This will prevent us from accidentally introducing a memory bloat
regression here in the future.
Signed-off-by: Eric Wong <normalperson@xxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>,
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 7 Mar 2013 10:32:40 +0000 Eric Wong <normalperson@xxxxxxxx> wrote:
>
> > Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > It's going to be hard to maintain this - someone will change something
> > > sometime and break it. I suppose we could add a runtime check if we
> > > cared enough. Adding a big fat comment to struct epitem might help.
> >
> > Thanks for looking at this patch. I'll send a patch with a comment
> > about keeping epitem size in check. Also, would adding (with comments):
> >
> > BUILD_BUG_ON(sizeof(struct epitem) > 128);
> >
> > ...be too heavy-handed? I used that in my testing. I'll check for:
> > sizeof(void *) <= 8 too; in case 128-bit machines appear...
>
> I guess such a check might avoid accidents in the future. If it
> becomes a problem, we can always delete it.
fs/eventpoll.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 9fec183..55028da 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -128,6 +128,8 @@ struct nested_calls {
/*
* Each file descriptor added to the eventpoll interface will
* have an entry of this type linked to the "rbr" RB tree.
+ * Avoid increasing the size of this struct, there can be many thousands
+ * of these on a server and we do not want this to take another cache line.
*/
struct epitem {
/* RB tree node used to link this structure to the eventpoll RB tree */
@@ -1964,6 +1966,12 @@ static int __init eventpoll_init(void)
/* Initialize the structure used to perform file's f_op->poll() calls */
ep_nested_calls_init(&poll_readywalk_ncalls);
+ /*
+ * We can have many thousands of epitems, so prevent this from
+ * using an extra cache line on 64-bit (and smaller) CPUs
+ */
+ BUILD_BUG_ON(sizeof(void *) <= 8 && sizeof(struct epitem) > 128);
+
/* Allocates slab cache used to allocate "struct epitem" items */
epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
--
Eric Wong
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
