Re: [PATCH 09/10] mm: refactor vma_merge() into modify-only vma_merge_modified()

Petr Tesařík
2024-08-06T13:42:51Z

On Mon, 5 Aug 2024 13:13:56 +0100
Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:

> The existing vma_merge() function is no longer required to handle what were
> previously referred to as cases 1-3 (i.e. the merging of a new VMA), as
> this is now handled by vma_merge_new_vma().
>
> Additionally, we simplify the convoluted control flow of the original,
> maintaining identical logic only expressed more clearly and doing away with
> a complicated set of cases, rather logically examining each possible
> outcome - merging of both the previous and subsequent VMA, merging of the
> previous VMA and merging of the subsequent VMA alone.
>
> We now utilise the previously implemented commit_merge() function to share
> logic with vma_expand() deduplicating code and providing less surface area
> for bugs and confusion.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
> mm/vma.c | 474 +++++++++++++++++++++++++++----------------------------
> mm/vma.h | 6 -
> 2 files changed, 232 insertions(+), 248 deletions(-)
>
> diff --git a/mm/vma.c b/mm/vma.c
> index b7e3c64d5d68..c55ae035f5d6 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -569,8 +569,7 @@ static int commit_merge(struct vma_merge_struct *vmg,
> struct vm_area_struct *adjust,
> struct vm_area_struct *remove,
> struct vm_area_struct *remove2,
> - long adj_start,
> - bool expanded)
> + long adj_start, bool expanded)

Um. Oops? ;-)

Otherwise LGTM.

Petr T

> {
> struct vma_prepare vp;
>
> @@ -607,6 +606,236 @@ static int commit_merge(struct vma_merge_struct *vmg,
> return 0;
> }
>
> +/*
> + * vma_merge_modified - Attempt to merge VMAs based on a VMA having its
> + * attributes modified.
> + *
> + * @vmg: Describes the modifications being made to a VMA and associated
> + * metadata.
> + *
> + * When the attributes of a range within a VMA change, then it might be possible
> + * for immediately adjacent VMAs to be merged into that VMA due to having
> + * identical properties.
> + *
> + * This function checks for the existence of any such mergeable VMAs and updates
> + * the maple tree describing the @vmg->vma->vm_mm address space to account for
> + * this, as well as any VMAs shrunk/expanded/deleted as a result of this merge.
> + *
> + * As part of this operation, if a merge occurs, the @vmg object will have its
> + * vma, start, end, and pgoff fields modified to execute the merge. Subsequent
> + * calls to this function should reset these fields.
> + *
> + * Returns: The merged VMA if merge succeeds, or NULL otherwise.
> + *
> + * ASSUMPTIONS:
> + * - The caller must assign the VMA to be modifed to vmg->vma.
> + * - The caller must have set vmg->prev to the previous VMA, if there is one.
> + * - The caller does not need to set vmg->next, as we determine this.
> + * - The caller must hold a WRITE lock on the mm_struct->mmap_lock.
> + */
> +static struct vm_area_struct *vma_merge_modified(struct vma_merge_struct *vmg)
> +{
> + struct vm_area_struct *vma = vmg->vma;
> + struct vm_area_struct *prev = vmg->prev;
> + struct vm_area_struct *next, *res;
> + struct vm_area_struct *anon_dup = NULL;
> + struct vm_area_struct *adjust = NULL;
> + unsigned long start = vmg->start;
> + unsigned long end = vmg->end;
> + bool left_side = vma && start == vma->vm_start;
> + bool right_side = vma && end == vma->vm_end;
> + bool merge_will_delete_vma, merge_will_delete_next;
> + bool merge_left, merge_right;
> + bool merge_both = false;
> + int err = 0;
> + long adj_start = 0;
> +
> + VM_WARN_ON(!vma); /* We are modifying a VMA, so caller must specify. */
> + VM_WARN_ON(vmg->next); /* We set this. */
> + VM_WARN_ON(prev && start <= prev->vm_start);
> + VM_WARN_ON(start >= end);
> + /*
> + * If vma == prev, then we are offset into a VMA. Otherwise, if we are
> + * not, we must span a portion of the VMA.
