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

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

On 1/15/2026 1:44 AM, David Hildenbrand (Red Hat) wrote:
> 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 :)

Thanks! I have send V5:
https://lore.kernel.org/linux-mm/20260118192253.9263-14-shivankg@amd.com

Best Regards,
Shivank