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

[Lance Yang <lance.yang@linux.dev>]

>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.
>
>What we conjured up was the following: since WP faults always happen on PTEs,
>you can simply dirty the block(s) corresponding to that particular PTE (which
>naturally points to a page inside the folio, whether it is large or not, doesn't
>matter). That way, you can avoid fully dirtying the folio's blocks. This is
>something that ATM isn't done by any filesystem, but it really should.
>
>Obviously this whole idea is thwarted if we keep writable shared file folios
>PMD-mapped - you can't get meaningful write notifications apart from "someone
>wrote to this folio", which isn't particularly useful once folios get 2M+ large.
>So PMD-mapping on a shared-write fault is antithetical to getting useful,
>granular write notifications to filesystems.

Just checking I got it right ... for a PMD-order file folio:

read-only mapping  -> PMD-mapped
writable mapping   -> PTE-mapped

Then ->page_mkwrite() could dirty only blocks corresponding to faulting
PTE, avoiding whole-folio write amplification.

Is that what you have in mind?

Cheers, Lance