linux-kernel - Re: [PATCH] mm/memory-failure: remove shake

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Date: Fri, 26 Apr 2024 20:05:18 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Sidhartha Kumar <sidhartha.kumar@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, linmiaohe@...wei.com,
	jane.chu@...cle.com, nao.horiguchi@...il.com, osalvador@...e.de
Subject: Re: [PATCH] mm/memory-failure: remove shake_page()

On Fri, Apr 26, 2024 at 11:53:01AM -0700, Sidhartha Kumar wrote:
> On 4/26/24 11:27 AM, Matthew Wilcox wrote:
> > On Fri, Apr 26, 2024 at 10:57:31AM -0700, Sidhartha Kumar wrote:
> > > On 4/26/24 10:34 AM, Matthew Wilcox wrote:
> > > > On Fri, Apr 26, 2024 at 10:15:11AM -0700, Sidhartha Kumar wrote:
> > > > > Use a folio in get_any_page() to save 5 calls to compound head and
> > > > > convert the last user of shake_page() to shake_folio(). This allows us
> > > > > to remove the shake_page() definition.
> > > > 
> > > > So I didn't do this before because I wasn't convinced it was safe.
> > > > We don't have a refcount on the folio, so the page might no longer
> > > > be part of this folio by the time we get the refcount on the folio.
> > > > 
> > > > I'd really like to see some argumentation for why this is safe.
> > > 
> > > If I moved down the folio = page_folio() line to after we verify
> > > __get_hwpoison_page() has returned 1, which indicates the reference count
> > > was successfully incremented via foliO_try_get(), that means the folio
> > > conversion would happen after we have a refcount. In the case we don't call
> > > __get_hwpoison_page(), that means the MF_COUNT_INCREASED flag is set. This
> > > means the page has existing users so that path would be safe as well. So I
> > > think this is safe after moving page_folio() after __get_hwpoison_page().
> > 
> > See if you can find a hole in this chain of reasoning ...
> > 
> > memory_failure()
> >          p = pfn_to_online_page(pfn);
> >          res = try_memory_failure_hugetlb(pfn, flags, &hugetlb);
> > (not a hugetlb)
> >          if (TestSetPageHWPoison(p)) {
> > (not already poisoned)
> >          if (!(flags & MF_COUNT_INCREASED)) {
> >                  res = get_hwpoison_page(p, flags);
> > 
> > get_hwpoison_page()
> >                  ret = get_any_page(p, flags);
> > 
> > get_any_page()
> > 	folio = page_folio(page)
> 
> That would be unsafe, the safe way would be if we moved page_folio() after
> the call to __get_hw_poison() in get_any_page() and there would still be one
> remaining user of shake_page() that we can't convert. A safe version of this
> patch would result in a removal of one use of PageHuge() and two uses of
> put_page(), would that be worth submitting?
> 
> get_any_page()
> 	if(__get_hwpoison_page())
> 		folio = page_folio() /* folio_try_get() returned 1, safe */

I think we should convert __get_hwpoison_page() to return either the folio
or an ERR_PTR or NULL.  Also, I think we should delete the "cannot catch
tail" part and just loop in __get_hwpoison_page() until we do catch it.
See try_get_folio() in mm/gup.c for inspiration (although you can't use
it exactly because that code knows that the page is mapped into a page
table, so has a refcount).

But that's just an immediate assessment; you might find a reason that
doesn't work.