Re: [PATCH 1/1] mm: prevent poison consumption when splitting THP

[Lance Yang]

On 2025/9/30 18:13, Lance Yang wrote:
> On 2025/9/30 16:53, David Hildenbrand wrote:
> > On 30.09.25 03:48, Lance Yang wrote:
> > > On Tue, Sep 30, 2025 at 3:07 AM David Hildenbrand <david@xxxxxxxxxx> 
> > > wrote:
> > > > On 29.09.25 18:30, Zhuo, Qiuxu wrote:
> > > > > Hi Tony,
> > > > >
> > > > > > From: Luck, Tony <tony.luck@xxxxxxxxx>
> > > > > > [...]
> > > > > > Subject: RE: [PATCH 1/1] mm: prevent poison consumption when 
> > > > > > splitting THP
> > > > > >
> > > > > > > Miaohe mentioned in another e-mail that there was an HWPoisoned flag
> > > > > > for the raw error 4K page.
> > > > > > > We could use that flag just to skip that raw error page and still 
> > > > > > > use
> > > > > > > the zeropage for other healthy sub-pages. I'll try that.
> > > > > >
> > > > > > That HWPoisoned flag is only set for raw pages where an error has 
> > > > > > been
> > > > > > detected. Maybe Linux could implement an
> > > > > > "is_this_page_all_zero_mc_safe()"[1] that would catch undetected 
> > > > > > poison
> > > > >
> > > > > This sounds like a great suggestion to me.
> > > > > Let's see what others think about this and the name (though the 
> > > > > name already LGTM 😊).
> > > >
> > > > The function name is just ... special. Not the good type of special 
> > > > IMHO. :)
> > > >
> > > > Note that we'll be moving to pages_identical() in [1]. Maybe we would
> > > > want a pages_identical_mc() or sth. like that as a follow up later.
> > > >
> > > >
> > > > So in any case, make that a follow-up work on top of a simple fix.
> > >
> > > Yeah. IIRC, as David suggested earlier, we can just check if a page is
> > > poisoned using PageHWPoison().
> > >
> > > Perhaps we should move this check into pages_identical()? This would 
> > > make
> > > it a central place to determine if pages are safe to access and merge ;)
> >
> > I would have to go into memcmp_pages(). Would be an option, but not 
> > sure if we should rather let callers deal with that.
> >
> > For example, in some cases it might be sufficient to just check if the 
> > large folio has any poisoned page and give up early.
>
> FWIW, one idea I had was to create a unified pre-flight checker, like
> folio_pages_identical_prepare(struct folio *folio). A caller could use
> it before a loop of pages_identical() calls to pre-check a folio :)

Forgot to add:

It would centralize all folio-level checks.

So if we ever need a new check in the future, we'd only modify the
prepare helper, not all the individual callers.