From: Will Deacon <will@kernel.org>
To: Barry Song <21cnbao@gmail.com>
Cc: "Yu Zhao" <yuzhao@google.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
Linux-MM <linux-mm@kvack.org>, "Andi Kleen" <ak@linux.intel.com>,
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huzhanyuan@oppo.com
Subject: Re: [PATCH v11 07/14] mm: multi-gen LRU: exploit locality in rmap
Date: Tue, 7 Jun 2022 11:43:58 +0100 [thread overview]
Message-ID: <20220607104358.GA32583@willie-the-truck> (raw)
In-Reply-To: <CAGsJ_4zGEdHDv0ObZ-5y8sFKLO7Y6ZjTsZFs0KvdLwA_-iGJ5A@mail.gmail.com>
On Tue, Jun 07, 2022 at 10:37:46AM +1200, Barry Song wrote:
> On Tue, Jun 7, 2022 at 10:21 PM Will Deacon <will@kernel.org> wrote:
> > On Tue, Jun 07, 2022 at 07:37:10PM +1200, Barry Song wrote:
> > > I can't really explain why we are getting a random app/java vm crash in monkey
> > > test by using ptep_test_and_clear_young() only in lru_gen_look_around() on an
> > > armv8-a machine without hardware PTE young support.
> > >
> > > Moving to ptep_clear_flush_young() in look_around can make the random
> > > hang disappear according to zhanyuan(Cc-ed).
> > >
> > > On x86, ptep_clear_flush_young() is exactly ptep_test_and_clear_young()
> > > after
> > > 'commit b13b1d2d8692 ("x86/mm: In the PTE swapout page reclaim case clear
> > > the accessed bit instead of flushing the TLB")'
> > >
> > > But on arm64, they are different. according to Will's comments in this
> > > thread which
> > > tried to make arm64 same with x86,
> > > https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1793881.html
> > >
> > > "
> > > This is blindly copied from x86 and isn't true for us: we don't invalidate
> > > the TLB on context switch. That means our window for keeping the stale
> > > entries around is potentially much bigger and might not be a great idea.
> > >
> > > If we roll a TLB invalidation routine without the trailing DSB, what sort of
> > > performance does that get you?
> > > "
> > > We shouldn't think ptep_clear_flush_young() is safe enough in LRU to
> > > clear PTE young? Any comments from Will?
> >
> > Given that this issue is specific to the multi-gen LRU work, I think Yu is
> > the best person to comment. However, looking quickly at your analysis above,
> > I wonder if the code is relying on this sequence:
> >
> >
> > ptep_test_and_clear_young(vma, address, ptep);
> > ptep_clear_flush_young(vma, address, ptep);
> >
> >
> > to invalidate the TLB. On arm64, that won't be the case, as the invalidation
> > in ptep_clear_flush_young() is predicated on the pte being young (and this
> > patches the generic implementation in mm/pgtable-generic.c. In fact, that
> > second function call is always going to be a no-op unless the pte became
> > young again in the middle.
>
> thanks for your reply, sorry for failing to let you understand my question.
> my question is actually as below,
> right now lru_gen_look_around() is using ptep_test_and_clear_young()
> only without flush to clear pte for a couple of pages including the specific
> address:
> void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
> {
> ...
>
> for (i = 0, addr = start; addr != end; i++, addr += PAGE_SIZE) {
> ...
>
> if (!ptep_test_and_clear_young(pvmw->vma, addr, pte + i))
> continue;
>
> ...
> }
>
> I wonder if it is safe to arm64. Do we need to move to ptep_clear_flush_young()
> in the loop?
I don't know what this code is doing, so Yu is the best person to answer
that. There's nothing inherently dangerous about eliding the TLB
maintenance; it really depends on the guarantees needed by the caller.
However, the snippet you posted from folio_referenced_one():
| if (pvmw.pte) {
| + if (lru_gen_enabled() && pte_young(*pvmw.pte) &&
| + !(vma->vm_flags & (VM_SEQ_READ | VM_RAND_READ))) {
| + lru_gen_look_around(&pvmw);
| + referenced++;
| + }
| +
| if (ptep_clear_flush_young_notify(vma, address,
Does seem to call lru_gen_look_around() *and*
ptep_clear_flush_young_notify(), which is what prompted my question as it
looks pretty suspicious to me.
Will
next prev parent reply other threads:[~2022-06-07 10:44 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-18 1:46 [PATCH v11 00/14] Multi-Gen LRU Framework Yu Zhao
2022-05-18 1:46 ` [PATCH v11 01/14] mm: x86, arm64: add arch_has_hw_pte_young() Yu Zhao
2022-05-18 1:46 ` [PATCH v11 02/14] mm: x86: add CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG Yu Zhao
2022-05-18 1:46 ` [PATCH v11 03/14] mm/vmscan.c: refactor shrink_node() Yu Zhao
2022-05-18 1:46 ` [PATCH v11 04/14] Revert "include/linux/mm_inline.h: fold __update_lru_size() into its sole caller" Yu Zhao
2022-05-18 1:46 ` [PATCH v11 05/14] mm: multi-gen LRU: groundwork Yu Zhao
2022-06-09 5:33 ` zhong jiang
2022-05-18 1:46 ` [PATCH v11 06/14] mm: multi-gen LRU: minimal implementation Yu Zhao
2022-06-09 12:34 ` zhong jiang
2022-06-09 14:46 ` zhong jiang
2022-05-18 1:46 ` [PATCH v11 07/14] mm: multi-gen LRU: exploit locality in rmap Yu Zhao
2022-06-06 9:25 ` Barry Song
2022-06-07 7:37 ` Barry Song
2022-06-07 10:21 ` Will Deacon
2022-06-06 22:37 ` Barry Song
2022-06-07 10:43 ` Will Deacon [this message]
2022-06-07 21:06 ` Yu Zhao
2022-06-08 0:43 ` Barry Song
2022-06-08 15:51 ` Linus Torvalds
2022-06-08 22:45 ` Barry Song
2022-06-16 21:55 ` Yu Zhao
2022-06-16 22:33 ` Barry Song
2022-06-16 23:29 ` Yu Zhao
2022-06-17 1:42 ` Yu Zhao
2022-06-17 2:01 ` Barry Song
2022-06-17 3:03 ` Yu Zhao
2022-06-17 3:17 ` Yu Zhao
2022-06-19 20:36 ` Yu Zhao
2022-06-19 21:56 ` Barry Song
2022-06-07 19:07 ` Yu Zhao
2022-06-08 7:48 ` Barry Song
2022-06-07 18:58 ` Yu Zhao
2022-05-18 1:46 ` [PATCH v11 08/14] mm: multi-gen LRU: support page table walks Yu Zhao
2022-05-18 1:46 ` [PATCH v11 09/14] mm: multi-gen LRU: optimize multiple memcgs Yu Zhao
2022-05-18 1:46 ` [PATCH v11 10/14] mm: multi-gen LRU: kill switch Yu Zhao
2022-05-18 1:46 ` [PATCH v11 11/14] mm: multi-gen LRU: thrashing prevention Yu Zhao
2022-05-18 1:46 ` [PATCH v11 12/14] mm: multi-gen LRU: debugfs interface Yu Zhao
2022-05-18 1:46 ` [PATCH v11 13/14] mm: multi-gen LRU: admin guide Yu Zhao
2022-05-18 1:46 ` [PATCH v11 14/14] mm: multi-gen LRU: design doc Yu Zhao
2022-05-18 2:05 ` [PATCH v11 00/14] Multi-Gen LRU Framework Jens Axboe
2022-06-07 22:47 ` Yu Zhao
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