Re: [PATCH] selftests/damon: prevent remaining cross-object state pollution

[SJ Park]

Hello zhaozhengzhuo,

On Thu, 27 Aug 2026 10:14:52 +0800 zhaozhengzhuo@xxxxxxxxxxxxx wrote:

> From: zhaozhengzhuo <zhaozhengzhuo@xxxxxxxxxxxxx>
> 
> _damon_sysfs.py defines constructors with mutable default arguments,
> including DamosAccessPattern(), DamosQuota(), DamosWatermarks(),
> DamosDests(), IntervalsGoal(), and empty lists.
> 
> Default arguments are evaluated once at function definition time.
> Damos() instances created without explicit arguments therefore share
> the same DamosQuota(), and the other default-constructed sub-objects
> and lists are shared in the same way.  The sub-objects keep
> back-pointers to their owner scheme, so constructing the second Damos()
> rebinds the shared quota's scheme pointer to the second object.  An
> item appended to one object's default contexts or filters list is also
> visible from other default-constructed objects.
> 
> The shared state can corrupt test configurations.  DamosQuota.sysfs_dir()
> derives the sysfs directory from its scheme pointer, so operating on
> the first scheme's default quota may write to the second scheme's
> directory.  The wrong values often match the defaults, so tests still
> pass, but the behavior depends on object creation order.

I agree all the points.  This should be fixed.  That said, these are only
possible future issues.  No existing test is broken, right?

> 
> Commit 8319dadcbd81 ("selftests/damon: prevent cross-context state
> pollution in DamonCtx") fixed the same pattern in DamonCtx only.  Fix
> the remaining constructors by defaulting to None and creating fresh
> objects or lists inside each constructor.  Explicit arguments keep
> their previous behavior.

Thank you for fixing this, looks good to me!

> 
> Signed-off-by: zhaozhengzhuo <zhaozhengzhuo@xxxxxxxxxxxxx>

Reviewed-by: SJ Park <sj@xxxxxxxxxx>

This patch is applied to damon/next [1] tree.  If this patch is not added to
mm.git in short term (~1 week?), I will ask mm.git maintainer (Andrew Morton)
to pick this.  So, no action from your side is needed for now.  If it seems I
also forgot doing that or you cannot wait for my action, please feel free to
directly ask that to Andrew.

[1] https://origin.kernel.org/doc/html/latest/mm/damon/maintainer-profile.html#scm-trees


Thanks,
SJ

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