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

["Garg, Shivank" <shivankg@amd.com>]

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 <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.

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:
>