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Message-ID: <lv2c4z5utzr37moyish53pmazcnyprhp7ncnrp3hwxyh2fxcvh@466zyeltaisr>
Date: Thu, 30 Oct 2025 09:29:19 -0300
From: Pedro Demarchi Gomes <pedrodemargomes@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Xu Xin <xu.xin16@....com.cn>, Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] ksm: perform a range-walk in break_ksm
On Wed, Oct 29, 2025 at 03:45:14PM +0100, David Hildenbrand wrote:
> On 28.10.25 14:19, Pedro Demarchi Gomes wrote:
> > Make break_ksm() receive an address range and change
> > break_ksm_pmd_entry() to perform a range-walk and return the address of
> > the first ksm page found.
> >
> > This change allows break_ksm() to skip unmapped regions instead of
> > iterating every page address. When unmerging large sparse VMAs, this
> > significantly reduces runtime, as confirmed by benchmark test (see
> > cover letter).
>
> Instead of referencing the cover letter, directly include the data here.
>
Ok
> >
> > Suggested-by: David Hildenbrand <david@...hat.com>
> > Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@...il.com>
> > ---
> > mm/ksm.c | 88 +++++++++++++++++++++++++++++++-------------------------
> > 1 file changed, 49 insertions(+), 39 deletions(-)
> >
> > diff --git a/mm/ksm.c b/mm/ksm.c
> > index 2a9a7fd4c777..1d1ef0554c7c 100644
> > --- a/mm/ksm.c
> > +++ b/mm/ksm.c
> > @@ -607,34 +607,54 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
> > return atomic_read(&mm->mm_users) == 0;
> > }
> > -static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
> > +struct break_ksm_arg {
> > + unsigned long addr;
> > +};
>
> You can avoid that by simply passing a pointer to addr.
>
> > +
> > +static int break_ksm_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned long end,
> > struct mm_walk *walk)
> > {
> > - struct page *page = NULL;
> > + struct page *page;
> > spinlock_t *ptl;
> > - pte_t *pte;
> > - pte_t ptent;
> > - int ret;
> > + pte_t *start_ptep = NULL, *ptep, pte;
>
> Is there a need to initialize start_ptep?
>
No, I will fix that.
> > + int ret = 0;
> > + struct mm_struct *mm = walk->mm;
> > + struct break_ksm_arg *private = (struct break_ksm_arg *) walk->private;
>
> Prefer reverse xmas tree:
>
> struct break_ksm_arg *private = (struct break_ksm_arg *) walk->private;
> struct mm_struct *mm = walk->mm;
> ...
>
> With the break_ksm_arg simplification you'd had
>
> unsigned long *found_addr = (unsigned long *)walk->private;
>
>
Ok
> > - pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> > - if (!pte)
> > + if (ksm_test_exit(walk->mm))
> > return 0;
> > - ptent = ptep_get(pte);
> > - if (pte_present(ptent)) {
> > - page = vm_normal_page(walk->vma, addr, ptent);
> > - } else if (!pte_none(ptent)) {
> > - swp_entry_t entry = pte_to_swp_entry(ptent);
> > - /*
> > - * As KSM pages remain KSM pages until freed, no need to wait
> > - * here for migration to end.
> > - */
> > - if (is_migration_entry(entry))
> > - page = pfn_swap_entry_to_page(entry);
> > + if (signal_pending(current))
> > + return -ERESTARTSYS;
>
> I assume that's not a problem for the other callsites that wouldn't check
> this before.
>
Correct.
> > +
> > + start_ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> > + if (!start_ptep)
> > + return 0;
> > +
> > + for (ptep = start_ptep; addr < end; ptep++, addr += PAGE_SIZE) {
>
> Best to declare pte and folio (see last mail) here in the loop:
>
> pte_t pte = ptep_get(ptep);
> struct folio *folio = NULL;
>
Ok.
> > + pte = ptep_get(ptep);
> > + page = NULL;
> > + if (pte_present(pte)) {
> > + page = vm_normal_page(walk->vma, addr, pte);
> > + } else if (!pte_none(pte)) {
> > + swp_entry_t entry = pte_to_swp_entry(pte);
> > +
> > + /*
> > + * As KSM pages remain KSM pages until freed, no need to wait
> > + * here for migration to end.
> > + */
> > + if (is_migration_entry(entry))
> > + page = pfn_swap_entry_to_page(entry);
> > + }
> > + /* return 1 if the page is an normal ksm page or KSM-placed zero page */
> > + ret = (page && folio_test_ksm(page_folio(page))) || is_ksm_zero_pte(pte);
> > + if (ret) {
> > + private->addr = addr;
> > + goto out_unlock;
> > + }
>
> I suggest you call "ret" "found" instead.
>
Ok.
> > }
> > - /* return 1 if the page is an normal ksm page or KSM-placed zero page */
> > - ret = (page && folio_test_ksm(page_folio(page))) || is_ksm_zero_pte(ptent);
> > - pte_unmap_unlock(pte, ptl);
> > +out_unlock:
> > + pte_unmap_unlock(ptep, ptl);
> > return ret;
> > }
> > @@ -661,9 +681,11 @@ static const struct mm_walk_ops break_ksm_lock_vma_ops = {
> > * of the process that owns 'vma'. We also do not want to enforce
> > * protection keys here anyway.
> > */
> > -static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_vma)
> > +static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
> > + unsigned long end, bool lock_vma)
> > {
> > vm_fault_t ret = 0;
> > + struct break_ksm_arg break_ksm_arg;
> > const struct mm_walk_ops *ops = lock_vma ?
> > &break_ksm_lock_vma_ops : &break_ksm_ops;
> > @@ -671,11 +693,10 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_v
> > int ksm_page;
> > cond_resched();
> > - ksm_page = walk_page_range_vma(vma, addr, addr + 1, ops, NULL);
> > - if (WARN_ON_ONCE(ksm_page < 0))
> > + ksm_page = walk_page_range_vma(vma, addr, end, ops, &break_ksm_arg);
> > + if (ksm_page <= 0)
> > return ksm_page;
> > - if (!ksm_page)
> > - return 0;
> > + addr = break_ksm_arg.addr;
> > ret = handle_mm_fault(vma, addr,
> > FAULT_FLAG_UNSHARE | FAULT_FLAG_REMOTE,
> > NULL);
> > @@ -761,7 +782,7 @@ static void break_cow(struct ksm_rmap_item *rmap_item)
> > mmap_read_lock(mm);
> > vma = find_mergeable_vma(mm, addr);
> > if (vma)
> > - break_ksm(vma, addr, false);
> > + break_ksm(vma, addr, addr + 1, false);
>
> Better to use addr + PAGE_SIZE
>
OK.
> --
> Cheers
>
> David / dhildenb
>
>
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