Re: [PATCH] mm/khugepaged: return -EAGAIN for SCAN_PAGE_HAS_PRIVATE in MADV_COLLAPSE

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

On 4/29/26 16:20, Dev Jain wrote:
> 
> 
> On 29/04/26 7:34 pm, Vineet Agarwal wrote:
>> MADV_COLLAPSE uses errno values to provide actionable feedback to
>> userspace. Temporary resource constraints are mapped to -EAGAIN so the
>> caller may retry, while intrinsic failures of the specified range are
>> mapped to -EINVAL.
>>
>> collapse_file() returns SCAN_PAGE_HAS_PRIVATE when
>> filemap_release_folio() fails while isolating file-backed folios for
>> collapse. This currently falls through the default case in
>> madvise_collapse_errno() and is reported to userspace as -EINVAL.
>>
>> However, filemap_release_folio() failure commonly reflects temporary
>> folio state rather than a permanently uncollapsible range.
>>
>> For example, ext4 returns false when a folio still has dirty
>> journalled data, btrfs returns false for dirty or writeback folios
>> before extent state release, and NFS may return false while reclaiming
>> filesystem-private folio state.
>>
>> In such cases, retrying MADV_COLLAPSE after writeback, reclaim or
>> journal progress may succeed. This matches the existing -EAGAIN
>> handling for SCAN_PAGE_DIRTY_OR_WRITEBACK and other transient collapse
>> failures more closely than -EINVAL.
>>
>> Therefore, map SCAN_PAGE_HAS_PRIVATE to -EAGAIN so userspace receives
>> retryable feedback for this temporary failure path.
>>
>> Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
>> ---
>>  mm/khugepaged.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index b8452dbdb043..58f55115d7d8 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -2808,6 +2808,7 @@ static int madvise_collapse_errno(enum scan_result r)
>>  	case SCAN_PAGE_LRU:
>>  	case SCAN_DEL_PAGE_LRU:
>>  	case SCAN_PAGE_FILLED:
>> +	case SCAN_PAGE_HAS_PRIVATE:
>>  	case SCAN_PAGE_DIRTY_OR_WRITEBACK:
>>  		return -EAGAIN;
>>  	/*
> 
> https://lore.kernel.org/all/4ab54ae0-2607-443d-8698-788d8e951bdd@arm.com/
> 
> I had mentioned at that time, other error codes may fit the -EAGAIN
> (or something else) definition as well, for example, probably SCAN_TRUNCATED.

In folio_split(), we such events as a mixture of EAGAIN and EBUSY.
filemap_release_folio() errors are, for example, handles as EBUSY.

So in that regard, using EAGAIN here (as we don't use EBUSY) makes sense for
SCAN_PAGE_HAS_PRIVATE.

-- 
Cheers,

David