Re: [PATCH v3 3/3] mm: don't miss the last page because of round-off error

[Roman Gushchin <guro@fb.com>]

On Mon, Aug 27, 2018 at 02:04:32PM -0700, Andrew Morton wrote:
> On Mon, 27 Aug 2018 09:26:21 -0700 Roman Gushchin <guro@fb.com> wrote:
> 
> > I've noticed, that dying memory cgroups are  often pinned
> > in memory by a single pagecache page. Even under moderate
> > memory pressure they sometimes stayed in such state
> > for a long time. That looked strange.
> > 
> > My investigation showed that the problem is caused by
> > applying the LRU pressure balancing math:
> > 
> >   scan = div64_u64(scan * fraction[lru], denominator),
> > 
> > where
> > 
> >   denominator = fraction[anon] + fraction[file] + 1.
> > 
> > Because fraction[lru] is always less than denominator,
> > if the initial scan size is 1, the result is always 0.
> > 
> > This means the last page is not scanned and has
> > no chances to be reclaimed.
> > 
> > Fix this by rounding up the result of the division.
> > 
> > In practice this change significantly improves the speed
> > of dying cgroups reclaim.
> > 
> > ...
> >
> > --- a/include/linux/math64.h
> > +++ b/include/linux/math64.h
> > @@ -281,4 +281,6 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
> >  }
> >  #endif /* mul_u64_u32_div */
> >  
> > +#define DIV64_U64_ROUND_UP(ll, d)	div64_u64((ll) + (d) - 1, (d))
> 
> This macro references arg `d' more than once.  That can cause problems
> if the passed expression has side-effects and is poor practice.  Can
> we please redo this with a temporary?

Argh, the original DIV_ROUND_UP can't be fixed this way, as it's used
in array's size declarations.

So, below is the patch for the new DIV64_U64_ROUND_UP macro only.

Thanks!

--

From d8237d3df222e6c5a98a74baa04bc52edf8a3677 Mon Sep 17 00:00:00 2001
From: Roman Gushchin <guro@fb.com>
Date: Wed, 29 Aug 2018 14:14:48 -0700
Subject: [PATCH] math64: prevent double calculation of DIV64_U64_ROUND_UP()
 arguments

Cause the DIV64_U64_ROUND_UP(ll, d) macro to cache
the result of (d) expression in a local variable to
avoid double calculation, which might bring unexpected
side effects.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 include/linux/math64.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 94af3d9c73e7..bb2c84afb80c 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -281,6 +281,7 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
 }
 #endif /* mul_u64_u32_div */
 
-#define DIV64_U64_ROUND_UP(ll, d)	div64_u64((ll) + (d) - 1, (d))
+#define DIV64_U64_ROUND_UP(ll, d)	\
+	({ u64 _tmp = (d); div64_u64((ll) + _tmp - 1, _tmp); })
 
 #endif /* _LINUX_MATH64_H */
-- 
2.17.1