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SubjectRe: [PATCH] mm/util.c: Make kvfree() safe for calling while holding spinlocks
On Sun, Dec 26, 2021 at 06:57:16PM +0100, Uladzislau Rezki wrote:
> On Sat, Dec 25, 2021 at 10:58:29PM +0000, Matthew Wilcox wrote:
> > On Sat, Dec 25, 2021 at 07:54:12PM +0100, Uladzislau Rezki wrote:
> > > +static void drain_vmap_area(struct work_struct *work)
> > > +{
> > > + if (mutex_trylock(&vmap_purge_lock)) {
> > > + __purge_vmap_area_lazy(ULONG_MAX, 0);
> > > + mutex_unlock(&vmap_purge_lock);
> > > + }
> > > +}
> > > +
> > > +static DECLARE_WORK(drain_vmap_area_work, drain_vmap_area);
> >
> > Presuambly if the worker fails to get the mutex, it should reschedule
> > itself? And should it even trylock or just always lock?
> >
> mutex_trylock() has no sense here. It should just always get the lock.
> Otherwise we can miss the point to purge. Agree with your opinion.
>
Below the patch that address Matthew's points:

<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d2a00ad4e1dd..b82db44fea60 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1717,17 +1717,10 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
return true;
}

-/*
- * Kick off a purge of the outstanding lazy areas. Don't bother if somebody
- * is already purging.
- */
-static void try_purge_vmap_area_lazy(void)
-{
- if (mutex_trylock(&vmap_purge_lock)) {
- __purge_vmap_area_lazy(ULONG_MAX, 0);
- mutex_unlock(&vmap_purge_lock);
- }
-}
+static void purge_vmap_area_lazy(void);
+static void drain_vmap_area(struct work_struct *work);
+static DECLARE_WORK(drain_vmap_area_work, drain_vmap_area);
+static atomic_t drain_vmap_area_work_in_progress;

/*
* Kick off a purge of the outstanding lazy areas.
@@ -1740,6 +1733,22 @@ static void purge_vmap_area_lazy(void)
mutex_unlock(&vmap_purge_lock);
}

+static void drain_vmap_area(struct work_struct *work)
+{
+ mutex_lock(&vmap_purge_lock);
+ __purge_vmap_area_lazy(ULONG_MAX, 0);
+ mutex_unlock(&vmap_purge_lock);
+
+ /*
+ * Check if rearming is still required. If not, we are
+ * done and can let a next caller to initiate a new drain.
+ */
+ if (atomic_long_read(&vmap_lazy_nr) > lazy_max_pages())
+ schedule_work(&drain_vmap_area_work);
+ else
+ atomic_set(&drain_vmap_area_work_in_progress, 0);
+}
+
/*
* Free a vmap area, caller ensuring that the area has been unmapped
* and flush_cache_vunmap had been called for the correct range
@@ -1766,7 +1775,8 @@ static void free_vmap_area_noflush(struct vmap_area *va)

/* After this point, we may free va at any time */
if (unlikely(nr_lazy > lazy_max_pages()))
- try_purge_vmap_area_lazy();
+ if (!atomic_xchg(&drain_vmap_area_work_in_progress, 1))
+ schedule_work(&drain_vmap_area_work);
}

/*
<snip>
Manfred, could you please have a look and if you have a time test it?
I mean if it solves your issue. You can take over this patch and resend
it, otherwise i can send it myself later if we all agree with it.

--
Vlad Rezki

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