Messages in this thread | | | Subject | Re: [PATCH v2] mm/ksm: ignore STABLE_FLAG of rmap_item->address in rmap_walk_ksm | From | Suzuki K Poulose <> | Date | Mon, 14 May 2018 10:45:02 +0100 |
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On 10/05/18 00:31, Andrew Morton wrote: > On Fri, 4 May 2018 11:11:46 +0800 Jia He <hejianet@gmail.com> wrote: > >> In our armv8a server(QDF2400), I noticed lots of WARN_ON caused by PAGE_SIZE >> unaligned for rmap_item->address under memory pressure tests(start 20 guests >> and run memhog in the host). >> >> ... >> >> In rmap_walk_ksm, the rmap_item->address might still have the STABLE_FLAG, >> then the start and end in handle_hva_to_gpa might not be PAGE_SIZE aligned. >> Thus it will cause exceptions in handle_hva_to_gpa on arm64. >> >> This patch fixes it by ignoring(not removing) the low bits of address when >> doing rmap_walk_ksm. >> >> Signed-off-by: jia.he@hxt-semitech.com > > I assumed you wanted this patch to be committed as > From:jia.he@hxt-semitech.com rather than From:hejianet@gmail.com, so I > made that change. Please let me know if this was inappropriate. > > You can do this yourself by adding an explicit From: line to the very > start of the patch's email text. > > Also, a storm of WARN_ONs is pretty poor behaviour. Is that the only > misbehaviour which this bug causes? Do you think the fix should be > backported into earlier kernels? >
I think its just not the WARN_ON(). We do more than what is probably intended with an unaligned address. i.e, We could be modifying the flags for other pages that were not affected.
e.g :
In the original report [0], the trace looked like :
[ 800.511498] [<ffff0000080b4f2c>] kvm_age_hva_handler+0xcc/0xd4 [ 800.517324] [<ffff0000080b4838>] handle_hva_to_gpa+0xec/0x15c [ 800.523063] [<ffff0000080b6c5c>] kvm_age_hva+0x5c/0xcc [ 800.528194] [<ffff0000080a7c3c>] kvm_mmu_notifier_clear_flush_young+0x54/0x90 [ 800.535324] [<ffff00000827a0e8>] __mmu_notifier_clear_flush_young+0x6c/0xa8 [ 800.542279] [<ffff00000825a644>] page_referenced_one+0x1e0/0x1fc [ 800.548279] [<ffff00000827e8f8>] rmap_walk_ksm+0x124/0x1a0 [ 800.553759] [<ffff00000825c974>] rmap_walk+0x94/0x98 [ 800.558717] [<ffff00000825ca98>] page_referenced+0x120/0x180 [ 800.564369] [<ffff000008228c58>] shrink_active_list+0x218/0x4a4 [ 800.570281] [<ffff000008229470>] shrink_node_memcg+0x58c/0x6fc [ 800.576107] [<ffff0000082296c4>] shrink_node+0xe4/0x328 [ 800.581325] [<ffff000008229c9c>] do_try_to_free_pages+0xe4/0x3b8 [ 800.587324] [<ffff00000822a094>] try_to_free_pages+0x124/0x234 [ 800.593150] [<ffff000008216aa0>] __alloc_pages_nodemask+0x564/0xf7c [ 800.599412] [<ffff000008292814>] khugepaged_alloc_page+0x38/0xb8 [ 800.605411] [<ffff0000082933bc>] collapse_huge_page+0x74/0xd70 [ 800.611238] [<ffff00000829470c>] khugepaged_scan_mm_slot+0x654/0xa98 [ 800.617585] [<ffff000008294e0c>] khugepaged+0x2bc/0x49c [ 800.622803] [<ffff0000080ffb70>] kthread+0x124/0x150 [ 800.627762] [<ffff0000080849f0>] ret_from_fork+0x10/0x1c [ 800.633066] ---[ end trace 944c130b5252fb01 ]---
Now, the ksm wants to mark *a page* as referenced via page_referenced_one(), passing it an unaligned address. This could eventually turn out to be one of :
ptep_clear_flush_young_notify(address, address + PAGE_SIZE)
or
pmdp_clear_flush_young_notify(address, address + PMD_SIZE)
which now spans two pages/pmds and the notifier consumer might take an action on the second page as well, which is not something intended. So, I do think that old behavior is wrong and has other side effects as mentioned above.
[0] https://lkml.kernel.org/r/1525244911-5519-1-git-send-email-hejianet@gmail.com
Suzuki
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