On Mon, May 22, 2017 at 10:11:51AM -0400, Jerome Glisse wrote: > On Mon, May 22, 2017 at 04:12:15PM +0300, Kirill A. Shutemov wrote: > > 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. > > Well then how do i avoid the BUG_ON above ? Because the init_mm pgd is > clear but none of the stall entry in any other mm. So if i unplug memory > and replug memory at exact same address it tries to allocate new p4d/pud > for struct page area and then when sync_global_pgds() is call it goes > over the list of pgd and BUG_ON() : > > if (!p4d_none(*p4d_ref) && !p4d_none(*p4d)) > BUG_ON(p4d_page_vaddr(*p4d) != p4d_page_vaddr(*p4d_ref)); > > > So to me either above check need to go and we should overwritte pgd no > matter what or we should restore previous behavior. I don't mind either > one. I would prefer to drop the BUG_ON. Ingo? -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
