Re: [PATCH v2 4/6] iomap: correct the dirty length in page mkwrite

[Matthew Wilcox]

On Mon, Aug 12, 2024 at 08:11:57PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
> 
> When doing page mkwrite, iomap_folio_mkwrite_iter() dirty the entire
> folio by folio_mark_dirty() even the map length is shorter than one
> folio. However, on the filesystem with more than one blocks per folio,
> we'd better to only set counterpart block's dirty bit according to
> iomap_length(), so open code folio_mark_dirty() and pass the correct
> length.

We shouldn't waste any time trying to optimise writing to files through
mmap().  People have written papers about what a terrible programming
model this is.  eg https://db.cs.cmu.edu/mmap-cidr2022/

There are some programs that do it, but they are few and far between.