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Shutemov" , =?iso-8859-1?B?Suly9G1l?= Glisse , Andrea Arcangeli , David Rientjes , "Aneesh Kumar K.V" , swkhack , "Potyra, Stefan" , Mike Rapoport , Stephen Rothwell , Colin Ian King , Jason Gunthorpe , Mauro Carvalho Chehab , Peng Fan , Nikolay Borisov , Ira Weiny , Kirill Tkhai , Yafang Shao Subject: Re: [PATCH v4 3/9] mm/lru: replace pgdat lru_lock with lruvec lock Message-ID: <20191121220613.GB487872@cmpxchg.org> References: <1574166203-151975-1-git-send-email-alex.shi@linux.alibaba.com> <1574166203-151975-4-git-send-email-alex.shi@linux.alibaba.com> <20191119160456.GD382712@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.12.2 (2019-09-21) Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Nov 20, 2019 at 07:41:44PM +0800, Alex Shi wrote: > =E5=9C=A8 2019/11/20 =E4=B8=8A=E5=8D=8812:04, Johannes Weiner =E5=86=99= =E9=81=93: > >> @@ -1246,6 +1245,46 @@ struct lruvec *mem_cgroup_page_lruvec(struct = page *page, struct pglist_data *pgd > >> return lruvec; > >> } > >> =20 > >> +struct lruvec *lock_page_lruvec_irq(struct page *page, > >> + struct pglist_data *pgdat) > >> +{ > >> + struct lruvec *lruvec; > >> + > >> +again: > >> + rcu_read_lock(); > >> + lruvec =3D mem_cgroup_page_lruvec(page, pgdat); > >> + spin_lock_irq(&lruvec->lru_lock); > >> + rcu_read_unlock(); > > The spinlock doesn't prevent the lruvec from being freed > >=20 > > You deleted the rules from the mem_cgroup_page_lruvec() documentation= , > > but they still apply: if the page is already !PageLRU() by the time > > you get here, it could get reclaimed or migrated to another cgroup, > > and that can free the memcg/lruvec. Merely having the lru_lock held > > does not prevent this. >=20 >=20 > Forgive my idiot, I still don't know the details of unsafe lruvec here. > From my shortsight, the spin_lock_irq(embedded a preempt_disable) could= block all rcu syncing thus, keep all memcg alive until the preempt_enabl= ed in unspinlock, is this right? > If so even the page->mem_cgroup is migrated to others cgroups, the new = and old cgroup should still be alive here. You are right about the freeing part, I missed this. And I should have read this email here before sending out my "fix" to the current code; thankfully Hugh re-iterated my mistake on that thread. My apologies. But I still don't understand how the moving part is safe. You look up the lruvec optimistically, lock it, then verify the lookup. What keeps page->mem_cgroup from changing after you verified it? lock_page_lruvec(): mem_cgroup_move_account(): again: rcu_read_lock() lruvec =3D page->mem_cgroup->lruvec isolate_lru_page() spin_lock_irq(&lruvec->lru_lock) rcu_read_unlock() if page->mem_cgroup->lruvec !=3D lruvec: spin_unlock_irq(&lruvec->lru_lock) goto again; page->mem_cgroup =3D new cgroup putback_lru_page() // new lruvec SetPageLRU() return lruvec; // old lruvec The caller assumes page belongs to the returned lruvec and will then change the page's lru state with a mismatched page and lruvec. If we could restrict lock_page_lruvec() to working only on PageLRU pages, we could fix the problem with memory barriers. But this won't work for split_huge_page(), which is AFAICT the only user that needs to freeze the lru state of a page that could be isolated elsewhere. So AFAICS the only option is to lock out mem_cgroup_move_account() entirely when the lru_lock is held. Which I guess should be fine.