Re: [PATCH 1/2] mm/khugepaged: do synchronous writeback for MADV_COLLAPSE

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

On 11.11.25 06:58, Garg, Shivank wrote:
> 
> 
> On 11/10/2025 7:58 PM, Matthew Wilcox wrote:
>> On Mon, Nov 10, 2025 at 07:50:17PM +0530, Garg, Shivank wrote:
>>> The issue is copying those binary to a freshly mounted filesystem.
>>> The page cache folios remain dirty until background writeback completes.
>>>
>>> Reproduces 100% for me: fresh XFS/EXT4 mount -> copy binary -> execute -> MADV_COLLAPSE fails.
>>
>> Yes, but this is an uncommon thing to do.  Really, it's the kind of
>> thing you do when you're testing something (like, whether ext4 supports
>> large folios, and whether that yields a performance improvement).
>> It's more reasonable to change userspace than the kernel to solve this
>> problem you're having.
> 
> Fair point.
> 
> You're right that this is primarily a testing scenario, though it may also
> potentially affect JIT compilers writing executables (also uncommon) but more
> research is needed.
> 
> For userspace workarounds, calling fsync() before MADV_COLLAPSE works.

Right. But do we want document that any caller of MADV_COLLAPSE should 
issue an fsync() if MADV_COLLAPSE fails to try again?

IMHO this just reveals a problem that might also be triggered in a 
container that just got downloaded or after upgrading a package, no?

-- 
Cheers

David