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Message-ID: <jbqygu4u22fd65pe5m4ilm6pz2cabgim7bi4vaasjp2jrqqepw@ozylhwth3atv>
Date: Sat, 17 Jan 2026 12:18:20 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, ziy@...dia.com,
dev.jain@....com, baohua@...nel.org, lance.yang@...ux.dev, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Vernon Yang <yanglincheng@...inos.cn>
Subject: Re: [PATCH mm-new v4 2/6] mm: khugepaged: refine scan progress number
On Wed, Jan 14, 2026 at 12:38:46PM +0100, David Hildenbrand (Red Hat) wrote:
> On 1/11/26 13:19, Vernon Yang wrote:
> > Currently, each scan always increases "progress" by HPAGE_PMD_NR,
> > even if only scanning a single pte.
> >
> > This patch does not change the original semantics of "progress", it
> > simply uses the exact number of PTEs counted to replace HPAGE_PMD_NR.
> >
> > Let me provide a detailed example:
> >
> > static int hpage_collapse_scan_pmd()
> > {
> > for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> > _pte++, addr += PAGE_SIZE) {
> > pte_t pteval = ptep_get(_pte);
> > ...
> > if (pte_uffd_wp(pteval)) { <-- first scan hit
> > result = SCAN_PTE_UFFD_WP;
> > goto out_unmap;
> > }
> > }
> > }
> >
> > During the first scan, if pte_uffd_wp(pteval) is true, the loop exits
> > directly. In practice, only one PTE is scanned before termination.
> > Here, "progress += 1" reflects the actual number of PTEs scanned, but
> > previously "progress += HPAGE_PMD_NR" always.
> >
> > Signed-off-by: Vernon Yang <yanglincheng@...inos.cn>
> > ---
> > mm/khugepaged.c | 28 ++++++++++++++++++++++------
> > 1 file changed, 22 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 2e570f83778c..5c6015ac7b5e 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1249,6 +1249,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> > static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
> > struct vm_area_struct *vma,
> > unsigned long start_addr, bool *mmap_locked,
> > + int *cur_progress,
> > struct collapse_control *cc)
> > {
> > pmd_t *pmd;
> > @@ -1264,19 +1265,27 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
> > VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);
> > result = find_pmd_or_thp_or_none(mm, start_addr, &pmd);
> > - if (result != SCAN_SUCCEED)
> > + if (result != SCAN_SUCCEED) {
> > + if (cur_progress)
> > + *cur_progress = HPAGE_PMD_NR;
> > goto out;
> > + }
> > memset(cc->node_load, 0, sizeof(cc->node_load));
> > nodes_clear(cc->alloc_nmask);
> > pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
> > if (!pte) {
> > + if (cur_progress)
> > + *cur_progress = HPAGE_PMD_NR;
> > result = SCAN_NO_PTE_TABLE;
> > goto out;
> > }
> > for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> > _pte++, addr += PAGE_SIZE) {
> > + if (cur_progress)
> > + *cur_progress += 1;
> > +
> > pte_t pteval = ptep_get(_pte);
> > if (pte_none_or_zero(pteval)) {
> > ++none_or_zero;
> > @@ -2297,6 +2306,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> > static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
> > struct file *file, pgoff_t start,
> > + int *cur_progress,
> > struct collapse_control *cc)
> > {
> > struct folio *folio = NULL;
> > @@ -2337,6 +2347,9 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
> > continue;
> > }
> > + if (cur_progress)
> > + *cur_progress += folio_nr_pages(folio);
> > +
>
> Okay, I had another look and I think the file path is confusing. We're
> scanning xarray entries. But then, we only count some entries and not
> others.
Thank you for the review.
Move PATCH #3 comments here
> Assume we found a single 4k folio in the xarray, but then collapse a 2M THP.
> Is the progress really "1" ?
This example is indeed a issue, I will use "xas->xa_index" to fix
these issues.
> What about shmem swap entries (xa_is_value)?
Sorry, I missed it. I will add "1 << xas_get_order(&xas)" to
"cur_progreess".
> Can we just keep that alone in this patch? That is, always indicate a
> progress of HPAGE_PMD_NR right at the start of the function?
Studying the implementation source code of xarray, I discovered that
these issues can be fixed by utilizing "xas->xa_index".
