linux-kernel - Re: [PATCH v3 0/2] iommu/iova: Make the rcache depot properly flexible

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Date: Wed, 10 Jan 2024 08:52:06 +0800
From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
To: Robin Murphy <robin.murphy@....com>, Ido Schimmel <idosch@...sch.org>
Cc: joro@...tes.org, will@...nel.org, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org, zhangzekun11@...wei.com,
 john.g.garry@...cle.com, dheerajkumar.srivastava@....com, jsnitsel@...hat.com
Subject: Re: [PATCH v3 0/2] iommu/iova: Make the rcache depot properly
 flexible


On 1/9/2024 7:26 PM, Robin Murphy wrote:
> On 2024-01-09 6:23 am, Ethan Zhao wrote:
>>
>> On 1/9/2024 1:54 PM, Ethan Zhao wrote:
>>>
>>> On 1/9/2024 1:35 AM, Robin Murphy wrote:
>>>> On 2023-12-28 12:23 pm, Ido Schimmel wrote:
>>>>> On Tue, Sep 12, 2023 at 05:28:04PM +0100, Robin Murphy wrote:
>>>>>> v2: 
>>>>>> https://lore.kernel.org/linux-iommu/cover.1692641204.git.robin.murphy@arm.com/
>>>>>>
>>>>>> Hi all,
>>>>>>
>>>>>> I hope this is good to go now, just fixed the locking (and threw
>>>>>> lockdep at it to confirm, which of course I should have done to 
>>>>>> begin
>>>>>> with...) and picked up tags.
>>>>>
>>>>> Hi,
>>>>>
>>>>> After pulling the v6.7 changes we started seeing the following memory
>>>>> leaks [1] of 'struct iova_magazine'. I'm not sure how to reproduce 
>>>>> it,
>>>>> which is why I didn't perform bisection. However, looking at the
>>>>> mentioned code paths, they seem to have been changed in v6.7 as 
>>>>> part of
>>>>> this patchset. I reverted both patches and didn't see any memory 
>>>>> leaks
>>>>> when running a full regression (~10 hours), but I will repeat it 
>>>>> to be
>>>>> sure.
>>>>>
>>>>> Any idea what could be the problem?
>>>>
>>>> Hmm, we've got what looks to be a set of magazines forming a 
>>>> plausible depot list (or at least the tail end of one):
>>>>
>>>> ffff8881411f9000 -> ffff8881261c1000
>>>>
>>>> ffff8881261c1000 -> ffff88812be26400
>>>>
>>>> ffff88812be26400 -> ffff8188392ec000
>>>>
>>>> ffff8188392ec000 -> ffff8881a5301000
>>>>
>>>> ffff8881a5301000 -> NULL
>>>>
>>>> which I guess has somehow become detached from its rcache->depot 
>>>> without being freed properly? However I'm struggling to see any 
>>>> conceivable way that could happen which wouldn't already be more 
>>>> severely broken in other ways as well (i.e. either general memory 
>>>> corruption or someone somehow still trying to use the IOVA domain 
>>>> while it's being torn down).
>>>>
>>>> Out of curiosity, does reverting just patch #2 alone make a 
>>>> difference? And is your workload doing anything "interesting" in 
>>>> relation to IOVA domain lifetimes, like creating and destroying 
>>>> SR-IOV virtual functions, changing IOMMU domain types via sysfs, or 
>>>> using that horrible vdpa thing, or are you seeing this purely from 
>>>> regular driver DMA API usage?
>>>
>>> There no lock held when free_iova_rcaches(), is it possible 
>>> free_iova_rcaches() race with the delayed cancel_delayed_work_sync() ?
>>>
>>> I don't know why not call cancel_delayed_work_sync(&rcache->work); 
>>> first in free_iova_rcaches() to avoid possible race.
>>>
>> between following functions pair, race possible ? if called cocurrently.
>>
>> 1. free_iova_rcaches() with iova_depot_work_func()
>>
>>     free_iova_rcaches() holds no lock, iova_depot_work_func() holds 
>> rcache->lock.
>
> Unless I've completely misunderstood the workqueue API, that can't 
> happen, since free_iova_rcaches() *does* synchronously cancel the work 
> before it starts freeing the depot list.

iova_depot_work_func() pop and free mag from depot. free_iova_rcaches() 
frees loaded and previous mag before syncronously cancelled.

different thing. okay here.

>
>> 2. iova_cpuhp_dead() with iova_depot_work_func()
>>
>>    iova_cpuhp_dead() holds per cpu lock cpu_rcache->lock, 
>> iova_depot_work_func() holds rcache->lock.
>
> That's not a race because those are touching completely different 
> things - the closest they come to interacting is where they both free 
> IOVAs back to the rbtree.

iova_cpuhp_dead() free pages with 
iova_magazine_free_pfns(cpu_rcache->loaded, iovad);

iova_depot_work_func() free mag from depot. iova_magazine_free_pfns() 
hold rbtree lock.

Okay, different thing.

>
>> 3. iova_cpuhp_dead() with free_iova_rcaches()
>>
>>     iova_cpuhp_dead() holds per cpu lock cpu_rcache->lock, 
>> free_iova_rcaches() holds no lock.
>
> See iova_domain_free_rcaches() - by the time we call 
> free_iova_rcaches(), the hotplug handler has already been removed (and 
> either way it couldn't account for *this* issue since it doesn't touch 
> the depot at all).
Yes, iova_cpuhp_dead() was removed before free_iova_rcaches().
>
>> 4. iova_cpuhp_dead() with free_global_cached_iovas()
>>
>>     iova_cpuhp_dead() holds per cpu lock cpu_rcache->lock and 
>> free_global_cached_iovas() holds rcache->lock.
>
> Again, they hold different locks because they're touching unrelated 
> things.

iova_cpuhp_dead() free loaded and previous pages. 
free_global_cached_iovas() free mags from depot.

Okay too.

then free_global_cached_iovas() with iova_depot_work_func() ? they all 
hold rcache->lock.

So there is no race at all, perfect.  out of imagination, that memory 
leak report.

kmemleak not always right.


Thanks,

Ethan

>
> Thanks,
> Robin.