Re: Subject: [BUG/RFC] write-open file THP cache purge can discard dirty page cache

[Zi Yan]

On Tue Jun 30, 2026 at 2:36 PM EDT, Matthew Wilcox wrote:
> On Tue, Jun 30, 2026 at 01:01:53PM -0400, Gregg Leventhal wrote:
>> On an affected 6.12 kernel with CONFIG_READ_ONLY_THP_FOR_FS=y, a file can
>> contain read-only file THPs installed by khugepaged / MADV_COLLAPSE. When that
>> same file is later opened for write, do_dentry_open() notices
>> filemap_nr_thps() and drops the page cache:
> [...]
>> 
>> This is unsafe if the mapping also contains dirty folios.
>
> But there shouldn't be any.  It should not be possible to have
> dirty folios and THPs in the same file unless the filesystem
> supports large folios natively.
>
> If the file is open for writing, the attempt to create THPs should fail.

Maybe he means order-0 dirty folios?

-- 
Best Regards,
Yan, Zi