[lxc-devel] [PATCH lxcfs 4/5] cgfs: fix dorealloc's batch allocation
Serge Hallyn
serge.hallyn at ubuntu.com
Thu Jan 7 19:20:24 UTC 2016
Quoting Wolfgang Bumiller (w.bumiller at proxmox.com):
>
> > On January 7, 2016 at 7:42 PM Serge Hallyn <serge.hallyn at ubuntu.com> wrote:
> >
> >
> > Quoting Wolfgang Bumiller (w.bumiller at proxmox.com):
> > > The initial check should use real lengths as with modulo a
> > > new required length of eg. 52 would be considered smaller
> > > than an old length of 48 (2 < 48).
> > >
> > > To get the 'batches' count 'newlen' must be divided and not
> > > taken modulo BATCH_SIZE. Otherwise '101', which would need a
> > > 3rd batch to reach 150, would end up with two (2*50 = 100
> > > bytes) and thereby be truncated instead.
> > >
> > > Signed-off-by: Wolfgang Bumiller <w.bumiller at proxmox.com>
> > > ---
> > > cgfs.c | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/cgfs.c b/cgfs.c
> > > index 0659e9e..681a478 100644
> > > --- a/cgfs.c
> > > +++ b/cgfs.c
> > > @@ -75,9 +75,9 @@ static inline void drop_trailing_newlines(char *s)
> > > static void dorealloc(char **mem, size_t oldlen, size_t newlen)
> > > {
> > > int batches;
> > > - if (newlen % BATCH_SIZE <= oldlen % BATCH_SIZE)
> > > + if (newlen <= oldlen)
> >
> > This will result in extra reallocs (though innocuous).
> > Is there any reason not to just do
> >
> > if (newlen / BATCH_SIZE <= oldlen / BATCH_SIZE)
>
> Ah yes I missed that the old length is a string length not the
> capacity, so yes, the division makes sense. Just not the modulus.
>
Ok, thanks - per your patch I think the following makes a nice cleanup:
>From a72193ff0dba319c3039372bc038708a75d0582f Mon Sep 17 00:00:00 2001
From: Serge Hallyn <serge.hallyn at ubuntu.com>
Date: Thu, 7 Jan 2016 11:17:17 -0800
Subject: [PATCH 1/1] dorealloc: avoid extra reallocs
The original check was very wrong, using % instead of /. However
the length we track is the actual used length, not the allocated
length, which is always (len / BATCH_SIZE) + 1. We don't want
to realloc when newlen is between oldlen and (oldlen / BATCH_SIZE) + 1)
Signed-off-by: Serge Hallyn <serge.hallyn at ubuntu.com>
---
cgfs.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/cgfs.c b/cgfs.c
index ec0f630..f37bc2b 100644
--- a/cgfs.c
+++ b/cgfs.c
@@ -74,18 +74,20 @@ static inline void drop_trailing_newlines(char *s)
#define BATCH_SIZE 50
static void dorealloc(char **mem, size_t oldlen, size_t newlen)
{
- int batches;
- if (newlen <= oldlen)
+ int newbatches = (newlen / BATCH_SIZE) + 1;
+ int oldbatches = (olden / BATCH_SIZE) + 1;
+
+ if (newbatches <= oldbatches)
return;
- batches = (newlen / BATCH_SIZE) + 1;
+
if (!*mem) {
do {
- *mem = malloc(batches * BATCH_SIZE);
+ *mem = malloc(newbatches * BATCH_SIZE);
} while (!*mem);
} else {
char *tmp;
do {
- tmp = realloc(*mem, batches * BATCH_SIZE);
+ tmp = realloc(*mem, newbatches * BATCH_SIZE);
} while (!tmp);
*mem = tmp;
}
--
2.5.0
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