Re: [PATCH] mm: hwpoison: deal with page cache THP

[HORIGUCHI NAOYA(堀口 直也)]

On Tue, Sep 07, 2021 at 08:14:26PM -0700, Yang Shi wrote:
...
> > > > > > > 5. We also could define a new FGP flag to return poisoned page, NULL
> > > > > > > or error pointer. This also should need significant code change since
> > > > > > > a lt callsites need to be contemplated.
> > > > > >
> > > > > > Could you explain a little more about which callers should use the flag?
> > > > >
> > > > > Just to solve the above invalidate/truncate problem and page fault
> > > > > doesn't expect an error pointer. But it seems the above
> > > > > invalidate/truncate paths don't matter. Page fault should be the only
> > > > > user since page fault may need unlock the page if poisoned page is
> > > > > returned.
> >
> > Orignally PG_hwpoison is detected in page fault handler for file-backed pages
> > like below,
> >
> >   static vm_fault_t __do_fault(struct vm_fault *vmf)
> >   ...
> >           ret = vma->vm_ops->fault(vmf);
> >           if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
> >                               VM_FAULT_DONE_COW)))
> >                   return ret;
> >
> >           if (unlikely(PageHWPoison(vmf->page))) {
> >                   if (ret & VM_FAULT_LOCKED)
> >                           unlock_page(vmf->page);
> >                   put_page(vmf->page);
> >                   vmf->page = NULL;
> >                   return VM_FAULT_HWPOISON;
> >           }
> >
> > so it seems to me that if a filesystem switches to the new scheme of keeping
> > error pages in page cache, ->fault() callback for the filesystem needs to be
> > aware of hwpoisoned pages inside it and returns VM_FAULT_HWPOISON when it
> > detects an error page (maybe by detecting pagecache_get_page() to return
> > PTR_ERR(-EIO or -EHWPOISON) or some other ways).  In the other filesystems
> > with the old scheme, fault() callback does not return VM_FAULT_HWPOISON so
> > that page fault handler falls back to the generic PageHWPoison check above.
> 
> Yes, exactly. If we return ERR_PTR we need modify the above code
> accordingly. But returning the page, no change is required.
> 
> >
> > > >
> > > > It seems page fault check IS_ERR(page) then just return
> > > > VM_FAULT_HWPOISON. But I found a couple of places in shmem which want
> > > > to return head page then handle subpage or just return the page but
> > > > don't care the content of the page. They should ignore hwpoison. So I
> > > > guess we'd better to have a FGP flag for such cases.
> >
> > At least currently thp are supposed to be split before error handling.
> 
> Not for file THP.
> 
> > We could loosen this assumption to support hwpoison on a subpage of thp,
> > but I'm not sure whether that has some benefit.
> 
> I don't quite get your point. Currently the hwpoison flag is set on
> specific subpage instead of head page.

Sorry, I miss the case when thp split fails, then the thp has hwpoison
flag set on any subpage.  I only thought of the successful case, where the
resulting error page is no longer a subpage.

> 
> > And also this discussion makes me aware that we need to have a way to
> > prevent hwpoisoned pages from being used to thp collapse operation.
> 
> Actually not. The refcount from hwpoison could prevent it from
> collapsing. But if we return ERR_PTR (#3) we need extra code in
> khugepaged to handle it.

OK, so we already prevent it.

Thanks,
Naoya Horiguchi

> 
> So I realized #1 would require fewer changes. And leaving the page
> state check to callers seem reasonable since the callers usually check
> other states, e.g. uptodate.