On Tue, Jun 7, 2022 at 1:37 AM Barry Song [off-list ref] wrote:

On Mon, Jun 6, 2022 at 9:25 PM Barry Song [off-list ref] wrote:

quoted

On Wed, May 18, 2022 at 4:49 PM Yu Zhao [off-list ref] 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).

This sounds too familiar -- let me ask again: was the following commit
included during the test?

  07509e10dcc7 arm64: pgtable: Fix pte_accessible()

If not, it will cause exactly the problem you described. And what
about this one?

  e914d8f00391 mm: fix unexpected zeroed page mapping with zram swap

Missing it also causes userspace memory corruption on Android, i.e.,
random app crashes.

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?

quoted

btw, lru_gen_look_around() has already included 'address', are we doing
pte check for 'address' twice here?

Explained in the previous reply. Hope that clarifies things.

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