I send the code as follow (patch #2 and #3 are squashed). Let's see if
it works? If not, please let me know, Thanks!
--
Thanks,
Vernon
diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index be850174e802..f77d97d7b957 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -1646,6 +1646,15 @@ static inline void xas_set(struct xa_state *xas, unsigned long index)
xas->xa_node = XAS_RESTART;
}
+/**
+ * xas_get_index() - Get XArray operation state for a different index.
+ * @xas: XArray operation state.
+ */
+static inline unsigned long xas_get_index(struct xa_state *xas)
+{
+ return xas->xa_index;
+}
+
/**
* xas_advance() - Skip over sibling entries.
* @xas: XArray operation state.
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 2e570f83778c..7d42035ece5b 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -68,7 +68,10 @@ enum scan_result {
static struct task_struct *khugepaged_thread __read_mostly;
static DEFINE_MUTEX(khugepaged_mutex);
-/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */
+/*
+ * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas
+ * every 10 second.
+ */
static unsigned int khugepaged_pages_to_scan __read_mostly;
static unsigned int khugepaged_pages_collapsed;
static unsigned int khugepaged_full_scans;
@@ -1249,6 +1252,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
struct vm_area_struct *vma,
unsigned long start_addr, bool *mmap_locked,
+ unsigned int *cur_progress,
struct collapse_control *cc)
{
pmd_t *pmd;
@@ -1263,6 +1267,9 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);
+ if (cur_progress)
+ *cur_progress += 1;
+
result = find_pmd_or_thp_or_none(mm, start_addr, &pmd);
if (result != SCAN_SUCCEED)
goto out;
@@ -1403,6 +1410,8 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
} else {
result = SCAN_SUCCEED;
}
+ if (cur_progress)
+ *cur_progress += _pte - pte;
out_unmap:
pte_unmap_unlock(pte, ptl);
if (result == SCAN_SUCCEED) {
@@ -2297,6 +2306,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
struct file *file, pgoff_t start,
+ unsigned int *cur_progress,
struct collapse_control *cc)
{
struct folio *folio = NULL;
@@ -2386,6 +2396,18 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
cond_resched_rcu();
}
}
+ if (cur_progress) {
+ unsigned long idx = xas_get_index(&xas) - start;
+
+ if (folio == NULL)
+ *cur_progress += HPAGE_PMD_NR;
+ else if (xa_is_value(folio))
+ *cur_progress += idx + (1 << xas_get_order(&xas));
+ else if (folio_order(folio) == HPAGE_PMD_ORDER)
+ *cur_progress += idx + 1;
+ else
+ *cur_progress += idx + folio_nr_pages(folio);
+ }
rcu_read_unlock();
if (result == SCAN_SUCCEED) {
@@ -2466,6 +2488,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
while (khugepaged_scan.address < hend) {
bool mmap_locked = true;
+ unsigned int cur_progress = 0;
cond_resched();
if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
@@ -2482,7 +2505,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
mmap_read_unlock(mm);
mmap_locked = false;
*result = hpage_collapse_scan_file(mm,
- khugepaged_scan.address, file, pgoff, cc);
+ khugepaged_scan.address, file, pgoff,
+ &cur_progress, cc);
fput(file);
if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
mmap_read_lock(mm);
@@ -2496,7 +2520,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
}
} else {
*result = hpage_collapse_scan_pmd(mm, vma,
- khugepaged_scan.address, &mmap_locked, cc);
+ khugepaged_scan.address, &mmap_locked,
+ &cur_progress, cc);
}
if (*result == SCAN_SUCCEED)
@@ -2504,7 +2529,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
/* move to next address */
khugepaged_scan.address += HPAGE_PMD_SIZE;
- progress += HPAGE_PMD_NR;
+ progress += cur_progress;
if (!mmap_locked)
/*
* We released mmap_lock so break loop. Note
@@ -2826,11 +2851,11 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
mmap_read_unlock(mm);
mmap_locked = false;
result = hpage_collapse_scan_file(mm, addr, file, pgoff,
- cc);
+ NULL, cc);
fput(file);
} else {
result = hpage_collapse_scan_pmd(mm, vma, addr,
- &mmap_locked, cc);
+ &mmap_locked, NULL, cc);
}
if (!mmap_locked)
*lock_dropped = true;
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