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

[Matthew Wilcox]

On Thu, Jan 20, 2022 at 04:39:55PM +0100, David Hildenbrand wrote:
> On 20.01.22 16:36, Matthew Wilcox wrote:
> > 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.
> 
> For example unless we are talking about *drumroll* COW handling.
> 
> > 
> >> 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.
> 
> Yes, we are talking past each other and no, I am talking about fully
> mapped THP, just mapped via PTEs.
> 
> Please refer to our THP COW logic: do_huge_pmd_wp_page()

You're going to have to be a bit more explicit.  That's clearly handling
the case where there's a PMD mapping.  If there is _also_ a PTE mapping,
then obviously the page is mapped more than once and can't be reused!

> > 
> >>> 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.
> 
> do_huge_pmd_wp_page()

I feel you could be a little more verbose about what you think is
going on here.  Are you talking about the fallback: path where we
call __split_huge_pmd()?