Re: [PATCH V4 2/2] mm/khugepaged: retry with sync writeback for MADV_COLLAPSE

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

On 1/11/2026 4:59 PM, David Hildenbrand (Red Hat) wrote:
> On 1/10/26 19:20, Garg, Shivank wrote:
>>
>>
>> On 1/9/2026 8:16 PM, David Hildenbrand (Red Hat) wrote:
>>> On 12/15/25 09:46, Shivank Garg wrote:

>>>
>>> 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:
> 
> Yeah, was a quick untested hack to see if this can be simplified :)
> 
>>
>> 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;
> 
> That whole logic of having two variables that express whether locks have been taken/dropped is just absolutely confusing. Any way we can clean that up?
>
>>
>>     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.
> 
> Ah, yes, makes sense. Having a single fput() would be nicer, but that would require yet another temporary variable.
>

I agree, that this interaction for lock taken/droped is confusing.
However, a proper clean-up would require refactoring the locking logic across multiple functions in the collapse call-flow path.
This seems significantly more invasive and risky.

I would like to handle this refactoring but in a separate TODO for later.
Could we please proceed with these minimal changes for now?

Since, V4 has been in the linux-next/mm-unstable for a while, should I send a v5 or an incremental clean-up on top for this?

Thanks,
Shivank