Re: [PATCH v2 14/14] arm64/mm: Add ptep_get_and_clear_full() to optimize process teardown

Alistair Popple
2023-11-27T07:43:46Z

Ryan Roberts <ryan.roberts@arm.com> writes:

> On 24/11/2023 01:35, Alistair Popple wrote:
>>
>> Ryan Roberts <ryan.roberts@arm.com> writes:
>>
>>> On 23/11/2023 05:13, Alistair Popple wrote:
>>>>
>>>> Ryan Roberts <ryan.roberts@arm.com> writes:
>>>>
>>>>> ptep_get_and_clear_full() adds a 'full' parameter which is not present
>>>>> for the fallback ptep_get_and_clear() function. 'full' is set to 1 when
>>>>> a full address space teardown is in progress. We use this information to
>>>>> optimize arm64_sys_exit_group() by avoiding unfolding (and therefore
>>>>> tlbi) contiguous ranges. Instead we just clear the PTE but allow all the
>>>>> contiguous neighbours to keep their contig bit set, because we know we
>>>>> are about to clear the rest too.
>>>>>
>>>>> Before this optimization, the cost of arm64_sys_exit_group() exploded to
>>>>> 32x what it was before PTE_CONT support was wired up, when compiling the
>>>>> kernel. With this optimization in place, we are back down to the
>>>>> original cost.
>>>>>
>>>>> This approach is not perfect though, as for the duration between
>>>>> returning from the first call to ptep_get_and_clear_full() and making
>>>>> the final call, the contpte block in an intermediate state, where some
>>>>> ptes are cleared and others are still set with the PTE_CONT bit. If any
>>>>> other APIs are called for the ptes in the contpte block during that
>>>>> time, we have to be very careful. The core code currently interleaves
>>>>> calls to ptep_get_and_clear_full() with ptep_get() and so ptep_get()
>>>>> must be careful to ignore the cleared entries when accumulating the
>>>>> access and dirty bits - the same goes for ptep_get_lockless(). The only
>>>>> other calls we might resonably expect are to set markers in the
>>>>> previously cleared ptes. (We shouldn't see valid entries being set until
>>>>> after the tlbi, at which point we are no longer in the intermediate
>>>>> state). Since markers are not valid, this is safe; set_ptes() will see
>>>>> the old, invalid entry and will not attempt to unfold. And the new pte
>>>>> is also invalid so it won't attempt to fold. We shouldn't see this for
>>>>> the 'full' case anyway.
>>>>>
>>>>> The last remaining issue is returning the access/dirty bits. That info
>>>>> could be present in any of the ptes in the contpte block. ptep_get()
>>>>> will gather those bits from across the contpte block. We don't bother
>>>>> doing that here, because we know that the information is used by the
>>>>> core-mm to mark the underlying folio as accessed/dirty. And since the
>>>>> same folio must be underpinning the whole block (that was a requirement
>>>>> for folding in the first place), that information will make it to the
>>>>> folio eventually once all the ptes have been cleared. This approach
>>>>> means we don't have to play games with accumulating and storing the
>>>>> bits. It does mean that any interleaved calls to ptep_get() may lack
>>>>> correct access/dirty information if we have already cleared the pte that
>>>>> happened to store it. The core code does not rely on this though.
>>>>
>>>> Does not *currently* rely on this. I can't help but think it is
>>>> potentially something that could change in the future though which would
>>>> lead to some subtle bugs.
>>>
>>> Yes, there is a risk, although IMHO, its very small.
>>>
>>>>
>>>> Would there be any may of avoiding this? Half baked thought but could
>>>> you for example copy the access/dirty information to the last (or
>>>> perhaps first, most likely invalid) PTE?
>>>
>>> I spent a long time thinking about this and came up with a number of
>>> possibilities, none of them ideal. In the end, I went for the simplest one
>>> (which works but suffers from the problem that it depends on the way it is
>>> called not changing).
>>
>> Ok, that answers my underlying question of "has someone thought about
>> this and are there any easy solutions". I suspected that was the case
>> given the excellent write up though!
>>
