Date:   Sat, 19 Nov 2022 22:20:20 +0800
From:   hev <r@....cc>
To:     Peter Xu <peterx@...hat.com>
Cc:     Huacai Chen <chenhuacai@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>, loongarch@...ts.linux.dev,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        "David S . Miller" <davem@...emloft.net>,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 04/47] LoongArch: Set _PAGE_DIRTY only if _PAGE_WRITE is
 set in {pmd,pte}_mkdirty()

Hi, Peter,

On Fri, Nov 18, 2022 at 2:53 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Thu, Nov 17, 2022 at 10:12:07AM -0500, Peter Xu wrote:
> > Hi, Huacai,
> >
> > On Thu, Nov 17, 2022 at 12:25:32PM +0800, Huacai Chen wrote:
> > > Now {pmd,pte}_mkdirty() set _PAGE_DIRTY bit unconditionally, this causes
> > > random segmentation fault after commit 0ccf7f168e17bb7e ("mm/thp: carry
> > > over dirty bit when thp splits on pmd").
> > >
> > > The reason is: when fork(), parent process use pmd_wrprotect() to clear
> > > huge page's _PAGE_WRITE and _PAGE_DIRTY (for COW);
> >
> > Is it safe to drop dirty bit when wr-protect?  It means the mm can reclaim
> > the page directly assuming the page contains rubbish.
> >
> > Consider after fork() and memory pressure kicks the kswapd, I don't see
> > anything stops the kswapd from recycling the pages and lose the data in
> > both processes.
>
> Feel free to ignore this question..  I think I got an answer from Hev (and
> I then got a follow up question):
>
> https://lore.kernel.org/all/Y3Z9Zf0jARMOkFBq@x1n/
>
> >
> > > then pte_mkdirty() set
> > > _PAGE_DIRTY as well as _PAGE_MODIFIED while splitting dirty huge pages;
> > > once _PAGE_DIRTY is set, there will be no tlb modify exception so the COW
> > > machanism fails; and at last memory corruption occurred between parent
> > > and child processes.
> > >
> > > So, we should set _PAGE_DIRTY only when _PAGE_WRITE is set in {pmd,pte}_
> > > mkdirty().
> > >
> > > Cc: stable@...r.kernel.org
> > > Cc: Peter Xu <peterx@...hat.com>
> > > Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> > > ---
> > > Note: CC sparc maillist because they have similar issues.
> >
> > I also had a look on sparc64, it seems to not do the same as loongarch
> > here (not removing dirty in wr-protect):
> >
> > static inline pmd_t pmd_wrprotect(pmd_t pmd)
> > {
> >       pte_t pte = __pte(pmd_val(pmd));
> >
> >       pte = pte_wrprotect(pte);
> >
> >       return __pmd(pte_val(pte));
> > }
> >
> > static inline pte_t pte_wrprotect(pte_t pte)
> > {
> >       unsigned long val = pte_val(pte), tmp;
> >
> >       __asm__ __volatile__(
> >       "\n661: andn            %0, %3, %0\n"
> >       "       nop\n"
> >       "\n662: nop\n"
> >       "       nop\n"
> >       "       .section        .sun4v_2insn_patch, \"ax\"\n"
> >       "       .word           661b\n"
> >       "       sethi           %%uhi(%4), %1\n"
> >       "       sllx            %1, 32, %1\n"
> >       "       .word           662b\n"
> >       "       or              %1, %%lo(%4), %1\n"
> >       "       andn            %0, %1, %0\n"
> >       "       .previous\n"
> >       : "=r" (val), "=r" (tmp)
> >       : "0" (val), "i" (_PAGE_WRITE_4U | _PAGE_W_4U),
> >         "i" (_PAGE_WRITE_4V | _PAGE_W_4V));
> >
> >       return __pte(val);
> > }
>
> (Same here; I just overlooked what does _PAGE_W_4U meant..)
>
> >
> > >
> > >  arch/loongarch/include/asm/pgtable.h | 8 ++++++--
> > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
> > > index 946704bee599..debbe116f105 100644
> > > --- a/arch/loongarch/include/asm/pgtable.h
> > > +++ b/arch/loongarch/include/asm/pgtable.h
> > > @@ -349,7 +349,9 @@ static inline pte_t pte_mkclean(pte_t pte)
> > >
> > >  static inline pte_t pte_mkdirty(pte_t pte)
> > >  {
> > > -   pte_val(pte) |= (_PAGE_DIRTY | _PAGE_MODIFIED);
> > > +   pte_val(pte) |= _PAGE_MODIFIED;
> > > +   if (pte_val(pte) & _PAGE_WRITE)
> > > +           pte_val(pte) |= _PAGE_DIRTY;
> >
> > I'm not sure whether mm has rule to always set write bit then set dirty
> > bit, need to be careful here because the outcome may differ when use:
> >
> >   pte_mkdirty(pte_mkwrite(pte))
> >   (expected)
> >
> > VS:
> >
> >   pte_mkwrite(pte_mkdirty(pte))
> >   (dirty not set)
> >
> > I had a feeling I miss some arch-specific details here on why loongarch
> > needs such implementation, but I can't quickly tell.
>
> After a closer look I think it's fine for loongarch as pte_mkwrite will
> also set the dirty bit unconditionally, so at least the two ways will still
> generate the same pte (DIRTY+MODIFIED+WRITE).
>
> But this whole thing is still confusing to me.  It'll still be great if
> anyone can help explain why the _DIRTY cannot be set only in pte_mkwrite()
> if that's the solo place in charge of "whether the pte is writable".
>
> The other follow up question is: how do we mark "this pte contains valid
> data" (the common definition of "dirty bit"), while "this pte is not
> writable" on loongarch?
>
> It can happen when we're installing a page with non-zero data meanwhile
> wr-protected.  That's actually a valid case for userfaultfd wr-protect mode
> where user specified UFFDIO_COPY ioctl with flag UFFDIO_COPY_MODE_WP, where
> we'll install a non-zero page from user buffer but don't grant write bit.
>
> From code-wise, I think it can be done currently with this on loongarch:
>
>   pte_wrprotect(pte_mkwrite(pte_mkdirty(pte)))
>
> Where pte_wrprotect(pte_mkwrite(pte)) is not a no-op but applying MODIFIED.

