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Howlett wrote: > > * Lorenzo Stoakes [240708 08:18]: > > > On Thu, Jul 04, 2024 at 02:27:15PM GMT, Liam R. Howlett wrote: > > > > From: "Liam R. Howlett" > > > > > > > > Instead of zeroing the vma tree and then overwriting the area, let the > > > > area be overwritten and then clean up the gathered vmas using > > > > vms_complete_munmap_vmas(). > > > > > > > > In the case of a driver mapping over existing vmas, the PTEs are cleared > > > > using the helper vms_complete_pte_clear(). > > > > > > > > Temporarily keep track of the number of pages that will be removed and > > > > reduce the charged amount. > > > > > > > > This also drops the validate_mm() call in the vma_expand() function. > > > > It is necessary to drop the validate as it would fail since the mm > > > > map_count would be incorrect during a vma expansion, prior to the > > > > cleanup from vms_complete_munmap_vmas(). > > > > > > > > Signed-off-by: Liam R. Howlett > > > > --- > > > > mm/internal.h | 1 + > > > > mm/mmap.c | 61 ++++++++++++++++++++++++++++++--------------------- > > > > 2 files changed, 37 insertions(+), 25 deletions(-) > > > > > > > > diff --git a/mm/internal.h b/mm/internal.h > > > > index 4c9f06669cc4..fae4a1bba732 100644 > > > > --- a/mm/internal.h > > > > +++ b/mm/internal.h > > > > @@ -1503,6 +1503,7 @@ struct vma_munmap_struct { > > > > unsigned long stack_vm; > > > > unsigned long data_vm; > > > > bool unlock; /* Unlock after the munmap */ > > > > + bool cleared_ptes; /* If the PTE are cleared already */ > > > > }; > > > > > > > > void __meminit __init_single_page(struct page *page, unsigned long pfn, > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > > index 5d458c5f080e..0c334eeae8cd 100644 > > > > --- a/mm/mmap.c > > > > +++ b/mm/mmap.c ... > > > > @@ -522,6 +526,7 @@ static inline void init_vma_munmap(struct vma_munmap_struct *vms, > > > > vms->exec_vm = vms->stack_vm = vms->data_vm = 0; > > > > vms->unmap_start = FIRST_USER_ADDRESS; > > > > vms->unmap_end = USER_PGTABLES_CEILING; > > > > + vms->cleared_ptes = false; > > > > } > > > > > > > > /* > > > > @@ -730,7 +735,6 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma, > > > > vma_iter_store(vmi, vma); > > > > > > > > vma_complete(&vp, vmi, vma->vm_mm); > > > > - validate_mm(vma->vm_mm); > > > > > > Since we're dropping this here, do we need to re-add this back somehwere > > > where we are confident the state will be consistent? > > > > The vma_expand() function is used in two places - one is in the mmap.c > > file which can no longer validate the mm until the munmap is complete. > > The other is in fs/exec.c which cannot call the validate_mm(). So > > to add this call back, I'd have to add a wrapper to vma_expand() to call > > the validate_mm() function for debug builds. > > > > Really all this code in fs/exec.c doesn't belong there so we don't need > > to do an extra function wrapper just to call validate_mm(). And you have > > a patch to do that which is out for review! > > Indeed :) perhaps we should add back to the wrapper? > ... > > > > > > > + if (!may_expand_vm(mm, vm_flags, (len >> PAGE_SHIFT) - nr_pages)) > > > > + return -ENOMEM; > > > > > > > > if (unlikely(!can_modify_mm(mm, addr, end))) > > > > return -EPERM; > > > > @@ -2971,14 +2974,12 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > > > > if (vms_gather_munmap_vmas(&vms, &mas_detach)) > > > > return -ENOMEM; > > > > > > > > - if (vma_iter_clear_gfp(&vmi, addr, end, GFP_KERNEL)) > > > > - return -ENOMEM; > > > > - > > > > - vms_complete_munmap_vmas(&vms, &mas_detach); > > > > next = vms.next; > > > > prev = vms.prev; > > > > vma = NULL; > > > > } else { > > > > + /* Minimal setup of vms */ > > > > + vms.nr_pages = 0; > > > ... > > > > @@ -3052,6 +3053,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > > > > > > > > if (file) { > > > > vma->vm_file = get_file(file); > > > > + /* call_mmap() map PTE, so ensure there are no existing PTEs */ ... > > > > + if (vms.nr_pages) > > > > + vms_complete_pte_clear(&vms, &mas_detach, true); > > > > error = call_mmap(file, vma); > > > > if (error) > > > > goto unmap_and_free_vma; > > > > @@ -3142,6 +3146,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > > > > expanded: > > > > perf_event_mmap(vma); > > > > > > > > + if (vms.nr_pages) > > > > + vms_complete_munmap_vmas(&vms, &mas_detach); > > > > + > > > > > > Hang on, if we already did this in the if (file) branch above, might we end > > > up calling this twice? I didn't see vms.nr_pages get set to zero or > > > decremented anywhere (unless I missed it)? > > > > No, we called the new helper vms_complete_pte_clear(), which will avoid > > clearing the ptes by the added flag vms->cleared_ptes in the second > > call. > > > > Above, I modified vms_complete_pte_clear() to check vms->cleared_ptes > > prior to clearing the ptes, so it will only be cleared if it needs > > clearing. > > > > I debated moving this nr_pages check within vms_complete_munmap_vmas(), > > but that would add an unnecessary check to the munmap() path. Avoiding > > both checks seemed too much code (yet another static inline, or such). > > I also wanted to keep the sanity of nr_pages checking to a single > > function - as you highlighted it could be a path to insanity. > > > > Considering I'll switch this ti a VMS_INIT(), I think that I could pass > > it through and do the logic within the static inline at the expense of > > the munmap() having a few extra instructions (but no cache hits, so not > > a really big deal). > > Yeah it's a bit confusing that the rest of vms_complete_munmap_vmas() is > potentially run twice even if the vms_complete_pte_clear() exits early due > to vms->cleared_ptes being set. vms_complete_munmap_vmas() is never run twice, it's only ever run once. vms_complete_pte_clear() is called from vms_complete_munmap_vmas(), but will do nothing if cleared_ptes == true, which is set at the end of the pte_clear() itself, and initialized as false. Hopefully this becomes more obvious with the change to an INIT_VMS() paradigm. I think I'll change the name of vms_complete_pte_clear() in an attempt to make this more obvious as well (remove the _complete, probably). Thanks, Liam