Re: More filesystem need this fix (xfs: use MMAPLOCK around filemap_map_pages())

[Matthew Wilcox]

On Mon, Sep 21, 2020 at 09:20:25AM -0700, Linus Torvalds wrote:
> On Mon, Sep 21, 2020 at 2:11 AM Jan Kara <jack@xxxxxxx> wrote:
> >
> > Except that on truncate, we have to unmap these
> > anonymous pages in private file mappings as well...
> 
> I'm actually not 100% sure we strictly would need to care.
> 
> Once we've faulted in a private file mapping page, that page is
> "ours". That's kind of what MAP_PRIVATE means.
> 
> If we haven't written to it, we do keep things coherent with the file,
> but that's actually not required by POSIX afaik - it's a QoI issue,
> and a lot of (bad) Unixes didn't do it at all.
> So as long as truncate _clears_ the pages it truncates, I think we'd
> actually be ok.

We don't even need to do that ...

"If the size of the mapped file changes after the call to mmap()
as a result of some other operation on the mapped file, the effect of
references to portions of the mapped region that correspond to added or
removed portions of the file is unspecified."

https://pubs.opengroup.org/onlinepubs/9699919799/functions/mmap.html

As you say, there's a QoI here, and POSIX permits some shockingly
bad and useless implementations.