Re: [PATCH 4/7] mm/mremap: initial refactor of move_vma()

From: Lorenzo Stoakes
Date: Wed Mar 05 2025 - 15:02:06 EST

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On Wed, Mar 05, 2025 at 02:20:37PM -0500, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [250303 06:08]:
> > Update move_vma() to use the threaded VRM object, de-duplicate code and
> > separate into smaller functions to aid readability and debug-ability.
> >
> > This in turn allows further simplification of expand_vma() as we can simply
> > thread VRM through the function.
> >
> > We also take the opportunity to abstract the account charging page count
> > into the VRM in order that we can correctly thread this through the
> > operation.
> >
> > We additionally do the same for tracking mm statistics - exec_vm, stack_vm,
> > data_vm, and locked_vm.
> >
> > As part of this change, we slightly modify when locked pages statistics are
> > counted for in mm_struct statistics. However this should cause no issues,
> > as there is no chance of underflow, nor will any rlimit failures occur as a
> > result.
> >
> > This is an intermediate step before a further refactoring of move_vma() in
> > order to aid review.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

Thanks!

>
> > ---
> > mm/mremap.c | 186 ++++++++++++++++++++++++++++++++++------------------
> > 1 file changed, 122 insertions(+), 64 deletions(-)
> >
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index 7f0c71aa9bb9..fdbf5515fc44 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -67,6 +67,7 @@ struct vma_remap_struct {
> > bool locked; /* Was the VMA mlock()'d (has the VM_LOCKED flag set). */
> > enum mremap_operation remap_type; /* expand, shrink, etc. */
> > bool mmap_locked; /* Is current->mm currently write-locked? */
> > + unsigned long charged; /* If VM_ACCOUNT, # pages to account. */
> > };
> >
> > static pud_t *get_old_pud(struct mm_struct *mm, unsigned long addr)
> > @@ -821,35 +822,88 @@ static unsigned long vrm_set_new_addr(struct vma_remap_struct *vrm)
> > return 0;
> > }
> >
> > -static unsigned long move_vma(struct vm_area_struct *vma,
> > - unsigned long old_addr, unsigned long old_len,
> > - unsigned long new_len, unsigned long new_addr,
> > - bool *locked, unsigned long flags,
> > - struct vm_userfaultfd_ctx *uf, struct list_head *uf_unmap)
> > +/*
> > + * Keep track of pages which have been added to the memory mapping. If the VMA
> > + * is accounted, also check to see if there is sufficient memory.
> > + *
> > + * Returns true on success, false if insufficient memory to charge.
> > + */
> > +static bool vrm_charge(struct vma_remap_struct *vrm)
> > {
> > - long to_account = new_len - old_len;
