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

[Naoya Horiguchi]

On Wed, Oct 13, 2021 at 11:01:33AM +0800, Peter Xu wrote:
> On Tue, Oct 12, 2021 at 07:48:39PM -0700, Yang Shi wrote:
> > On Tue, Oct 12, 2021 at 3:10 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Oct 12, 2021 at 11:02:09AM -0700, Yang Shi wrote:
> > > > On Mon, Oct 11, 2021 at 6:44 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Oct 11, 2021 at 08:55:26PM -0400, Peter Xu wrote:
> > > > > > Another thing is I noticed soft_offline_in_use_page() will still ignore file
> > > > > > backed split.  I'm not sure whether it means we'd better also handle that case
> > > > > > as well, so shmem thp can be split there too?
> > > > >
> > > > > Please ignore this paragraph - I somehow read "!PageHuge(page)" as
> > > > > "PageAnon(page)"...  So I think patch 5 handles soft offline too.
> > > >
> > > > Yes, exactly. And even though the split is failed (or file THP didn't
> > > > get split before patch 5/5), soft offline would just return -EBUSY
> > > > instead of calling __soft_offline_page->page_handle_poison(). So
> > > > page_handle_poison() should not see THP at all.
> > >
> > > I see, so I'm trying to summarize myself on what I see now with the new logic..
> > >
> > > I think the offline code handles hwpoison differently as it sets PageHWPoison
> > > at the end of the process, IOW if anything failed during the offline process
> > > the hwpoison bit is not set.
> > >
> > > That's different from how the memory failure path is handling this, as in that
> > > case the hwpoison bit on the subpage is set firstly, e.g. before split thp.  I
> > > believe that's also why memory failure requires the extra sub-page-hwpoison bit
> > > while offline code shouldn't need to: because for soft offline split happens
> > > before setting hwpoison so we just won't ever see a "poisoned file thp", while
> > > for memory failure it could happen, and the sub-page-hwpoison will be a temp
> > > bit anyway only exist for a very short period right after we set hwpoison on
> > > the small page but before we split the thp.
> > >
> > > Am I right above?
> > 
> > Yeah, you are right. I noticed this too, only successfully migrated
> > page is marked as hwpoison. But TBH I'm not sure why it does this way.
> 
> My wild guess is that unlike memory failures, soft offline is best-effort. Say,
> the data on the page is still consistent, so even if offline failed for some
> reason we shouldn't stop the program from execution.  That's not true for
> memory failures via MCEs, afaict, as the execution could read/write wrong data
> and that'll be a serious mistake, so we set hwpoison 1st there first before
> doing anything else, making sure "this page is broken" message delivered and
> user app won't run with risk.
> 
> But yeah it'll be great if Naoya could help confirm that.

Yes, these descriptions are totally correct, how PG_hwpoison flag is set
is different between hwpoison/soft-offline mechanisms from the beginning.

Thanks,
Naoya Horiguchi