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/[email protected]
Best Regards,
Shivank