> > - struct mm_struct *mm = vma->vm_mm;
> > - struct vm_area_struct *new_vma;
> > - unsigned long vm_flags = vma->vm_flags;
> > - unsigned long new_pgoff;
> > - unsigned long moved_len;
> > - bool account_start = false;
> > - bool account_end = false;
> > - unsigned long hiwater_vm;
> > - int err = 0;
> > - bool need_rmap_locks;
> > - struct vma_iterator vmi;
> > + unsigned long charged;
> > +
> > + if (!(vrm->vma->vm_flags & VM_ACCOUNT))
> > + return true;
> > +
> > + /*
> > + * If we don't unmap the old mapping, then we account the entirety of
> > + * the length of the new one. Otherwise it's just the delta in size.
> > + */
> > + if (vrm->flags & MREMAP_DONTUNMAP)
> > + charged = vrm->new_len >> PAGE_SHIFT;
> > + else
> > + charged = vrm->delta >> PAGE_SHIFT;
> > +
> > +
> > + /* This accounts 'charged' pages of memory. */
> > + if (security_vm_enough_memory_mm(current->mm, charged))
> > + return false;
> > +
> > + vrm->charged = charged;
> > + return true;
> > +}
> > +
> > +/*
> > + * an error has occurred so we will not be using vrm->charged memory. Unaccount
> > + * this memory if the VMA is accounted.
> > + */
> > +static void vrm_uncharge(struct vma_remap_struct *vrm)
> > +{
> > + if (!(vrm->vma->vm_flags & VM_ACCOUNT))
> > + return;
> > +
> > + vm_unacct_memory(vrm->charged);
> > + vrm->charged = 0;
> > +}
> > +
> > +/*
> > + * Update mm exec_vm, stack_vm, data_vm, and locked_vm fields as needed to
> > + * account for 'bytes' memory used, and if locked, indicate this in the VRM so
> > + * we can handle this correctly later.
> > + */
> > +static void vrm_stat_account(struct vma_remap_struct *vrm,
> > + unsigned long bytes)
> > +{
> > + unsigned long pages = bytes >> PAGE_SHIFT;
> > + struct mm_struct *mm = current->mm;
> > + struct vm_area_struct *vma = vrm->vma;
> > +
> > + vm_stat_account(mm, vma->vm_flags, pages);
> > + if (vma->vm_flags & VM_LOCKED) {
> > + mm->locked_vm += pages;
> > + vrm->locked = true;
> > + }
> > +}
> > +
> > +/*
> > + * Perform checks before attempting to write a VMA prior to it being
> > + * moved.
> > + */
> > +static unsigned long prep_move_vma(struct vma_remap_struct *vrm,
> > + unsigned long *vm_flags_ptr)
> > +{
> > + unsigned long err;
> > + struct vm_area_struct *vma = vrm->vma;
> > + unsigned long old_addr = vrm->addr;
> > + unsigned long old_len = vrm->old_len;
> >
> > /*
> > * We'd prefer to avoid failure later on in do_munmap:
> > * which may split one vma into three before unmapping.
> > */
> > - if (mm->map_count >= sysctl_max_map_count - 3)
> > + if (current->mm->map_count >= sysctl_max_map_count - 3)
> > return -ENOMEM;
>
> With the refactoring, it is pointing out some things that need to be
> reworked at a later date.

