looks good to me but I do not understand _why_ it makes any > > > difference. There must be somebody to put (memblock) reserved memory > > > available to the page allocator behind our backs. > > > > Thats exactly right, and I am also not sure why this is happening, > > there must be some overlapping happening that just should not. I will > > study it later. > > Maybe a stupid question, but I do not see it from the code (this init > code is just to complex to keep it cached in head so I always have to > study the code again and again, sigh). So what exactly prevents > memmap_init_zone to stumble over reserved regions? We do play some ugly > games to find a first !reserved pfn in the node but I do not really see > anything in the init path to properly skip over reserved holes inside > the node. I think we are not really skiping reserved regions in memmap_init_zone(). memmap_init_zone() gets just called being size the subtract of zone_end_pfn - zone_start_pfn, and I don't see that we're checking if those pfn's fall in reserved regions. To get a better insight, I just put a couple of printk's: kernel: zero_resv_unavail: start-end: 0x9f000-0x100000 kernel: zero_resv_unavail: pfn: 0x9f kernel: zero_resv_unavail: pfn: 0xa0 kernel: zero_resv_unavail: pfn: 0xa1 kernel: zero_resv_unavail: pfn: 0xa2 kernel: zero_resv_unavail: pfn: 0xa3 kernel: zero_resv_unavail: pfn: 0xa4 kernel: zero_resv_unavail: pfn: 0xa5 kernel: zero_resv_unavail: pfn: 0xa6 kernel: zero_resv_unavail: pfn: 0xa7 kernel: zero_resv_unavail: pfn: 0xa8 kernel: zero_resv_unavail: pfn: 0xa9 ... ... kernel: memmap_init_zone: pfn: 9f kernel: memmap_init_zone: pfn: a0 kernel: memmap_init_zone: pfn: a1 kernel: memmap_init_zone: pfn: a2 kernel: memmap_init_zone: pfn: a3 kernel: memmap_init_zone: pfn: a4 kernel: memmap_init_zone: pfn: a5 kernel: memmap_init_zone: pfn: a6 kernel: memmap_init_zone: pfn: a7 kernel: memmap_init_zone: pfn: a8 kernel: memmap_init_zone: pfn: a9 kernel: memmap_init_zone: pfn: aa kernel: memmap_init_zone: pfn: ab kernel: memmap_init_zone: pfn: ac kernel: memmap_init_zone: pfn: ad kernel: memmap_init_zone: pfn: ae kernel: memmap_init_zone: pfn: af kernel: memmap_init_zone: pfn: b0 kernel: memmap_init_zone: pfn: b1 kernel: memmap_init_zone: pfn: b2 The printk from memmap_init_zone has already passed the checks about early_pfn_ etc. So, reverting Pavel's fix would twist this, and we'd end up zeroing pages that are already set up in memmap_init_zone() (as we already had). -- Oscar Salvador SUSE L3