> + */
> + VM_WARN_ON(vma && ((vma != prev && vmg->start != vma->vm_start) ||
> + vmg->end > vma->vm_end));
> +
> + /*
> + * If a special mapping or neither at the furthermost left or right side
> + * of the VMA, then we have no chance of merging and should abort.
> + *
> + * We later require that vma->vm_flags == vm_flags, so this tests
> + * vma->vm_flags & VM_SPECIAL, too.
> + */
> + if (vmg->flags & VM_SPECIAL || (!left_side && !right_side))
> + return NULL;
> +
> + if (left_side && prev && prev->vm_end == start && can_vma_merge_after(vmg)) {
> + merge_left = true;
> + vma_prev(vmg->vmi);
> + } else {
> + merge_left = false;
> + }
> +
> + if (right_side) {
> + next = vmg->next = vma_lookup(vma->vm_mm, end);
> +
> + /*
> + * We can merge right if there is a subsequent VMA, if it is
> + * immediately adjacent, and if it is compatible with vma.
> + */
> + merge_right = next && end == next->vm_start &&
> + can_vma_merge_before(vmg);
> +
> + /*
> + * We can only merge both if the anonymous VMA of the previous
> + * VMA is compatible with the anonymous VMA of the subsequent
> + * VMA.
> + *
> + * Otherwise, we default to merging only the left.
> + */
> + if (merge_left && merge_right)
> + merge_right = merge_both =
> + is_mergeable_anon_vma(prev->anon_vma,
> + next->anon_vma, NULL);
> + } else {
> + merge_right = false;
> + next = NULL;
> + }
> +
> + /* If we have nothing to merge, abort. */
> + if (!merge_left && !merge_right)
> + return NULL;
> +
> + /* If we span the entire VMA, a merge implies it will be deleted. */
> + merge_will_delete_vma = left_side && right_side;
> + /* If we merge both VMAs, then next is also deleted. */
> + merge_will_delete_next = merge_both;
> +
> + /* No matter what happens, we will be adjusting vma. */
> + vma_start_write(vma);
> +
> + if (merge_left)
> + vma_start_write(prev);
> +
> + if (merge_right)
> + vma_start_write(next);
> +
> + if (merge_both) {
> + /*
> + * |<----->|
> + * |-------*********-------|
> + * prev vma next
> + * extend delete delete
> + */
> +
> + vmg->vma = prev;
> + vmg->start = prev->vm_start;
> + vmg->end = next->vm_end;
> + vmg->pgoff = prev->vm_pgoff;
> +
> + /*
> + * We already ensured anon_vma compatibility above, so now it's
> + * simply a case of, if prev has no anon_vma object, which of
> + * next or vma contains the anon_vma we must duplicate.
> + */
> + err = dup_anon_vma(prev, next->anon_vma ? next : vma, &anon_dup);
> + } else if (merge_left) {
> + /*
> + * |<----->| OR
> + * |<--------->|
> + * |-------*************
> + * prev vma
> + * extend shrink/delete
> + */
> +
> + unsigned long end = vmg->end;
> +
> + vmg->vma = prev;
> + vmg->start = prev->vm_start;
> + vmg->pgoff = prev->vm_pgoff;
> +
> + if (merge_will_delete_vma) {
> + /*
> + * can_vma_merge_after() assumed we would not be
> + * removing vma, so it skipped the check for
> + * vm_ops->close, but we are removing vma.
> + */
> + if (vma->vm_ops && vma->vm_ops->close)
> + err = -EINVAL;
> + } else {
> + adjust = vma;
> + adj_start = end - vma->vm_start;
> + }
> +
> + if (!err)
> + err = dup_anon_vma(prev, vma, &anon_dup);
> + } else { /* merge_right */
> + /*
> + * |<----->| OR
> + * |<--------->|
> + * *************-------|
> + * vma next
> + * shrink/delete extend
> + */
> +
> + pgoff_t pglen = PHYS_PFN(vmg->end - vmg->start);
> +
> + VM_WARN_ON(!merge_right);
> + /* If we are offset into a VMA, then prev must be vma. */
> + VM_WARN_ON(vmg->start > vma->vm_start && prev && vma != prev);
> +
> + if (merge_will_delete_vma) {
> + vmg->vma = next;
> + vmg->end = next->vm_end;
> + vmg->pgoff = next->vm_pgoff - pglen;
> + } else {
> + /*
> + * We shrink vma and expand next.
