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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 1/14/26 20:47, Garg, Shivank wrote:
> 
> 
> 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?

Sure, fine with me.

> 
> 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?

Just send a v4, unless Andrew tells you otherwise :)

-- 
Cheers

David