On Wed, Mar 14, 2018 at 04:57:29PM +0530, Chintan Pandya wrote: > > > On 3/14/2018 4:23 PM, Mark Rutland wrote: > > On Wed, Mar 14, 2018 at 02:18:24PM +0530, Chintan Pandya wrote: > > > While setting huge page, we need to take care of > > > previously existing next level mapping. Since, > > > we are going to overrite previous mapping, the > > > only reference to next level page table will get > > > lost and the next level page table will be zombie, > > > occupying space forever. So, free it before > > > overriding. > > > > > @@ -939,6 +940,9 @@ int pud_set_huge(pud_t *pudp, phys_addr_t phys, pgprot_t prot) > > > return 0; > > > BUG_ON(phys & ~PUD_MASK); > > > + if (pud_val(*pud) && !pud_huge(*pud)) > > > + free_page((unsigned long)__va(pud_val(*pud))); > > > + > > > set_pud(pudp, pfn_pud(__phys_to_pfn(phys), sect_prot)); > > > return 1; > > > } > > > @@ -953,6 +957,9 @@ int pmd_set_huge(pmd_t *pmdp, phys_addr_t phys, pgprot_t prot) > > > return 0; > > > BUG_ON(phys & ~PMD_MASK); > > > + if (pmd_val(*pmd) && !pmd_huge(*pmd)) > > > + free_page((unsigned long)__va(pmd_val(*pmd))); > > > + > > > > As Marc noted, (assuming the subsequent revert is applied) in both of > > these cases, these tables are still live, and thus it is not safe to > > free them. > > > > Consider that immediately after freeing the pages, they may get > > re-allocated elsewhere, where they may be modified. If this happens > > before TLB invalidation, page table walks may allocate junk into TLBs, > > resulting in a number of problems. > Ah okay. What about this sequence, > 1) I store old PMD/PUD values > 2) Update the PMD/PUD with section mapping > 3) Invalidate TLB > 4) Then free the *leaked* page You must invalidate the TLB *before* setting the new entry: 1) store the old entry value 2) clear the entry 3) invalidate the TLB ... then you can either: 4) update the entry 5) free the old table ... or: 4) free the old table 5) update the entry Thanks, Mark.