> + *
> + * IMPORTANT: This is the ONLY case where the final
> + * merged VMA is NOT vmg->vma, but rather vmg->next.
> + */
> +
> + vmg->start = vma->vm_start;
> + vmg->end = start;
> + vmg->pgoff = vma->vm_pgoff;
> +
> + adjust = next;
> + adj_start = -(vma->vm_end - start);
> + }
> +
> + err = dup_anon_vma(next, vma, &anon_dup);
> + }
> +
> + if (err)
> + goto abort;
> +
> + if (commit_merge(vmg, adjust,
> + merge_will_delete_vma ? vma : NULL,
> + merge_will_delete_next ? next : NULL,
> + adj_start,
> + /*
> + * In nearly all cases, we expand vmg->vma. There is
> + * one exception - merge_right where we partially span
> + * the VMA. In this case we shrink the end of vmg->vma
> + * and adjust the start of vmg->next accordingly.
> + */
> + !merge_right || merge_will_delete_vma))
> + return NULL;
> +
> + res = merge_left ? prev : next;
> + khugepaged_enter_vma(res, vmg->flags);
> +
> + return res;
> +
> +abort:
> + vma_iter_set(vmg->vmi, start);
> + vma_iter_load(vmg->vmi);
> + return NULL;
> +}
> +
> /*
> * vma_merge_new_vma - Attempt to merge a new VMA into address space
> *
> @@ -1022,245 +1251,6 @@ int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
> return do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
> }
>
> -/*
> - * Given a mapping request (addr,end,vm_flags,file,pgoff,anon_name),
> - * figure out whether that can be merged with its predecessor or its
> - * successor. Or both (it neatly fills a hole).
> - *
> - * In most cases - when called for mmap, brk or mremap - [addr,end) is
> - * certain not to be mapped by the time vma_merge is called; but when
> - * called for mprotect, it is certain to be already mapped (either at
> - * an offset within prev, or at the start of next), and the flags of
> - * this area are about to be changed to vm_flags - and the no-change
> - * case has already been eliminated.
> - *
> - * The following mprotect cases have to be considered, where **** is
> - * the area passed down from mprotect_fixup, never extending beyond one
> - * vma, PPPP is the previous vma, CCCC is a concurrent vma that starts
> - * at the same address as **** and is of the same or larger span, and
> - * NNNN the next vma after ****:
> - *
> - * **** **** ****
> - * PPPPPPNNNNNN PPPPPPNNNNNN PPPPPPCCCCCC
> - * cannot merge might become might become
> - * PPNNNNNNNNNN PPPPPPPPPPCC
> - * mmap, brk or case 4 below case 5 below
> - * mremap move:
> - * **** ****
> - * PPPP NNNN PPPPCCCCNNNN
> - * might become might become
> - * PPPPPPPPPPPP 1 or PPPPPPPPPPPP 6 or
> - * PPPPPPPPNNNN 2 or PPPPPPPPNNNN 7 or
> - * PPPPNNNNNNNN 3 PPPPNNNNNNNN 8
> - *
> - * It is important for case 8 that the vma CCCC overlapping the
> - * region **** is never going to extended over NNNN. Instead NNNN must
> - * be extended in region **** and CCCC must be removed. This way in
> - * all cases where vma_merge succeeds, the moment vma_merge drops the
> - * rmap_locks, the properties of the merged vma will be already
> - * correct for the whole merged range. Some of those properties like
> - * vm_page_prot/vm_flags may be accessed by rmap_walks and they must
> - * be correct for the whole merged range immediately after the
> - * rmap_locks are released. Otherwise if NNNN would be removed and
> - * CCCC would be extended over the NNNN range, remove_migration_ptes
> - * or other rmap walkers (if working on addresses beyond the "end"
> - * parameter) may establish ptes with the wrong permissions of CCCC
> - * instead of the right permissions of NNNN.
