Re: [LSF/MM/BPF TOPIC] Improving large folio writeback performance

[Vlastimil Babka <vbabka@suse.cz>]

On 1/15/25 01:50, Joanne Koong wrote:
> Hi all,
> 
> I would like to propose a discussion topic about improving large folio
> writeback performance. As more filesystems adopt large folios, it
> becomes increasingly important that writeback is made to be as
> performant as possible. There are two areas I'd like to discuss:
> 
> 
> == Granularity of dirty pages writeback ==
> Currently, the granularity of writeback is at the folio level. If one
> byte in a folio is dirty, the entire folio will be written back. This
> becomes unscalable for larger folios and significantly degrades
> performance, especially for workloads that employ random writes.
> 
> One idea is to track dirty pages at a smaller granularity using a
> 64-bit bitmap stored inside the folio struct where each bit tracks a
> smaller chunk of pages (eg for 2 MB folios, each bit would track 32k
> pages), and only write back dirty chunks rather than the entire folio.

I think this might be tricky in some cases? I.e. with 2 MB and pmd-mapped
folio, it's possible to write-protect only the whole pmd, not individual 32k
chunks in order to catch the first write to a chunk to mark it dirty.