Re: [v3 PATCH 2/5] mm: filemap: check if THP has hwpoisoned subpage for PMD page fault

[Yang Shi <shy828301@gmail.com>]

On Fri, Oct 1, 2021 at 12:23 AM Naoya Horiguchi
<naoya.horiguchi@linux.dev> wrote:
>
> On Thu, Sep 30, 2021 at 02:53:08PM -0700, Yang Shi wrote:
> > When handling shmem page fault the THP with corrupted subpage could be PMD
> > mapped if certain conditions are satisfied.  But kernel is supposed to
> > send SIGBUS when trying to map hwpoisoned page.
> >
> > There are two paths which may do PMD map: fault around and regular fault.
> >
> > Before commit f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths")
> > the thing was even worse in fault around path.  The THP could be PMD mapped as
> > long as the VMA fits regardless what subpage is accessed and corrupted.  After
> > this commit as long as head page is not corrupted the THP could be PMD mapped.
> >
> > In the regular fault path the THP could be PMD mapped as long as the corrupted
> > page is not accessed and the VMA fits.
> >
> > This loophole could be fixed by iterating every subpage to check if any
> > of them is hwpoisoned or not, but it is somewhat costly in page fault path.
> >
> > So introduce a new page flag called HasHWPoisoned on the first tail page.  It
> > indicates the THP has hwpoisoned subpage(s).  It is set if any subpage of THP
> > is found hwpoisoned by memory failure and cleared when the THP is freed or
> > split.
> >
> > Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> > Cc: <stable@vger.kernel.org>
> > Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> ...
> > @@ -668,6 +673,20 @@ PAGEFLAG_FALSE(DoubleMap)
> >       TESTSCFLAG_FALSE(DoubleMap)
> >  #endif
> >
> > +#if defined(CONFIG_MEMORY_FAILURE) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
> > +/*
> > + * PageHasPoisoned indicates that at least on subpage is hwpoisoned in the
>
> Maybe you meant as follow?
>
> + * PageHasHWPoisoned indicates that at least one subpage is hwpoisoned in the

Yeah, thanks for catching it. It is a typo because the flag was called
PageHasPoisoned. But "poisoned" seems ambiguous for some cases since,
for example, some memory sanitizers use "poisoned", so I renamed it to
PageHasHWPoisoned to make it less ambiguous.

>
> Thanks,
> Naoya Horiguchi