> - *
> - * In the code below:
> - * PPPP is represented by *prev
> - * CCCC is represented by *curr or not represented at all (NULL)
> - * NNNN is represented by *next or not represented at all (NULL)
> - * **** is not represented - it will be merged and the vma containing the
> - * area is returned, or the function will return NULL
> - */
> -static struct vm_area_struct *vma_merge(struct vma_merge_struct *vmg)
> -{
> - struct mm_struct *mm = container_of(vmg->vmi->mas.tree, struct mm_struct, mm_mt);
> - struct vm_area_struct *prev = vmg->prev;
> - struct vm_area_struct *curr, *next, *res;
> - struct vm_area_struct *vma, *adjust, *remove, *remove2;
> - struct vm_area_struct *anon_dup = NULL;
> - struct vma_prepare vp;
> - pgoff_t vma_pgoff;
> - int err = 0;
> - bool merge_prev = false;
> - bool merge_next = false;
> - bool vma_expanded = false;
> - unsigned long addr = vmg->start;
> - unsigned long end = vmg->end;
> - unsigned long vma_start = addr;
> - unsigned long vma_end = end;
> - pgoff_t pglen = PHYS_PFN(end - addr);
> - long adj_start = 0;
> -
> - /*
> - * We later require that vma->vm_flags == vm_flags,
> - * so this tests vma->vm_flags & VM_SPECIAL, too.
> - */
> - if (vmg->flags & VM_SPECIAL)
> - return NULL;
> -
> - /* Does the input range span an existing VMA? (cases 5 - 8) */
> - curr = find_vma_intersection(mm, prev ? prev->vm_end : 0, end);
> -
> - if (!curr || /* cases 1 - 4 */
> - end == curr->vm_end) /* cases 6 - 8, adjacent VMA */
> - next = vmg->next = vma_lookup(mm, end);
> - else
> - next = vmg->next = NULL; /* case 5 */
> -
> - if (prev) {
> - vma_start = prev->vm_start;
> - vma_pgoff = prev->vm_pgoff;
> -
> - /* Can we merge the predecessor? */
> - if (addr == prev->vm_end && can_vma_merge_after(vmg)) {
> - merge_prev = true;
> - vma_prev(vmg->vmi);
> - }
> - }
> -
> - /* Can we merge the successor? */
> - if (next && can_vma_merge_before(vmg)) {
> - merge_next = true;
> - }
> -
> - /* Verify some invariant that must be enforced by the caller. */
> - VM_WARN_ON(prev && addr <= prev->vm_start);
> - VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> - VM_WARN_ON(addr >= end);
> -
> - if (!merge_prev && !merge_next)
> - return NULL; /* Not mergeable. */
> -
> - if (merge_prev)
> - vma_start_write(prev);
> -
> - res = vma = prev;
> - remove = remove2 = adjust = NULL;
> -
> - /* Can we merge both the predecessor and the successor? */
> - if (merge_prev && merge_next &&
> - is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL)) {
> - vma_start_write(next);
> - remove = next; /* case 1 */
> - vma_end = next->vm_end;
> - err = dup_anon_vma(prev, next, &anon_dup);
> - if (curr) { /* case 6 */
> - vma_start_write(curr);
> - remove = curr;
> - remove2 = next;
> - /*
> - * Note that the dup_anon_vma below cannot overwrite err
> - * since the first caller would do nothing unless next
> - * has an anon_vma.