Yeah agreed, let's revisit this!

>
> >
> > - if (unlikely(flags & MREMAP_DONTUNMAP))
> > - to_account = new_len;
> > -
> > if (vma->vm_ops && vma->vm_ops->may_split) {
> > if (vma->vm_start != old_addr)
> > err = vma->vm_ops->may_split(vma, old_addr);
> > @@ -867,22 +921,46 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> > * so KSM can come around to merge on vma and new_vma afterwards.
> > */
> > err = ksm_madvise(vma, old_addr, old_addr + old_len,
> > - MADV_UNMERGEABLE, &vm_flags);
> > + MADV_UNMERGEABLE, vm_flags_ptr);
> > if (err)
> > return err;
> >
> > - if (vm_flags & VM_ACCOUNT) {
> > - if (security_vm_enough_memory_mm(mm, to_account >> PAGE_SHIFT))
> > - return -ENOMEM;
> > - }
> > + return 0;
> > +}
> > +
> > +static unsigned long move_vma(struct vma_remap_struct *vrm)
> > +{
> > + struct mm_struct *mm = current->mm;
> > + struct vm_area_struct *vma = vrm->vma;
> > + struct vm_area_struct *new_vma;
> > + unsigned long vm_flags = vma->vm_flags;
> > + unsigned long old_addr = vrm->addr, new_addr = vrm->new_addr;
> > + unsigned long old_len = vrm->old_len, new_len = vrm->new_len;
> > + unsigned long new_pgoff;
> > + unsigned long moved_len;
> > + unsigned long account_start = false;
> > + unsigned long account_end = false;
> > + unsigned long hiwater_vm;
> > + int err;
> > + bool need_rmap_locks;
> > + struct vma_iterator vmi;
> > +
> > + err = prep_move_vma(vrm, &vm_flags);
> > + if (err)
> > + return err;
> > +
> > + /* If accounted, charge the number of bytes the operation will use. */
> > + if (!vrm_charge(vrm))
> > + return -ENOMEM;
> >
> > vma_start_write(vma);
> > new_pgoff = vma->vm_pgoff + ((old_addr - vma->vm_start) >> PAGE_SHIFT);
> > - new_vma = copy_vma(&vma, new_addr, new_len, new_pgoff,
> > + new_vma = copy_vma(&vrm->vma, new_addr, new_len, new_pgoff,
> > &need_rmap_locks);
> > + /* This may have been updated. */
> > + vma = vrm->vma;
> > if (!new_vma) {
> > - if (vm_flags & VM_ACCOUNT)
> > - vm_unacct_memory(to_account >> PAGE_SHIFT);
> > + vrm_uncharge(vrm);
> > return -ENOMEM;
> > }
> >
> > @@ -907,7 +985,7 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> > old_addr = new_addr;
> > new_addr = err;
> > } else {
> > - mremap_userfaultfd_prep(new_vma, uf);
> > + mremap_userfaultfd_prep(new_vma, vrm->uf);
> > }
> >
> > if (is_vm_hugetlb_page(vma)) {
> > @@ -915,7 +993,7 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> > }
> >
> > /* Conceal VM_ACCOUNT so old reservation is not undone */
> > - if (vm_flags & VM_ACCOUNT && !(flags & MREMAP_DONTUNMAP)) {
> > + if (vm_flags & VM_ACCOUNT && !(vrm->flags & MREMAP_DONTUNMAP)) {
> > vm_flags_clear(vma, VM_ACCOUNT);
> > if (vma->vm_start < old_addr)
> > account_start = true;
> > @@ -933,13 +1011,12 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> > * If this were a serious issue, we'd add a flag to do_munmap().
> > */
> > hiwater_vm = mm->hiwater_vm;
> > - vm_stat_account(mm, vma->vm_flags, new_len >> PAGE_SHIFT);
> >
> > /* Tell pfnmap has moved from this vma */
> > if (unlikely(vma->vm_flags & VM_PFNMAP))
> > untrack_pfn_clear(vma);
> >
> > - if (unlikely(!err && (flags & MREMAP_DONTUNMAP))) {
> > + if (unlikely(!err && (vrm->flags & MREMAP_DONTUNMAP))) {
> > /* We always clear VM_LOCKED[ONFAULT] on the old vma */
> > vm_flags_clear(vma, VM_LOCKED_MASK);
> >
> > @@ -952,22 +1029,20 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> > unlink_anon_vmas(vma);
> >
> > /* Because we won't unmap we don't need to touch locked_vm */
> > + vrm_stat_account(vrm, new_len);
> > return new_addr;
> > }
> >
> > + vrm_stat_account(vrm, new_len);
> > +
> > vma_iter_init(&vmi, mm, old_addr);
