Re: [PATCH v19 00/40] DEPT(DEPendency Tracker)

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/20/26 19:51, Matthew Wilcox wrote:
> On Thu, Aug 20, 2026 at 07:16:05PM +0200, David Hildenbrand (Arm) wrote:
>>> Consider this real deadlock pattern that lockdep cannot detect:
>>>
>>>    context X              context Y              context Z
>>>
>>>                           mutex_lock A
>>>    folio_lock B
>>>                           folio_lock B <- DEADLOCK
>>>                                                  mutex_lock A <- DEADLOCK
>>>                                                  folio_unlock B
>>>                           folio_unlock B
>>>                           mutex_unlock A
>>>                                                  mutex_unlock A
>>
>> But that really just boils down to folio lock being implemented as a PG_lock +
>> some advanced wait mechanism. And we must do that because of lack of bits in
>> struct page.
>>
>> Willy mentioned in a previous version [1]: "I don't think it makes sense to
>> track lock state in the page (nor folio).  Partly because there's just so many
>> of them, but also because the locking rules don't really apply to individual
>> folios so much as they do to the mappings (or anon_vmas) that contain folios."
>>
>> Given that lockdep is a debug feature, and we will at some point allocate struct
>> folio separately, I assume we could just squeeze a "struct lockdep_map" in there
>> in such debug configs and the world would not collapse.
>>
>> Doing that today (one "struct lockdep_map" in each "struct page") wouldn't work
>> as mm_zero_struct_page() would not expect such large "struct page". But
>> conceptually, for a debug kernel with a special CONFIG_LOCKDEP_PAGE_LOCK, maybe
>> that would already be ok and we could just do that (and optimize it as we
>> allocate folios separately).
>>
>> Not that it's ideal, but for a debug feature to at least check PG_lock, probably
>> an easier way to achieve it than some completely new infrastructure.
>>
>> Now, Willy said "locking rules don't really apply to individual folios", I
>> wonder if that could just help to also let lockdep check PG_lock with less
>> metadata? (didn't fully wrap my head around the implications)
>>
>> [1]
>> https://lore.kernel.org/all/aR3WHf9QZ_dizNun@casper.infradead.org/?utm_source=chatgpt.com
> 
> There are a few things going on that make PG_lock special.  Let me try
> to explain again, only better this time.
> 

Thanks for that information :)

> 1. The current lifetime of a struct page is the lifetime of the system.
> But the semantics of its PG_lock bit change each time it is freed and
> allocated.

Well, yeah, but that will effectively change once we dynamically allocate struct
folio.

Until then, don't we have the following two events:

(a) Allocator handing out a page
(b) Page getting returned to page allocator

And while in the allocator, the PG_locked is essentially unused/unusable (and so
would be lockdep data)?

I'd argue there is a well defined lifetime for PG_locked and any additional
lockdep metadata. At least theoretically ;)

> 
> 2. The position of PG_lock in the locking hierarchy only depend on
> what the folio is currently being used for.  That is, all folios in
> a given xfs inode behave exactly the same from a locking perspective.
> There's no need to build up state about how each PG_lock is used;
> they can all share.  Arguably all xfs file inodes are the same as
> each other (directory inodes might be different from file inodes),
> so we might want to go further than telling DEPT that "this folio
> belongs to this inode" and go to "this folio belongs to this xfs file
> inode".

"There is no need", I am not entirely sure when it comes to anonymous folios.

Even if there is actually no need, given that we are talking about debugging
mechanisms, I am not sure if optimizing for less memory consumption is our
highest priority. Debug kernels already consume plenty of memory on other things
(page_ext, for example), so consuming some more to support the folio lock with
lockdep does not sound too crazy for me.

(maybe lockdep data could even life in page_ext)

I understand that Dept applies to more things than just PG_locked, though.

> 
> 3. PG_lock can be taken in task context then released in interrupt
> context.  For full points, we need to mark the exact point at which
> we submit the folio for read.  Otherwise we can get into the situation
> alluded to by f2c817bed58d and better discussed at
> https://lore.kernel.org/linux-mm/20200127150024.GN1183@dhcp22.suse.cz/
> where we have the folio locked but haven't yet submitted it for I/O
> so it doesn't matter how long we wait, it will never come unlocked.

Yeah, it seems hard to squeeze different locking contexts into existing lockdep.
I cannot tell whether lockdep could be adjusted to allow for that, or how much
work it would involve.

So yeah, Dept might be a more natural fit for this scenario.

-- 
Cheers,

David