> On Nov 5, 2021, at 1:38 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > From: Nadav Amit <namit@xxxxxxxxxx> > Subject: mm/memory.c: use correct VMA flags when freeing page-tables > > Consistent use of the mmu_gather interface requires a call to > tlb_start_vma() and tlb_end_vma() for each VMA. free_pgtables() does not > follow this pattern. > > Certain architectures need tlb_start_vma() to be called in order for > tlb_update_vma_flags() to update the VMA flags (tlb->vma_exec and > tlb->vma_huge), which are later used for the proper TLB flush to be > issued. Since tlb_start_vma() is not called, this can lead to the wrong > VMA flags being used when the flush is performed. > > Specifically, the munmap syscall would call unmap_region(), which unmaps > the VMAs and then frees the page-tables. A flush is needed after the > page-tables are removed to prevent page-walk caches from holding stale > entries, but this flush would use the flags of the VMA flags of the last > VMA that was flushed. This does not appear to be right. > > Use tlb_start_vma() and tlb_end_vma() to prevent this from happening. > This might lead to unnecessary calls to flush_cache_range() on certain > arch's. If needed, a new flag can be added to mmu_gather to indicate that > the flush is not needed. Hugh correctly indicated that I made a silly bug, and this patch is not healping. Nothing would explode, but I assumed the patch would be dropped for me to submit v2. I’ll send a fix to this fix instead unless it is dropped.
