Re: [PATCH V3 2/2] mm/khugepaged: retry with sync writeback for MADV_COLLAPSE

[Lance Yang <lance.yang@linux.dev>]

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 <Branden.Moore@amd.com>
> Closes: https://lore.kernel.org/all/4e26fe5e-7374-467c-a333-9dd48f85d7cc@amd.com
> Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to MADV_COLLAPSE")
> Suggested-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Shivank Garg <shivankg@amd.com>
> ---
>   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.

>   handle_result:
>   		switch (result) {
>   		case SCAN_SUCCEED: