On 12/2/2025 10:20 AM, Lance Yang wrote:
>
>
> On 2025/12/2 02:56, 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]>
>> Signed-off-by: Shivank Garg <[email protected]>
>> ---
>> mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 41 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 219dfa2e523c..7a12e9ef30b4 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:
>> cond_resched();
>> mmap_read_lock(mm);
>> mmap_locked = true;
>> @@ -2819,6 +2822,44 @@ 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;
>> + }
>> + }
>> +
>> +
>
> Nit: spurious blank line.
Ah, I completely missed this. I’ll fix it in the next version.
Hope the rest of the patch looks reasonable. Thanks for the review.
Thanks,
Shivank
>
>> handle_result:
>> switch (result) {
>> case SCAN_SUCCEED:
>
