Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page

[Matthew Wilcox]

On Thu, Jan 20, 2022 at 04:26:22PM +0100, David Hildenbrand wrote:
> On 20.01.22 15:39, Matthew Wilcox wrote:
> > On Thu, Jan 20, 2022 at 03:15:37PM +0100, David Hildenbrand wrote:
> >> On 17.01.22 14:31, zhangliang (AG) wrote:
> >>> Sure, I will do that :)
> >>
> >> I'm polishing up / testing the patches and might send something out for discussion shortly.
> >> Just a note that on my branch was a version with a wrong condition that should have been fixed now.
> >>
> >> I am still thinking about PTE mapped THP. For these, we'll always
> >> have page_count() > 1, essentially corresponding to the number of still-mapped sub-pages.
> >>
> >> So if we end up with a R/O mapped part of a THP, we'll always have to COW and cannot reuse ever,
> >> although it's really just a single process mapping the THP via PTEs.
> >>
> >> One approach would be to scan the currently locked page table for entries mapping
> >> this same page. If page_count() corresponds to that value, we know that only we are
> >> mapping the THP and there are no additional references. That would be a special case
> >> if we find an anon THP in do_wp_page(). Hm.
> > 
> > You're starting to optimise for some pretty weird cases at that point.
> 
> So your claim is that read-only, PTE mapped pages are weird? How do you
> come to that conclusion?

Because normally anon THP pages are PMD mapped.  That's rather
the point of anon THPs.

> If we adjust the THP reuse logic to split on additional references
> (page_count() == 1) -- similarly as suggested by Linus to fix the CVE --
> we're going to end up with exactly that more frequently.

I don't understand.  Are we talking past each other?  As I understand
the situation we're talking about here, a process has created a THP,
done something to cause it to be partially mapped (or mapped in a
misaligned way) in its own address space, then forked, and we're
trying to figure out if it's safe to reuse it?  I say that situation is
rare enough that it's OK to always allocate an order-0 page and
copy into it.

> > Anon THP is always going to start out aligned (and can be moved by
> > mremap()).  Arguably it should be broken up if it's moved so it can be
> > reformed into aligned THPs by khugepaged.
> 
> Can you elaborate, I'm missing the point where something gets moved. I
> don't care about mremap() at all here.
> 
> 
> 1. You have a read-only, PTE mapped THP
> 2. Write fault on the THP
> 3. We PTE-map the THP because we run into a false positive in our COW
>    logic to handle COW on PTE
> 4. Write fault on the PTE
> 5. We always have to COW each and every sub-page and can never reuse,
>    because page_count() > 1
> 
> That's essentially what reuse_swap_page() tried to handle before.
> Eventually optimizing for this is certainly the next step, but I'd like
> to document which effect the removal of reuse_swap_page() will have to THP.

I'm talking about step 0.  How do we get a read-only, PTE-mapped THP?
Through mremap() or perhaps through an mprotect()/mmap()/munmap() that
failed to split the THP.