Re: [PATCH] mm/huge_memory: transfer the pmd dirty bit to the folio on zap

[David Hildenbrand (Arm)]

On 8/25/26 07:59, Christoph Hellwig wrote:
> On Thu, Aug 20, 2026 at 03:10:11PM +0100, Pedro Falcato wrote:
>>> Why? Write batching from large folios is a win.
>>
>> For some context: we were discussing (off-list) the recent report that
>> systemd-journald had horrible write amplification, worsed quite a bit
>> by large folios. At the moment, there is quite a lot of write amplification,
>> but _only_ on mmap writes (if you look at the write(2) paths, you'll see
>> write_begin and write_end which tactically only dirty what you actually wrote
>> to, block-wise in the BHs or iomap IFS). This doesn't need to be true.
> 
> So don't use mmap to write to storage.  That is always a bad idea for
> many, many reasons.  We should not work around broken applications that
> do this, but change them.  It only took about 10 years of bad reputation
> for MongoDB to finally fix their act, so it should be possible for
> systemd as well.

I was in contact with systemd folks (and pulled in Willy and Jan) regarding
that, and nothing should really block that.

-- 
Cheers,

David