Re: [PATCH V5 2/2] mm/khugepaged: retry with sync writeback for MADV_COLLAPSE

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

On 1/22/26 12:07, Dev Jain wrote:
> 
> On 19/01/26 12:39 am, 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>
>> Tested-by: Lance Yang <lance.yang@linux.dev>
>> Signed-off-by: Shivank Garg <shivankg@amd.com>
>> ---
>>   mm/khugepaged.c | 15 +++++++++++++++
>>   1 file changed, 15 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 219dfa2e523c..16582bdcb6ff 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"
>> @@ -2788,7 +2789,9 @@ 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);
>> @@ -2809,8 +2812,20 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>>   
>>   			mmap_read_unlock(mm);
>>   			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);
> 
> So we don't care about the return value here because this is best-effort.
> I really wish we had in our coding-style.rst to typecast such things to (void),
> so we know explicitly that we are ignoring the return value, and not that the
> function itself returns void.

That makes functions like bitmap_and() hard (and ugly) to use that just 
return some value for the caller's convenience.

For functions where we really want callers to think about this, we can 
enforce such checks through __must_check.

Here, it's rather obvious that we don't care about the result as we only 
retry once to then give up.

-- 
Cheers

David