On 1/9/2026 8:16 PM, David Hildenbrand (Red Hat) wrote:
> On 12/15/25 09:46, Shivank Garg wrote:
>> When MADV_COLLAPSE is called on file-backed mappings (e.g., executable
>> text sections), the pages may still be dirty from recent writes.
>> collapse_file() will trigger async writeback and fail with
>> SCAN_PAGE_DIRTY_OR_WRITEBACK (-EAGAIN).
>>
>> MADV_COLLAPSE is a synchronous operation where userspace expects
>> immediate results. If the collapse fails due to dirty pages, perform
>> synchronous writeback on the specific range and retry once.
>>
>> This avoids spurious failures for freshly written executables while
>> avoiding unnecessary synchronous I/O for mappings that are already clean.
>>
>> Reported-by: Branden Moore <[email protected]>
>> Closes:
>> https://lore.kernel.org/all/[email protected]
>> Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to
>> MADV_COLLAPSE")
>> Suggested-by: David Hildenbrand <[email protected]>
>> Tested-by: Lance Yang <[email protected]>
>> Signed-off-by: Shivank Garg <[email protected]>
>> ---
>> mm/khugepaged.c | 40 ++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 40 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 219dfa2e523c..6c8c35d3e0c9 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -22,6 +22,7 @@
>> #include <linux/dax.h>
>> #include <linux/ksm.h>
>> #include <linux/pgalloc.h>
>> +#include <linux/backing-dev.h>
>> #include <asm/tlb.h>
>> #include "internal.h"
>> @@ -2787,9 +2788,11 @@ int madvise_collapse(struct vm_area_struct *vma,
>> unsigned long start,
>> hend = end & HPAGE_PMD_MASK;
>> for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
>> + bool retried = false;
>> int result = SCAN_FAIL;
>> if (!mmap_locked) {
>> +retry:
>
> Jumping into an if block is nasty :)
>
>> cond_resched();
>> mmap_read_lock(mm);
>> mmap_locked = true;
>> @@ -2819,6 +2822,43 @@ int madvise_collapse(struct vm_area_struct *vma,
>> unsigned long start,
>> if (!mmap_locked)
>> *lock_dropped = true;
>> + /*
>> + * If the file-backed VMA has dirty pages, the scan triggers
>> + * async writeback and returns SCAN_PAGE_DIRTY_OR_WRITEBACK.
>> + * Since MADV_COLLAPSE is sync, we force sync writeback and
>> + * retry once.
>> + */
>> + if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !retried) {
>> + /*
>> + * File scan drops the lock. We must re-acquire it to
>> + * safely inspect the VMA and hold the file reference.
>> + */
>> + if (!mmap_locked) {
>> + cond_resched();
>> + mmap_read_lock(mm);
>> + mmap_locked = true;
>> + result = hugepage_vma_revalidate(mm, addr, false, &vma, cc);
>> + if (result != SCAN_SUCCEED)
>> + goto handle_result;
>> + }
>> +
>> + if (!vma_is_anonymous(vma) && vma->vm_file &&
>> + mapping_can_writeback(vma->vm_file->f_mapping)) {
>> + struct file *file = get_file(vma->vm_file);
>> + pgoff_t pgoff = linear_page_index(vma, addr);
>> + loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
>> + loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
>> +
>> + mmap_read_unlock(mm);
>> + mmap_locked = false;
>> + *lock_dropped = true;
>> + filemap_write_and_wait_range(file->f_mapping, lstart, lend);
>> + fput(file);
>> + retried = true;
>> + goto retry;
>> + }
>> + }
>> +
>
> This looks a bit complicated. Can't we move that handing up, where we have
> most of that
> information already? Or am I missing something important?
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 97d1b2824386f..c7271877c5220 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -22,6 +22,7 @@
> #include <linux/dax.h>
> #include <linux/ksm.h>
> #include <linux/pgalloc.h>
> +#include <linux/backing-dev.h>
>
> #include <asm/tlb.h>
> #include "internal.h"
> @@ -2786,7 +2787,9 @@ int madvise_collapse(struct vm_area_struct *vma,
> unsigned long start,
>
> for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
> int result = SCAN_FAIL;
> + bool triggered_wb = false;
>
> +retry:
> if (!mmap_locked) {
> cond_resched();
> mmap_read_lock(mm);
> @@ -2809,6 +2812,16 @@ int madvise_collapse(struct vm_area_struct *vma,
> unsigned long start,
> mmap_locked = false;
*lock_dropped = true;
> result = hpage_collapse_scan_file(mm, addr, file,
> pgoff,
> cc);
> +
> + if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
> !triggered_wb &&
> + mapping_can_writeback(file->f_mapping)) {
> + loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
> + loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
> +
> + filemap_write_and_wait_range(file->f_mapping,
> lstart, lend);
> + triggered_wb = true;
fput(file);
> + goto retry;
> + }
> fput(file);
> } else {
> result = hpage_collapse_scan_pmd(mm, vma, addr,
>
>
Thank you for the suggestion, this approach looks much simpler.
There are two small nits I observed:
1. In the retry loop, it is possible that we reacquire the mmap_lock and set
mmap_locked to true. This can cause issues later when we do:
if (!mmap_locked)
*lock_dropped = true;
because the caller would no longer see that the lock was dropped earlier.
2. We need an fput() to balance the file reference taken at line 2795.
fixed patch: (tested it no longer reproduce the issue):
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 87ca577c2668..b88bbb54f109 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -21,6 +21,7 @@
#include <linux/shmem_fs.h>
#include <linux/dax.h>
#include <linux/ksm.h>
+#include <linux/backing-dev.h>
#include <asm/tlb.h>
#include <asm/pgalloc.h>
@@ -2771,7 +2772,9 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned
long start,
for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
int result = SCAN_FAIL;
+ bool triggered_wb = false;
+retry:
if (!mmap_locked) {
cond_resched();
mmap_read_lock(mm);
@@ -2794,8 +2797,20 @@ int madvise_collapse(struct vm_area_struct *vma,
unsigned long start,
mmap_read_unlock(mm);
mmap_locked = false;
+ *lock_dropped = true;
result = hpage_collapse_scan_file(mm, addr, file, pgoff,
cc);
+
+ if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
!triggered_wb &&
+ mapping_can_writeback(file->f_mapping)) {
+ loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
+ loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
+
+ filemap_write_and_wait_range(file->f_mapping,
lstart, lend);
+ triggered_wb = true;
+ fput(file);
+ goto retry;
+ }
fput(file);
} else {
result = hpage_collapse_scan_pmd(mm, vma, addr,
