Re: [PATCH v9 1/2] mm/khugepaged: recover from poisoned anonymous memory

[Jiaqi Yan <jiaqiyan@google.com>]

On Wed, Feb 8, 2023 at 3:45 AM Alexander Potapenko <glider@google.com> wrote:
>
> On Tue, Feb 7, 2023 at 7:20 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
> >
> > Pinging KMSAN experts, for the general guidance of
> > kmsan_copy_page_meta vs kmsan_unpoison_memory
>
> Oh, sorry, I've missed the previous email.

NP, thanks for jumping in :)

>
> copy_mc_user_highpage() is expected to copy data from the user page,
> for which no metadata is ever allocated.
> Therefore we just initialize the destination shadow with zeros instead
> of copying anything.
>
> kmsan_copy_page_meta() is used when the metadata is copied between two
> kernel pages, therefore it handles the cases when page->kmsan_shadow
> is NULL for the source and destination pages.
>
> It might be a good idea to use kmsan_copy_page_meta() in both cases,
> but to do that I want to better understand what happens when
> kmap_local_page(from) is called in copy_mc_user_highpage().
> Where does the corresponding struct page come from?

Kirill can correct me, but I think khugepaged always copies user pages
because it is trying to convert raw pages to THP for better userspace
application performance. Therefore khugepaged should only need
copy_mc_user_highpage(), for both file-backed and anonymous memory
pages.

However, copy_mc_user_highpage() needs both vaddr and vma, so it is a
little bit hard to use it in collapse_file (i.e. in the file-backed
case):
1. vma is not carried over to collapse_file from khugepaged_scan_mm_slot
2. collapse_file is not directly iterating with vaddrs of pages to be copied

(Although both vaddr and vma are unused auguments in
copy_mc_user_highpage, I think for cleanness, the caller e.g.
khugepaged should feed valid values).

So my patchset uses copy_mc_page(and kmsan_copy_page_meta) for both
file-backed and anon memory pages. I guess as long as
kmsan_copy_page_meta doesn't do anything unexpected for user pages (at
least from my reading), we are good?