> - */
> - if (!next->anon_vma)
> - err = dup_anon_vma(prev, curr, &anon_dup);
> - }
> - } else if (merge_prev) { /* case 2 */
> - if (curr) {
> - vma_start_write(curr);
> - if (end == curr->vm_end) { /* case 7 */
> - /*
> - * can_vma_merge_after() assumed we would not be
> - * removing prev vma, so it skipped the check
> - * for vm_ops->close, but we are removing curr
> - */
> - if (curr->vm_ops && curr->vm_ops->close)
> - err = -EINVAL;
> - remove = curr;
> - } else { /* case 5 */
> - adjust = curr;
> - adj_start = end - curr->vm_start;
> - }
> - if (!err)
> - err = dup_anon_vma(prev, curr, &anon_dup);
> - }
> - } else { /* merge_next */
> - vma_start_write(next);
> - res = next;
> - if (prev && addr < prev->vm_end) { /* case 4 */
> - vma_start_write(prev);
> - vma_end = addr;
> - adjust = next;
> - adj_start = -(prev->vm_end - addr);
> - err = dup_anon_vma(next, prev, &anon_dup);
> - } else {
> - /*
> - * Note that cases 3 and 8 are the ONLY ones where prev
> - * is permitted to be (but is not necessarily) NULL.
> - */
> - vma = next; /* case 3 */
> - vma_start = addr;
> - vma_end = next->vm_end;
> - vma_pgoff = next->vm_pgoff - pglen;
> - if (curr) { /* case 8 */
> - vma_pgoff = curr->vm_pgoff;
> - vma_start_write(curr);
> - remove = curr;
> - err = dup_anon_vma(next, curr, &anon_dup);
> - }
> - }
> - }
> -
> - /* Error in anon_vma clone. */
> - if (err)
> - goto anon_vma_fail;
> -
> - if (vma_start < vma->vm_start || vma_end > vma->vm_end)
> - vma_expanded = true;
> -
> - if (vma_expanded) {
> - vma_iter_config(vmg->vmi, vma_start, vma_end);
> - } else {
> - vma_iter_config(vmg->vmi, adjust->vm_start + adj_start,
> - adjust->vm_end);
> - }
> -
> - if (vma_iter_prealloc(vmg->vmi, vma))
> - goto prealloc_fail;
> -
> - init_multi_vma_prep(&vp, vma, adjust, remove, remove2);
> - VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
> - vp.anon_vma != adjust->anon_vma);
> -
> - vma_prepare(&vp);
> - vma_adjust_trans_huge(vma, vma_start, vma_end, adj_start);
> - vma_set_range(vma, vma_start, vma_end, vma_pgoff);
> -
> - if (vma_expanded)
> - vma_iter_store(vmg->vmi, vma);
> -
> - if (adj_start) {
> - adjust->vm_start += adj_start;
> - adjust->vm_pgoff += adj_start >> PAGE_SHIFT;
> - if (adj_start < 0) {
> - WARN_ON(vma_expanded);
> - vma_iter_store(vmg->vmi, next);
> - }
> - }
> -
> - vma_complete(&vp, vmg->vmi, mm);
> - khugepaged_enter_vma(res, vmg->flags);
> - return res;
> -
> -prealloc_fail:
> - if (anon_dup)
> - unlink_anon_vmas(anon_dup);
> -
> -anon_vma_fail:
> - vma_iter_set(vmg->vmi, addr);
> - vma_iter_load(vmg->vmi);
> - return NULL;
> -}
> -
> -struct vm_area_struct *vma_merge_modified(struct vma_merge_struct *vmg)
> -{
> - return vma_merge(vmg);
> -}
> -
> /*
> * We are about to modify one or multiple of a VMA's flags, policy, userfaultfd
> * context and anonymous VMA name within the range [start, end).
> @@ -1280,7 +1270,7 @@ static struct vm_area_struct *vma_modify(struct vma_merge_struct *vmg)
> struct vm_area_struct *merged;
>
> /* First, try to merge. */
> - merged = vma_merge(vmg);
> + merged = vma_merge_modified(vmg);
> if (merged)
> return merged;
>
> diff --git a/mm/vma.h b/mm/vma.h
> index bbb173053f34..bf29ff569a3d 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -110,12 +110,6 @@ struct vm_area_struct
>
> struct vm_area_struct *vma_merge_new_vma(struct vma_merge_struct *vmg);
>
> -/*
> - * Temporary wrapper around vma_merge() so we can have a common interface for
> - * tests.
> - */
> -struct vm_area_struct *vma_merge_modified(struct vma_merge_struct *vmg);
> -
> struct vm_area_struct *vma_merge_extend(struct vma_iterator *vmi,
> struct vm_area_struct *vma,
> unsigned long delta);