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

From: Peter Xu <peterx@redhat.com>
To: Yang Shi <shy828301@gmail.com>
Cc: "HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>,
	"Hugh Dickins" <hughd@google.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Oscar Salvador" <osalvador@suse.de>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"Linux FS-devel Mailing List" <linux-fsdevel@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [v3 PATCH 2/5] mm: filemap: check if THP has hwpoisoned subpage for PMD page fault
Date: Thu, 14 Oct 2021 07:13:18 +0800	[thread overview]
Message-ID: <YWdoDn4uEYG8jpXH@t490s> (raw)
In-Reply-To: <CAHbLzkrcOpG5AHk934hDJb2d+FocYjUc6nhBRofhTbTxLVWtYA@mail.gmail.com>

On Wed, Oct 13, 2021 at 02:42:42PM -0700, Yang Shi wrote:
> On Tue, Oct 12, 2021 at 8:41 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Tue, Oct 12, 2021 at 08:27:06PM -0700, Yang Shi wrote:
> > > > But this also reminded me that shouldn't we be with the page lock already
> > > > during the process of "setting hwpoison-subpage bit, split thp, clear
> > > > hwpoison-subpage bit"?  If it's only the small window that needs protection,
> > > > while when looking up the shmem pagecache we always need to take the page lock
> > > > too, then it seems already safe even without the extra bit?  Hmm?
> > >
> > > I don't quite get your point. Do you mean memory_failure()? If so the
> > > answer is no, outside the page lock. And the window may be indefinite
> > > since file THP doesn't get split before this series and the split may
> > > fail even after this series.
> >
> > What I meant is that we could extend the page_lock in try_to_split_thp_page()
> > to cover setting hwpoison-subpage too (and it of course covers the clearing if
> > thp split succeeded, as that's part of the split process).  But yeah it's a
> > good point that the split may fail, so the extra bit seems still necessary.
> >
> > Maybe that'll be something worth mentioning in the commit message too?  The
> > commit message described very well on the overhead of looping over 512 pages,
> > however the reader can easily overlook the real reason for needing this bit -
> > IMHO it's really for the thp split failure case, as we could also mention that
> > if thp split won't fail, page lock should be suffice (imho).  We could also
> 
> Not only for THP split failure case. Before this series, shmem THP
> does't get split at all. And this commit is supposed to be backported
> to the older versions, so saying "page lock is sufficient" is not
> precise and confusing.

Sure, please feel free to use any wording you prefer as long as the other side
of the lock besides the performance impact could be mentioned.  Thanks,

-- 
Peter Xu