We would like to note that on LoongArch (for misunderstanding naming):
* _PAGE_DIRTY meaning hardware writable.
* _PAGE_WRITE meaning software writable.
* _PAGE_MODIFIED meaning software dirty, this page contains updated valid data.

PTE APIs:
* pte_mkwrite: Allow to write, only needs set _PAGE_WRITE.
* pte_mkdirty: Mark as dirty, only needs set _PAGE_MODIFIED.
* pte_dirty: Test is dirty, only test _PAGE_MODIFIED.
* pte_wrprotect: Clear both writable, force to raise exception to
handle_mm_fault.

If a pte is only set _PAGE_WRITE without _PAGE_DIRTY by pte_mkwrite,
then a write memory access will cause mmu exception, and the
(_PAGE_DIRTY|_PAGE_MODIFIED) will be set in this exception handler. I
think the _PAGE_DIRTY is also possible to set in pte_mkwrite for
speedup, then _PAGE_MODIFIED must be set at the same time. To avoid
the page data being modified but not detected by pte_dirty. (Current
code may needs to fix

pte_mkdirty mark pte as dirty is the main function, It can also make
pte writeable by hardware(_PAGE_DIRTY) for speedup (too) if and only
if the pte is writable(_PAGE_WRITE). (mkdirty sets _PAGE_DIRTY
unconditionally is the root cause of the huge page COW issue.

For write-protection, pte_wrprotect will clear both writable(software
and hardware) in pte to force a MMU exception to handle_mm_fault.

So yeah, the pte marked as dirty(_PAGE_MODIFIED) and without any
writable in the following code:

  pte_wrprotect(pte_mkwrite(pte_mkdirty(pte)))

Regards,
Ray

>
> While on many other archs it'll be as simple as:
>
>   pte_mkdirty(pte)
>
> But that's really error-prone and not obvious.
>
> Copying Hev too.
>
> Thanks,
>
> --
> Peter Xu
>

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