Re: [PATCH] mm/page_alloc: Add a reason for reserved pages in has_unmovable_pages()

[Michal Hocko]

[Cc Ralph]

On Tue 14-01-20 13:49:14, Anshuman Khandual wrote:
> 
> 
> On 10/04/2019 01:55 PM, David Hildenbrand wrote:
> > On 03.10.19 14:14, Qian Cai wrote:
> >>
> >>
> >>> On Oct 3, 2019, at 8:01 AM, Anshuman Khandual <Anshuman.Khandual@xxxxxxx> wrote:
> >>>
> >>> Will something like this be better ?
> >>
> >> Not really. dump_page() will dump PageCompound information anyway, so it is trivial to figure out if went in that path.
> >>
> > 
> > I agree, I use the dump_page() output frequently to identify PG_reserved
> > pages. No need to duplicate that.
> 
> Here in this path there is a reserved page which is preventing
> offlining a memory section but unfortunately dump_page() does
> not print page->flags for a reserved page pinned there possibly
> through memblock_reserve() during boot.
> 
> __offline_pages()
> 	start_isolate_page_range()
> 		set_migratetype_isolate()
> 			has_unmovable_pages()
> 				dump_page() 
> 
> [   64.920970] ------------[ cut here ]------------
> [   64.921718] WARNING: CPU: 16 PID: 1116 at mm/page_alloc.c:8298 has_unmovable_pages+0x274/0x2a8
> [   64.923110] Modules linked in:
> [   64.923634] CPU: 16 PID: 1116 Comm: bash Not tainted 5.5.0-rc6-00006-gca544f2a11ae-dirty #281
> [   64.925102] Hardware name: linux,dummy-virt (DT)
> [   64.925905] pstate: 60400085 (nZCv daIf +PAN -UAO)
> [   64.926742] pc : has_unmovable_pages+0x274/0x2a8
> [   64.927554] lr : has_unmovable_pages+0x298/0x2a8
> [   64.928359] sp : ffff800014fd3a00
> [   64.928944] x29: ffff800014fd3a00 x28: fffffe0017640000 
> [   64.929875] x27: 0000000000000000 x26: ffff0005fcfcda00 
> [   64.930810] x25: 0000000000640000 x24: 0000000000000003 
> [   64.931736] x23: 0000000019840000 x22: 0000000000001380 
> [   64.932667] x21: ffff800011259000 x20: ffff0005fcfcda00 
> [   64.933588] x19: 0000000000661000 x18: 0000000000000010 
> [   64.934514] x17: 0000000000000000 x16: 0000000000000000 
> [   64.935454] x15: ffffffffffffffff x14: ffff8000118498c8 
> [   64.936377] x13: ffff800094fd3797 x12: ffff800014fd379f 
> [   64.937304] x11: ffff800011861000 x10: ffff800014fd3720 
> [   64.938226] x9 : 00000000ffffffd0 x8 : ffff8000106a60d0 
> [   64.939156] x7 : 0000000000000000 x6 : ffff0005fc6261b0 
> [   64.940078] x5 : ffff0005fc6261b0 x4 : 0000000000000000 
> [   64.941003] x3 : ffff0005fc62cf80 x2 : ffffffffffffec80 
> [   64.941927] x1 : ffff800011141b58 x0 : ffff0005fcfcda00 
> [   64.942857] Call trace:
> [   64.943298]  has_unmovable_pages+0x274/0x2a8
> [   64.944056]  start_isolate_page_range+0x258/0x360
> [   64.944879]  __offline_pages+0xf4/0x9e8
> [   64.945554]  offline_pages+0x10/0x18
> [   64.946189]  memory_block_action+0x40/0x1a0
> [   64.946929]  memory_subsys_offline+0x4c/0x78
> [   64.947679]  device_offline+0x98/0xc8
> [   64.948328]  unprobe_store+0xa8/0x158
> [   64.948976]  dev_attr_store+0x14/0x28
> [   64.949628]  sysfs_kf_write+0x40/0x50
> [   64.950273]  kernfs_fop_write+0x108/0x218
> [   64.950983]  __vfs_write+0x18/0x40
> [   64.951592]  vfs_write+0xb0/0x1d0
> [   64.952175]  ksys_write+0x64/0xe8
> [   64.952761]  __arm64_sys_write+0x18/0x20
> [   64.953451]  el0_svc_common.constprop.2+0x88/0x150
> [   64.954293]  el0_svc_handler+0x20/0x80
> [   64.954963]  el0_sync_handler+0x118/0x188
> [   64.955669]  el0_sync+0x140/0x180
> [   64.956256] ---[ end trace b162b4d1cbea304d ]---
> [   64.957063] page:fffffe0017640000 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0
> [   64.958489] raw: 1ffff80000001000 fffffe0017640008 fffffe0017640008 0000000000000000
> [   64.959839] raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
> [   64.961174] page dumped because: unmovable page
> 
> The reason is dump_page() does not print page->flags universally
> and only does so for KSM, Anon and File pages while excluding
> reserved pages at boot. Wondering should not we make printing
> page->flags universal ?

We used to do that and this caught me as a surprise when looking again.
This is a result of 76a1850e4572 ("mm/debug.c: __dump_page() prints an
extra line") which is a cleanup patch and I suspect this result was not
anticipated.

The following will do the trick but I cannot really say I like the code
duplication. pr_cont in this case sounds like a much cleaner solution to
me.

diff --git a/mm/debug.c b/mm/debug.c
index 0461df1207cb..4cbf30d03c88 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -89,6 +89,8 @@ void __dump_page(struct page *page, const char *reason)
 		} else
 			pr_warn("%ps\n", mapping->a_ops);
 		pr_warn("flags: %#lx(%pGp)\n", page->flags, &page->flags);
+	} else {
+		pr_warn("flags: %#lx(%pGp)\n", page->flags, &page->flags);
 	}
 	BUILD_BUG_ON(ARRAY_SIZE(pageflag_names) != __NR_PAGEFLAGS + 1);
 

-- 
Michal Hocko
SUSE Labs