> > - if (do_vmi_munmap(&vmi, mm, old_addr, old_len, uf_unmap, false) < 0) {
> > + if (do_vmi_munmap(&vmi, mm, old_addr, old_len, vrm->uf_unmap, false) < 0) {
> > /* OOM: unable to split vma, just get accounts right */
> > - if (vm_flags & VM_ACCOUNT && !(flags & MREMAP_DONTUNMAP))
> > + if (vm_flags & VM_ACCOUNT && !(vrm->flags & MREMAP_DONTUNMAP))
> > vm_acct_memory(old_len >> PAGE_SHIFT);
> > account_start = account_end = false;
> > }
> >
> > - if (vm_flags & VM_LOCKED) {
> > - mm->locked_vm += new_len >> PAGE_SHIFT;
> > - *locked = true;
> > - }
> > -
> > mm->hiwater_vm = hiwater_vm;
> >
> > /* Restore VM_ACCOUNT if one or two pieces of vma left */
> > @@ -1138,9 +1213,7 @@ static unsigned long mremap_to(struct vma_remap_struct *vrm)
> > if (err)
> > return err;
> >
> > - return move_vma(vrm->vma, vrm->addr, vrm->old_len, vrm->new_len,
> > - vrm->new_addr, &vrm->locked, vrm->flags,
> > - vrm->uf, vrm->uf_unmap);
> > + return move_vma(vrm);
> > }
> >
> > static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
> > @@ -1245,17 +1318,11 @@ static unsigned long check_mremap_params(struct vma_remap_struct *vrm)
> > static unsigned long expand_vma_in_place(struct vma_remap_struct *vrm)
> > {
> > struct mm_struct *mm = current->mm;
> > - long pages = vrm->delta >> PAGE_SHIFT;
> > struct vm_area_struct *vma = vrm->vma;
> > VMA_ITERATOR(vmi, mm, vma->vm_end);
> > - long charged = 0;
> > -
> > - if (vma->vm_flags & VM_ACCOUNT) {
> > - if (security_vm_enough_memory_mm(mm, pages))
> > - return -ENOMEM;
> >
> > - charged = pages;
> > - }
> > + if (!vrm_charge(vrm))
> > + return -ENOMEM;
> >
> > /*
> > * Function vma_merge_extend() is called on the
> > @@ -1268,15 +1335,11 @@ static unsigned long expand_vma_in_place(struct vma_remap_struct *vrm)
> > */
> > vma = vrm->vma = vma_merge_extend(&vmi, vma, vrm->delta);
> > if (!vma) {
> > - vm_unacct_memory(charged);
> > + vrm_uncharge(vrm);
> > return -ENOMEM;
> > }
> >
> > - vm_stat_account(mm, vma->vm_flags, pages);
> > - if (vma->vm_flags & VM_LOCKED) {
> > - mm->locked_vm += pages;
> > - vrm->locked = true;
> > - }
> > + vrm_stat_account(vrm, vrm->delta);
> >
> > return 0;
> > }
> > @@ -1316,11 +1379,7 @@ static bool align_hugetlb(struct vma_remap_struct *vrm)
> > static unsigned long expand_vma(struct vma_remap_struct *vrm)
> > {
> > unsigned long err;
> > - struct vm_area_struct *vma = vrm->vma;
> > unsigned long addr = vrm->addr;
> > - unsigned long old_len = vrm->old_len;
> > - unsigned long new_len = vrm->new_len;
> > - unsigned long flags = vrm->flags;
> >
> > err = resize_is_valid(vrm);
> > if (err)
> > @@ -1353,7 +1412,7 @@ static unsigned long expand_vma(struct vma_remap_struct *vrm)
> > */
> >
> > /* We're not allowed to move the VMA, so error out. */
> > - if (!(flags & MREMAP_MAYMOVE))
> > + if (!(vrm->flags & MREMAP_MAYMOVE))
> > return -ENOMEM;
> >
> > /* Find a new location to move the VMA to. */
> > @@ -1361,8 +1420,7 @@ static unsigned long expand_vma(struct vma_remap_struct *vrm)
> > if (err)
> > return err;
> >
> > - return move_vma(vma, addr, old_len, new_len, vrm->new_addr,
> > - &vrm->locked, flags, vrm->uf, vrm->uf_unmap);
> > + return move_vma(vrm);
> > }
> >
> > /*
> > --
> > 2.48.1
> >

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Previous message: Shyam Saini: "Re: [PATCH v4 0/4] Properly handle module_kobject creation"
In reply to: Liam R. Howlett: "Re: [PATCH 4/7] mm/mremap: initial refactor of move_vma()"
Next in thread: Lorenzo Stoakes: "[PATCH 6/7] mm/mremap: refactor move_page_tables(), abstracting state"
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