>>> 1) copy the access/dirty flags into all the remaining uncleared ptes within the
>>> contpte block. This is how I did it in v1; although it was racy. I think this
>>> could be implemented correctly but its extremely complex.
>>>
>>> 2) batch calls from the core-mm (like I did for pte_set_wrprotects()) so that we
>>> can clear 1 or more full contpte blocks in a single call - the ptes are never in
>>> an intermediate state. This is difficult because ptep_get_and_clear_full()
>>> returns the pte that was cleared so its difficult to scale that up to multiple ptes.
>>>
>>> 3) add ptep_get_no_access_dirty() and redefine the interface to only allow that
>>> to be called while ptep_get_and_clear_full() calls are on-going. Then assert in
>>> the other functions that ptep_get_and_clear_full() is not on-going when they are
>>> called. So we would get a clear sign that usage patterns have changed. But there
>>> is no easy place to store that state (other than scanning a contpte block
>>> looking for pte_none() amongst pte_valid_cont() entries) and it all felt ugly.
>>>
>>> 4) The simple approach I ended up taking; I thought it would be best to keep it
>>> simple and see if anyone was concerned before doing something more drastic.
>>>
>>> What do you think? If we really need to solve this, then option 1 is my
>>> preferred route, but it would take some time to figure out and reason about a
>>> race-free scheme.
>>
>> Well I like simple, and I agree the risk is small. But I can't help feel
>> the current situation is too subtle, mainly because it is architecture
>> specific and the assumptions are not communicated in core-mm code
>> anywhere. But also none of the aternatives seem much better.
>>
>> However there are only three callers of ptep_get_and_clear_full(), and
>> all of these hold the PTL. So if I'm not mistaken that should exclude
>> just about all users of ptep_get*() which will take the ptl before hand.
>
> The problem isn't racing threads because as you say, the PTL is already
> serializing all calls except ptep_get_lockless(). And although there are 3
> callers to ptep_get_and_clear_full(), only the caller in zap_pte_range() ever
> calls it with full=1, as I recall.
>
> The problem is that the caller in zap_pte_range() does this:
>
> ptl = lock_page_table()
> for each pte {
> ptent = ptep_get(pte);
> if (pte_present(ptent) {
> ptent = ptep_get_and_clear_full(ptent);
> if (pte_dirty(ptent))
> ...
> if (pte_young(ptent))
> ...
> }
> }
> unlock_page_table(ptl)
>
> It deliberately interleves calls to ptep_get() and ptep_get_and_clear_full()
> under the ptl. So if the loop is iterating over a contpte block and the HW
> happens to be storing the access/dirty info in the first pte entry, then the
> first time through the loop, ptep_get() will return the correct access/dirty
> info, as will ptep_get_and_clear_full(). The next time through the loop though,
> the access/dirty info which was in the previous pte is now cleared so ptep_get()
> and ptep_get_and_clear_full() will return old/clean. It all works, but is fragile.

So if ptep_get_lockless() isn't a concern what made the option posted in
v1 racy (your option 1 above)? Is there something else reading PTEs or
clearing PTE bits without holding the PTL that I'm missing?

>>
>> So really that only leaves ptep_get_lockless() that could/should
>> interleave right?
>
> Yes, but ptep_get_lockless() is special. Since it is called without the PTL, it
> is very careful to ensure that the contpte block is in a consistent state and it
> keeps trying until it is. So this will always return the correct consistent
> information.
>
>> From a quick glance of those users none look at the
>> young/dirty information anyway, so I wonder if we can just assert in the
>> core-mm that ptep_get_lockless() does not return young/dirty information
>> and clear it in the helpers? That would make things explicit and
>> consistent which would address my concern (although I haven't looked too
>> closely at the details there).
>
> As per explanation above, its not ptep_get_lockless() that is the problem so I
> don't think this helps.
>
> Thanks,
> Ryan
>
>>
>>> Thanks,
>>> Ryan
>>