On 24 Dec 2024, at 7:37, David Hildenbrand wrote:
> On 23.12.24 23:14, Shakeel Butt wrote:
>> On Sat, Dec 21, 2024 at 05:18:20PM +0100, David Hildenbrand wrote:
>> [...]
>>>
>>> Yes, so I can see fuse
>>>
>>> (1) Breaking memory reclaim (memory cannot get freed up)
>>>
>>> (2) Breaking page migration (memory cannot be migrated)
>>>
>>> Due to (1) we might experience bigger memory pressure in the system I guess.
>>> A handful of these pages don't really hurt, I have no idea how bad having
>>> many of these pages can be. But yes, inherently we cannot throw away the
>>> data as long as it is dirty without causing harm. (maybe we could move it to
>>> some other cache, like swap/zswap; but that smells like a big and
>>> complicated project)
>>>
>>> Due to (2) we turn pages that are supposed to be movable possibly for a long
>>> time unmovable. Even a *single* such page will mean that CMA allocations /
>>> memory unplug can start failing.
>>>
>>> We have similar situations with page pinning. With things like O_DIRECT, our
>>> assumption/experience so far is that it will only take a couple of seconds
>>> max, and retry loops are sufficient to handle it. That's why only long-term
>>> pinning ("indeterminate", e.g., vfio) migrate these pages out of
>>> ZONE_MOVABLE/MIGRATE_CMA areas in order to long-term pin them.
>>>
>>>
>>> The biggest concern I have is that timeouts, while likely reasonable it many
>>> scenarios, might not be desirable even for some sane workloads, and the
>>> default in all system will be "no timeout", letting the clueless admin of
>>> each and every system out there that might support fuse to make a decision.
>>>
>>> I might have misunderstood something, in which case I am very sorry, but we
>>> also don't want CMA allocations to start failing simply because a network
>>> connection is down for a couple of minutes such that a fuse daemon cannot
>>> make progress.
>>>
>>
>> I think you have valid concerns but these are not new and not unique to
>> fuse. Any filesystem with a potential arbitrary stall can have similar
>> issues. The arbitrary stall can be caused due to network issues or some
>> faultly local storage.
>
> What concerns me more is that this is can be triggered by even unprivileged user space, and that there is no default protection as far as I understood, because timeouts cannot be set universally to a sane defaults.
>
> Again, please correct me if I got that wrong.
>
>
> BTW, I just looked at NFS out of interest, in particular nfs_page_async_flush(), and I spot some logic about re-dirtying pages + canceling writeback. IIUC, there are default timeouts for UDP and TCP, whereby the TCP default one seems to be around 60s (* retrans?), and the privileged user that mounts it can set higher ones. I guess one could run into similar writeback issues?
>
> So I wonder why we never required AS_WRITEBACK_INDETERMINATE for nfs? Not sure if I grasped all details about NFS and writeback and when it would redirty+end writeback, and if there is some other handling in there.
>
>>
>> Regarding the reclaim, I wouldn't say fuse or similar filesystem are
>> breaking memory reclaim as the kernel has mechanism to throttle the
>> threads dirtying the file memory to reduce the chance of situations
>> where most of memory becomes unreclaimable due to being dirty.
>
> Yes, likely even cgroups can easily limit the amount.
>
>>
>> Please note that such filesystems are mostly used in environments like
>> data center or hyperscalar and usually have more advanced mechanisms to
>> handle and avoid situations like long delays. For such environment
>> network unavailability is a larger issue than some cma allocation
>> failure. My point is: let's not assume the disastrous situaion is normal
>> and overcomplicate the solution.
>
> Let me summarize my main point: ZONE_MOVABLE/MIGRATE_CMA must only be used for movable allocations.
Exactly this.
>
> Mechanisms that possible turn these folios unmovable for a long/indeterminate time must either fail or migrate these folios out of these regions, otherwise we start violating the very semantics why ZONE_MOVABLE/MIGRATE_CMA was added in the first place.
Totally agree.
>
> Yes, there are corner cases where we cannot guarantee movability (e.g., OOM when allocating a migration destination), but these are not cases that can be triggered by (unprivileged) user space easily.
>
> That's why FOLL_LONGTERM pinning does exactly that: even if user space would promise that this is really only "short-term", we will treat it as "possibly forever", because it's under user-space control.
>
>
> Instead of having more subsystems violate these semantics because "performance" ... I would hope we would do better. Maybe it's an issue for NFS as well ("at least" only for privileged user space)? In which case, again, I would hope we would do better.
Another issue with the proposed AS_WRITEBACK_INDETERMINATE approach is that FUSE
used to use temp pages from MIGRATE_UNMOVABLE to write back dirty pages, which
confines these unmovable pages within certain pageblocks, but now any dirty page
can become unmovable due to AS_WRITEBACK_INDETERMINATE and they can spread across
the entire physical space. This means memory can be fragmented much easier, namely
with the same 512 dirty pages, previously, all could be confined in 1 pageblock,
but now in the worse scenario they can appear in 512 pageblocks.
--
Best Regards,
Yan, Zi
