Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios

[Barry Song <21cnbao@gmail.com>]

On Sat, Nov 18, 2023 at 3:37 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Nov 17, 2023 at 07:47:00AM +0800, Barry Song wrote:
> > This has been discussed. Steven, Ryan and I all don't think this is a good
> > option. in case we have a large folio with 16 basepages, as do_swap_page
> > can only map one base page for each page fault, that means we have
> > to restore 16(tags we restore in each page fault) * 16(the times of page faults)
> > for this large folio.
>
> That doesn't seem all that hard to fix?  Call set_ptes() instead of
> set_pte_at().  The biggest thing, I guess, is making sure that all
> the PTEs you're going to set up are still pte_none().

I guess you mean all are still swap entries in ptes. some risks  I can see

1. vma might be splitted after folios added into swapcache, for example
unmap or mprotect a part of large folios from userspace

2. vma is not splitted, but some basepages are MADV_DONTNEED
within the folios.

3. basepages in the large folio might become having different permissions
on R/W/X.

for example, if a large folio has 16 basepages, as userspace is still
working at 4kb, userspace can mprotect RD_ONLY for a part of them,
in this case, 16PTEs will still be swap entries, but the re-use for
write fault can't work at folio granularity.

I need to consider all the above DoubleMap/split risks rather than simply
checking PTEs as userspace is still 4KB.

>

Thanks
Barry