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

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

On 2025/12/4 02:25, Garg, Shivank wrote:
> 
> 
> 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.

Apart from that nit, nothing else jumped out at me :)

Confirmed that the spurious EINVAL is gone, and it works as expected ;p

Tested-by: Lance Yang <lance.yang@linux.dev>

[...]

Cheers,
Lance