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From: Lance Yang <ioworker0@gmail.com>
Date: Fri, 12 Apr 2024 09:48:19 +0800
Message-ID: <CAK1f24=iMEcD43AVv7Xbhviv_K=3CBAidTwT3bOTRMW8yhKt4w@mail.gmail.com>
Subject: Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in madvise_free
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: akpm@linux-foundation.org, david@redhat.com, 21cnbao@gmail.com, 
	mhocko@suse.com, fengwei.yin@intel.com, zokeefe@google.com, 
	shy828301@gmail.com, xiehuan09@gmail.com, wangkefeng.wang@huawei.com, 
	songmuchun@bytedance.com, peterx@redhat.com, minchan@kernel.org, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Thu, Apr 11, 2024 at 10:39=E2=80=AFPM Ryan Roberts <ryan.roberts@arm.com=
> wrote:
>
> On 11/04/2024 15:07, Lance Yang wrote:
> > On Thu, Apr 11, 2024 at 9:48=E2=80=AFPM Ryan Roberts <ryan.roberts@arm.=
com> wrote:
> >>
> >> [...]
> >>
> >>>>> +
> >>>>> +             if (!folio_trylock(folio))
> >>>>> +                     continue;
> >>>>
> >>>> This is still wrong. This should all be protected by the "if
> >>>> (folio_test_swapcache(folio) || folio_test_dirty(folio))" as it was =
previously
> >>>> so that you only call folio_trylock() if that condition is true. You=
 are
> >>>> unconditionally locking here, then unlocking, then relocking below i=
f the
> >>>> condition is met. Just put everything inside the condition and lock =
once.
> >>>
> >>> I'm not sure if it's safe to call folio_mapcount() without holding th=
e
> >>> folio lock.
> >>>
> >>> As mentioned earlier by David in the v2[1]
> >>>> What could work for large folios is making sure that #ptes that map =
the
> >>>> folio here correspond to the folio_mapcount(). And folio_mapcount()
> >>>> should be called under folio lock, to avoid racing with swapout/migr=
ation.
> >>>
> >>> [1] https://lore.kernel.org/all/5cc05529-eb80-410e-bc26-233b0ba0b21f@=
redhat.com/
> >>
> >> But I'm not suggesting that you should call folio_mapcount() without t=
he lock.
> >> I'm proposing this:
> >>
> >>                 if (folio_test_swapcache(folio) || folio_test_dirty(fo=
lio)) {
> >>                         if (!folio_trylock(folio))
> >>                                 continue;
> >>                         /*
> >> -                        * If folio is shared with others, we mustn't =
clear
> >> -                        * the folio's dirty flag.
> >> +                        * If we have a large folio at this point, we =
know it is
> >> +                        * fully mapped so if its mapcount is the same=
 as its
> >> +                        * number of pages, it must be exclusive.
> >>                          */
> >> -                       if (folio_mapcount(folio) !=3D 1) {
> >> +                       if (folio_mapcount(folio) !=3D folio_nr_pages(=
folio)) {
> >>                                 folio_unlock(folio);
> >>                                 continue;
> >>                         }
> >
> > IIUC, if the folio is clean and not in the swapcache, we still need to
> > compare the number of batched PTEs against folio_mapcount().
>
> Why? That's not how the old code worked. In fact the comment says that th=
e
> reason for the exclusive check is to avoid marking a dirty *folio* as cle=
an if
> shared; that would be bad because we could throw away data that others re=
lied
> upon. It's perfectly safe to clear the dirty flag from the *pte* even if =
it is
> shared; the ptes are private to the process so that won't affect sharers.
>
> You should just follow the pattern already estabilished by the original c=
ode.
> The only difference is that because the folio is now (potentially) large,=
 you
> have to change the way to detect exclusivity.

Thanks a lot for your patience and help!

My bad for the oversight and mistake :(
I'll take another look at the original code and make adjustments following =
the
established pattern.

Thanks,
Lance

>
> >
> > Thanks,
> > Lance
> >
> >>
> >> What am I missing?
> >>
>