Messages in this thread | | | Date | Mon, 22 May 2017 16:12:15 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH] x86/mm: pgds getting out of sync after memory hot remove |
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On Fri, May 19, 2017 at 02:01:26PM -0400, Jérôme Glisse wrote: > After memory hot remove it seems we do not synchronize pgds for kernel > virtual memory range (on vmemmap_free()). This seems bogus to me as it > means we are left with stall entry for process with mm != mm_init > > Yet i am puzzle by the fact that i am only now hitting this issue. It > never was an issue with 4.12 or before ie HMM never triggered following > BUG_ON inside sync_global_pgds(): > > if (!p4d_none(*p4d_ref) && !p4d_none(*p4d)) > BUG_ON(p4d_page_vaddr(*p4d) != p4d_page_vaddr(*p4d_ref)); > > > It seems that Kirill 5 level page table changes play a role in this > behavior change. I could not bisect because HMM is painfull to rebase > for each bisection step so that is just my best guess. > > > Am i missing something here ? Am i wrong in assuming that should sync > pgd on vmemmap_free() ? If so anyone have a good guess on why i am now > seeing the above BUG_ON ?
What would we gain by syncing pgd on free? Stale pgds are fine as long as they are not referenced (use-after-free case). Syncing is addtional work.
See af2cf278ef4f ("x86/mm/hotplug: Don't remove PGD entries in remove_pagetable()") and 5372e155a28f ("x86/mm: Drop unused argument 'removed' from sync_global_pgds()").
-- Kirill A. Shutemov
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