Re: [PATCH v5 3/6] userfaultfd: convert copy_huge_page_from_user() to copy_folio_from_user()

Andrew Morton
2023-04-18T22:21:15Z

On Tue, 11 Apr 2023 04:40:17 +0100 Matthew Wilcox <willy@infradead.org> wrote:

> On Thu, Apr 06, 2023 at 07:28:44PM -0700, Vishal Moola wrote:
> > > - flush_dcache_page(subpage);
> > > -
> > > cond_resched();
> > > }
> > > + flush_dcache_folio(dst_folio);
> > > return ret_val;
> > > }
> >
> > Moving the flush_dcache_page() outside the loop to be
> > flush_dcache_folio() changes the behavior of the function.
> >
> > Initially, if it fails to copy the entire page, the function breaks out
> > of the loop and returns the number of unwritten bytes without
> > flushing the page from the cache. Now if it fails, it will still flush
> > out the page it failed on, as well as any later pages it may not
> > have gotten to yet.
>
> I'm not sure this is worth worrying about. Failing to copy the entire
> folio is unlikely, and if we do, flushing the entire folio instead of just
> a few pages in it is harmless. Plus I have patches which significantly
> optiise flush_dcache_folio() over flush_dcache_page() (for the majority
> of architectures) and so I think this change is actually beneficial in
> the long term.

Thanks, I'll send the series in for the next merge window as-is. If
others remain unhappy with the flushing issue, please propose something
during the next -rc cycle.