Re: [PATCH v5 3/4] mm: support large folios swapin as a whole for zRAM-like swapfile

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

On Tue, Jul 30, 2024 at 12:55 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Jul 29, 2024 at 02:36:38PM +0800, Chuanhua Han wrote:
> > Matthew Wilcox <willy@infradead.org> 于2024年7月29日周一 11:51写道：
> > >
> > > On Fri, Jul 26, 2024 at 09:46:17PM +1200, Barry Song wrote:
> > > > -                     folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> > > > -                                             vma, vmf->address, false);
> > > > +                     folio = alloc_swap_folio(vmf);
> > > >                       page = &folio->page;
> > >
> > > This is no longer correct.  You need to set 'page' to the precise page
> > > that is being faulted rather than the first page of the folio.  It was
> > > fine before because it always allocated a single-page folio, but now it
> > > must use folio_page() or folio_file_page() (whichever has the correct
> > > semantics for you).
> > >
> > > Also you need to fix your test suite to notice this bug.  I suggest
> > > doing that first so that you know whether you've got the calculation
> > > correct.
> >
> > >
> > >
> > This is no problem now, we support large folios swapin as a whole, so
> > the head page is used here instead of the page that is being faulted.
> > You can also refer to the current code context, now support large
> > folios swapin as a whole, and previously only support small page
> > swapin is not the same.
>
> You have completely failed to understand the problem.  Let's try it this
> way:
>
> We take a page fault at address 0x123456789000.
> If part of a 16KiB folio, that's page 1 of the folio at 0x123456788000.
> If you now map page 0 of the folio at 0x123456789000, you've
> given the user the wrong page!  That looks like data corruption.
>
> The code in
>         if (folio_test_large(folio) && folio_test_swapcache(folio)) {
> as Barry pointed out will save you -- but what if those conditions fail?
> What if the mmap has been mremap()ed and the folio now crosses a PMD
> boundary?  mk_pte() will now be called on the wrong page.

Chuanhua understood everything correctly. I think you might have missed
that we have very strict checks both before allocating large folios and before
mapping them for this new allocated mTHP swap-in case.

to allocate a large folio, we check all alignment requirements;  PTEs have
aligned swap offset and all physically contiguous, that is how mTHP
is swapped out. if a mTHP has been mremap() to be unaligned, we won't
swap them in as mTHP.  two reasons: 1. we have no way to figure out
what is the start address of a previous mTHP for non-swapcache case;
2. mremap() to unaligned addresses is rare.

to map a large folio, we check all PTEs are still there by double confirming
can_swapin_thp() is true. if PTEs have changed, this is a "goto out_nomap"
case.

        /* allocated large folios for SWP_SYNCHRONOUS_IO */
        if (folio_test_large(folio) && !folio_test_swapcache(folio)) {
                unsigned long nr = folio_nr_pages(folio);
                unsigned long folio_start = ALIGN_DOWN(vmf->address,
nr * PAGE_SIZE);
                unsigned long idx = (vmf->address - folio_start) / PAGE_SIZE;
                pte_t *folio_ptep = vmf->pte - idx;

                if (!can_swapin_thp(vmf, folio_ptep, nr))
                        goto out_nomap;

                page_idx = idx;
                address = folio_start;
                ptep = folio_ptep;
                goto check_folio;
        }

Thanks
Barry